How did ancient astronomers know to order the planets from the closest to the farthest from the Sun?

How did ancient astronomers know to order the planets from the closest to the farthest from the Sun?

If we assume a geocentric model of the universe (like ancient astronomers did) how could we ever find out (again, like ancient astronomers did) the right distances of the planets? For example, how did they know that Jupiter is further than Mars, if Jupiter is brighter?

According to the Cambridge Concise History of Astronomy (p 33 of my edition), essentially the Greeks took the (not unreasonable) view that the planets that moved more slowly were further away and were orbiting on larger spheres.

That's obviously not the same as suggesting they knew "the right distances" to the planets, merely the order. They did develop a mathematically sound means of estimating the distance to the Moon, though their observational data let them down on a correct estimate.

They also thought the Sun was in the fourth sphere around the Earth (to explain the observed behaviour of Mercury and Venus).

Not all Greeks thought the Sun revolved round the Earth and those who argued for a heliocentric universe said the reasons the stars did not move as the Earth did was because they were very, very far away.

How did ancient astronomers know to order the planets from the closest to the farthest from the Sun? - Astronomy

Beginnings of Astronomy

Key points: Why ancient humans pursued astronomy apparent motions of stars and planets on the sky examples of major astronomical monuments

Knowing the time of year relative to the seasons is critical to determine when to plant crops. It was obvious that the month was about 30 days and the year was about 365 days or 12 months, but none of these relations is exact so any counting scheme had to be "reset" so it wouldn’t drift.

The first observatories we know addressed this problem

Stonehenge is a famous example. Among other things, it was used to identify the first day of summer, the "summer solstice." Ancient astronomers could use it like a giant gunsight to determine when the sun was rising in a certain direction. (From Getty Trust,
This picture gives you an idea of how it might have looked in its prime. It is of a reconstructed Druid ceremony at Stonehenge (From C. Witcombe

An even earlier example can be found at Newgrange, just north of Dublin

Through such observations, the ancient astronomers developed calendars that eventually evolved into the one we use today

Early man also knew that what stars could be seen and how they appeared to move during the night depended on where he was on the earth:

The further north one travels, the higher the North star appears in the sky and stars appear to move closer to parallel to the horizon.

Here is how the motions look from Tucson (assuming we are looking south and have super-wide-angle eyes that can see all the way from east to west) (animation by G. Rieke) you may have to reload to start it.
and here is how they would look from the North Pole (assuming our super-wide-angle eyes take in the pole star at the top of the animation) (by G. Rieke) .

It is this behavior that suggested that the stars and planets were placed on great crystal spheres that were centered on the earth and rotated around it.

The dependence of apparent positions on the sky on north-south position resulted in using stars for navigation. Reconstructed Greek Trireme, Olympios , (from Perseus Encyclopedia,
" Glorious Odysseus, happy with the wind, spread sails and taking his seat artfully with the steering oar he held her on her course ['her' is a raft he built to sail away from the nymph Kalypso], nor did sleep ever descend on his eyelids as he kept his eye on the Pleiades [a cluster of stars] and late-setting Bootes [a constellation, an identifiable pattern of stars in the sky] and the Bear [the constellation now known as the Big Dipper], to whom men give also the name of the Wagon, who turns about in a fixed place and looks at Orion [another easily identified constellation], and she alone is never plunged in the wash of the Ocean. For so Kalypso, bright among goddesses, had told him to make his way over the sea, keeping the Bear on his left hand."

Some objects appeared to be points of light that move with respect to the stars. The Greeks gave this type of object the name "planet" for wanderer. Mercury, Venus, Mars, Jupiter, and Saturn have been known since pre-historic times.

These complications led to sophisticated systems of astronomy/astrology. To predict dramatic celestial events, the astronomers/priests/astrologers developed accurate long term calendars and texts.

The Maya of meso-America provide an example of great accomplishments in astronomy, which they embodied into religious/ceremonial aspects of their culture.

Building such large monuments therefore required substantial investments in astronomy as well as in architecture An example is El Caracol, a Mayan observatory at Chichen Itza, not far from El Castillo (G. Rieke )
Temples at Tikal, Guatemala, are elevated above the treetops to allow unobstructed viewing of the stars. Temple IV at Tikal, built in about 470AD, rises to a height of 212 feet above the jungle floor . (G. Rieke)
Modern (days) Maya in days
Lunar (synodic) month 29.53059 29.53086
Synodic period of Venus 583.93 583.92027
Synodic period of Mars 779.94 780
Solar (tropical) year 365.24198 365.242

However, this accomplishment was addressed more toward prediction of propitious times for rulers to take action, that is, predicting the future. It belongs more in the realm of a very sophisticated astrology than to that of astronomy. The failure of the Mayas to seek underlying causes for the processes they measured so precisely meant that their approach was a dead end scientifically.

Not all ancient societies were as accomplished in astronomy as the Maya. However, virtually all had myths associated with the sky and earth, particularly creation myths In general, such myths are not refined through detailed measurements (such as those in the table above by the Maya) and they may not even be updated as new astronomical knowledge is acquired. Thus, they are basically not examples of a scientific approach.

1. The planets orbit the Sun

A few centuries later, there had been a lot of progress. Aristarchus of Samos (310BC to 230BC) argued that the Sun was the “central fire” of the cosmos and he placed all of the then known planets in their correct order of distance around it. This is the earliest known heliocentric theory of the solar system.

Unfortunately, the original text in which he makes this argument has been lost to history, so we cannot know for certain how he worked it out. Aristarchus knew the Sun was much bigger than the Earth or the Moon, and he may have surmised that it should therefore have the central position in the solar system.

Nevertheless it is a jawdropping finding, especially when you consider that it wasn’t rediscovered until the 16th century, by Nicolaus Copernicus, who even acknowledged Aristarchus during the development of his own work.

What's the Order of the Planets in the Solar System?

Over the past 60 years, humans have begun to explore our solar system in earnest. From the first launches in the late 1950s until today, we've sent probes, orbiters, landers and even rovers (like NASA's Perseverance Rover that touched down on Mars in February 2021) to every planet in our solar system. But can you name all eight of those planets? (Yes, there's only eight – not nine. Pluto got "demoted" in 2006.) And can you put them in the correct order?

In case you're a little rusty, we'll break down some common ways to order the planets plus a few tricks to help you remember them going forward. Let's start with distance from the sun.

The Order of the Planets by Distance

The most common way to order the planets is by their distance from the sun. Using this method, the planets are listed in the following order:

  • Mercury - 0.39 AU from the sun
  • Venus - 0.72 AU
  • Earth - 1.00 AU
  • Mars - 1.52 AU
  • Jupiter - 5.20 AU
  • Saturn - 9.54 AU
  • Uranus - 19.20 AU
  • Neptune - 30.06 AU

AU stands for astronomical units – it's the equivalent to the average distance from Earth to the sun (which is why Earth is 1 AU from the sun). It's a common way astronomers measure distances in the solar system that accounts for the large scale of these distances. To put it another way, Mercury, which is closest, is 35.98 million miles from the sun, while Neptune, the farthest, is 2.79 billion miles from the sun. Earth is 92.96 million miles from the sun.

How to Remember the Order of the Planets

There are many handy expressions to remember the order of the planets. These are typically mnemonics which use the first letter of each planet's name to come up with a phrase that's easier to remember.

  • My Very Excellent Mother Just Served Us Noodles (or Nachos)
  • My Very Easy Method Just Speeds Up Names
  • My Very Expensive Malamute Jumped Ship Up North

In each case, "M" stands for "Mercury," "V" for "Venus," and so on. You can also try to remember them with a few rhyming verses:

Finally, if you are musically inclined, there are a few songs that may help you remember. Two popular ones are Mr. R's Planet Song and The Planet Song from Kids Learning Tube.

You Can Order the Planets in Other Ways

While most people want to know the order of the planets by distance, there are other ways to order the planets that you might be curious about.

For example, if you order the planets by size (radius) from biggest to smallest, then the list would be:

  • Jupiter (43,441 miles/69,911 kilometers)
  • Saturn (36,184 miles/58,232 km)
  • Uranus (15,759 miles (25,362 km)
  • Neptune (15,299 miles/24,622 km)
  • Earth (3,959 miles/6,371 km)
  • Venus (3,761 miles/6,052 km)
  • Mars (2,460 miles/3,390 km)
  • Mercury (1,516 miles/2,440 km)

Or you could order the planets by weight (mass). Then, the list from most massive to least massive would be: Jupiter (1.8986 x 10 27 kilograms), Saturn (5.6846 x 10 26 kg), Neptune (10.243 x 10 25 kg), Uranus (8.6810 x 10 25 kg), Earth (5.9736 x 10 24 kg), Venus (4.8685 x 10 24 kg), Mars (6.4185 x 10 23 kg), and Mercury (3.3022 x 10 23 kg). Interestingly, Neptune has more mass than Uranus, even though Uranus is larger! Scientists can't put a planet on a scale, so to determine mass, they look at how long it takes nearby objects to orbit the planet and how far from the planet those objects are. The heavier the planet, the stronger it pulls on nearby objects.

Finally, a fun way to order the planets is by the number of moons they have. We'll start with the planet that has the most:

  • Saturn (82)
  • Jupiter (79)
  • Uranus (27)
  • Neptune (14)
  • Mars (2)
  • Earth (1)
  • Venus and Mercury (both zero)

(Note that these numbers include provisional moons that are still being confirmed by astronomers.)

In short, there are a number of ways to order and reorder the planets based on different facts about them as long as you remember there are eight in total, that's what counts. (Sorry, Pluto!)

Speaking of Pluto, what's the deal? After its discovery in 1930, Pluto was classified as a planet. However, in 2006, the International Astronomical Union downgraded Pluto from "planet" to "dwarf planet." This is because the definition of a planet means that it has cleared its orbit of other objects (which Pluto has not done, as it shares its space with many Kuiper Belt objects). Pluto is one of five dwarf planets in our solar system – and it's not even the largest one (that's Eris).

2.2 Ancient Astronomy

Let us now look briefly back into history. Much of modern Western civilization is derived in one way or another from the ideas of the ancient Greeks and Romans, and this is true in astronomy as well. However, many other ancient cultures also developed sophisticated systems for observing and interpreting the sky.

Astronomy around the World

Ancient Babylonian, Assyrian, and Egyptian astronomers knew the approximate length of the year. The Egyptians of 3000 years ago, for example, adopted a calendar based on a 365-day year. They kept careful track of the rising time of the bright star Sirius in the predawn sky, which has a yearly cycle that corresponded with the flooding of the Nile River. The Chinese also had a working calendar they determined the length of the year at about the same time as the Egyptians. The Chinese also recorded comets, bright meteors, and dark spots on the Sun. (Many types of astronomical objects were introduced in Science and the Universe: A Brief Tour. If you are not familiar with terms like comets and meteors, you may want to review that chapter.) Later, Chinese astronomers kept careful records of “guest stars”—those that are normally too faint to see but suddenly flare up to become visible to the unaided eye for a few weeks or months. We still use some of these records in studying stars that exploded a long time ago.

The Mayan culture in Mexico and Central America developed a sophisticated calendar based on the planet Venus, and they made astronomical observations from sites dedicated to this purpose a thousand years ago. The Polynesians learned to navigate by the stars over hundreds of kilometers of open ocean—a skill that enabled them to colonize new islands far away from where they began.

In Britain, before the widespread use of writing, ancient people used stones to keep track of the motions of the Sun and Moon. We still find some of the great stone circles they built for this purpose, dating from as far back as 2800 BCE. The best known of these is Stonehenge, which is discussed in Earth, Moon, and Sky. 1

Early Greek and Roman Cosmology

Our concept of the cosmos—its basic structure and origin—is called cosmology , a word with Greek roots. Before the invention of telescopes, humans had to depend on the simple evidence of their senses for a picture of the universe. The ancients developed cosmologies that combined their direct view of the heavens with a rich variety of philosophical and religious symbolism.

At least 2000 years before Columbus, educated people in the eastern Mediterranean region knew Earth was round. Belief in a spherical Earth may have stemmed from the time of Pythagoras , a philosopher and mathematician who lived 2500 years ago. He believed circles and spheres to be “perfect forms” and suggested that Earth should therefore be a sphere. As evidence that the gods liked spheres, the Greeks cited the fact that the Moon is a sphere, using evidence we describe later.

The writings of Aristotle (384–322 BCE), the tutor of Alexander the Great, summarize many of the ideas of his day. They describe how the progression of the Moon’s phases—its apparent changing shape—results from our seeing different portions of the Moon’s sunlit hemisphere as the month goes by (see Earth, Moon, and Sky). Aristotle also knew that the Sun has to be farther away from Earth than is the Moon because occasionally the Moon passed exactly between Earth and the Sun and hid the Sun temporarily from view. We call this a solar eclipse.

Aristotle cited convincing arguments that Earth must be round. First is the fact that as the Moon enters or emerges from Earth’s shadow during an eclipse of the Moon, the shape of the shadow seen on the Moon is always round (Figure 2.9). Only a spherical object always produces a round shadow. If Earth were a disk, for example, there would be some occasions when the sunlight would strike it edge-on and its shadow on the Moon would be a line.

As a second argument, Aristotle explained that travelers who go south a significant distance are able to observe stars that are not visible farther north. And the height of the North Star—the star nearest the north celestial pole—decreases as a traveler moves south. On a flat Earth, everyone would see the same stars overhead. The only possible explanation is that the traveler must have moved over a curved surface on Earth, showing stars from a different angle. (See the How Do We Know Earth Is Round? feature for more ideas on proving Earth is round.)

One Greek thinker, Aristarchus of Samos (310–230 BCE), even suggested that Earth was moving around the Sun, but Aristotle and most of the ancient Greek scholars rejected this idea. One of the reasons for their conclusion was the thought that if Earth moved about the Sun, they would be observing the stars from different places along Earth’s orbit. As Earth moved along, nearby stars should shift their positions in the sky relative to more distant stars. In a similar way, we see foreground objects appear to move against a more distant background whenever we are in motion. When we ride on a train, the trees in the foreground appear to shift their position relative to distant hills as the train rolls by. Unconsciously, we use this phenomenon all of the time to estimate distances around us.

The apparent shift in the direction of an object as a result of the motion of the observer is called parallax . We call the shift in the apparent direction of a star due to Earth’s orbital motion stellar parallax. The Greeks made dedicated efforts to observe stellar parallax, even enlisting the aid of Greek soldiers with the clearest vision, but to no avail. The brighter (and presumably nearer) stars just did not seem to shift as the Greeks observed them in the spring and then again in the fall (when Earth is on the opposite side of the Sun).

This meant either that Earth was not moving or that the stars had to be so tremendously far away that the parallax shift was immeasurably small. A cosmos of such enormous extent required a leap of imagination that most ancient philosophers were not prepared to make, so they retreated to the safety of the Earth-centered view, which would dominate Western thinking for nearly two millennia.

Astronomy Basics

How Do We Know Earth Is Round?

In addition to the two ways (from Aristotle’s writings) discussed in this chapter, you might also reason as follows:

  1. Let’s watch a ship leave its port and sail into the distance on a clear day. On a flat Earth, we would just see the ship get smaller and smaller as it sails away. But this isn’t what we actually observe. Instead, ships sink below the horizon, with the hull disappearing first and the mast remaining visible for a while longer. Eventually, only the top of the mast can be seen as the ship sails around the curvature of Earth. Finally, the ship disappears under the horizon.
  2. The International Space Station circles Earth once every 90 minutes or so. Photographs taken from the shuttle and other satellites show that Earth is round from every perspective.
  3. Suppose you made a friend in each time zone of Earth. You call all of them at the same hour and ask, “Where is the Sun?” On a flat Earth, each caller would give you roughly the same answer. But on a round Earth you would find that, for some friends, the Sun would be high in the sky whereas for others it would be rising, setting, or completely out of sight (and this last group of friends would be upset with you for waking them up).

Measurement of Earth by Eratosthenes

The Greeks not only knew Earth was round, but also they were able to measure its size. The first fairly accurate determination of Earth’s diameter was made in about 200 BCE by Eratosthenes (276–194 BCE), a Greek living in Alexandria, Egypt. His method was a geometric one, based on observations of the Sun.

The Sun is so distant from us that all the light rays that strike our planet approach us along essentially parallel lines. To see why, look at Figure 2.10. Take a source of light near Earth—say, at position A. Its rays strike different parts of Earth along diverging paths. From a light source at B, or at C (which is still farther away), the angle between rays that strike opposite parts of Earth is smaller. The more distant the source, the smaller the angle between the rays. For a source infinitely distant, the rays travel along parallel lines.

Of course, the Sun is not infinitely far away, but given its distance of 150 million kilometers, light rays striking Earth from a point on the Sun diverge from one another by an angle far too small to be observed with the unaided eye. As a consequence, if people all over Earth who could see the Sun were to point at it, their fingers would, essentially, all be parallel to one another. (The same is also true for the planets and stars—an idea we will use in our discussion of how telescopes work.)

Eratosthenes was told that on the first day of summer at Syene, Egypt (near modern Aswan), sunlight struck the bottom of a vertical well at noon. This indicated that the Sun was directly over the well—meaning that Syene was on a direct line from the center of Earth to the Sun. At the corresponding time and date in Alexandria, Eratosthenes observed the shadow a column made and saw that the Sun was not directly overhead, but was slightly south of the zenith, so that its rays made an angle with the vertical equal to about 1/50 of a circle (7°). Because the Sun’s rays striking the two cities are parallel to one another, why would the two rays not make the same angle with Earth’s surface? Eratosthenes reasoned that the curvature of the round Earth meant that “straight up” was not the same in the two cities. And the measurement of the angle in Alexandria, he realized, allowed him to figure out the size of Earth. Alexandria, he saw, must be 1/50 of Earth’s circumference north of Syene (Figure 2.11). Alexandria had been measured to be 5000 stadia north of Syene. (The stadium was a Greek unit of length, derived from the length of the racetrack in a stadium.) Eratosthenes thus found that Earth’s circumference must be 50 × 5000, or 250,000 stadia.

It is not possible to evaluate precisely the accuracy of Eratosthenes solution because there is doubt about which of the various kinds of Greek stadia he used as his unit of distance. If it was the common Olympic stadium, his result is about 20% too large. According to another interpretation, he used a stadium equal to about 1/6 kilometer, in which case his figure was within 1% of the correct value of 40,000 kilometers. Even if his measurement was not exact, his success at measuring the size of our planet by using only shadows, sunlight, and the power of human thought was one of the greatest intellectual achievements in history.

Hipparchus and Precession

Perhaps the greatest astronomer of antiquity was Hipparchus , born in Nicaea in what is present-day Turkey. He erected an observatory on the island of Rhodes around 150 BCE, when the Roman Republic was expanding its influence throughout the Mediterranean region. There he measured, as accurately as possible, the positions of objects in the sky, compiling a pioneering star catalog with about 850 entries. He designated celestial coordinates for each star, specifying its position in the sky, just as we specify the position of a point on Earth by giving its latitude and longitude.

He also divided the stars into apparent magnitudes according to their apparent brightness. He called the brightest ones “stars of the first magnitude” the next brightest group, “stars of the second magnitude” and so forth. This rather arbitrary system, in modified form, still remains in use today (although it is less and less useful for professional astronomers).

By observing the stars and comparing his data with older observations, Hipparchus made one of his most remarkable discoveries: the position in the sky of the north celestial pole had altered over the previous century and a half. Hipparchus deduced correctly that this had happened not only during the period covered by his observations, but was in fact happening all the time: the direction around which the sky appears to rotate changes slowly but continuously. Recall from the section on celestial poles and the celestial equator that the north celestial pole is just the projection of Earth’s North Pole into the sky. If the north celestial pole is wobbling around, then Earth itself must be doing the wobbling. Today, we understand that the direction in which Earth’s axis points does indeed change slowly but regularly—a motion we call precession . If you have ever watched a spinning top wobble, you observed a similar kind of motion. The top’s axis describes a path in the shape of a cone, as Earth’s gravity tries to topple it (Figure 2.12).

Because our planet is not an exact sphere, but bulges a bit at the equator, the pulls of the Sun and Moon cause it to wobble like a top. It takes about 26,000 years for Earth’s axis to complete one circle of precession. As a result of this motion, the point where our axis points in the sky changes as time goes on. While Polaris is the star closest to the north celestial pole today (it will reach its closest point around the year 2100), the star Vega in the constellation of Lyra will be the North Star in 14,000 years.

Ptolemy’s Model of the Solar System

The last great astronomer of the Roman era was Claudius Ptolemy (or Ptolemaeus), who flourished in Alexandria in about the year 140. He wrote a mammoth compilation of astronomical knowledge, which today is called by its Arabic name, Almagest (meaning “The Greatest”). Almagest does not deal exclusively with Ptolemy’s own work it includes a discussion of the astronomical achievements of the past, principally those of Hipparchus. Today, it is our main source of information about the work of Hipparchus and other Greek astronomers.

Ptolemy’s most important contribution was a geometric representation of the solar system that predicted the positions of the planets for any desired date and time. Hipparchus, not having enough data on hand to solve the problem himself, had instead amassed observational material for posterity to use. Ptolemy supplemented this material with new observations of his own and produced a cosmological model that endured more than a thousand years, until the time of Copernicus.

The complicating factor in explaining the motions of the planets is that their apparent wandering in the sky results from the combination of their own motions with Earth’s orbital revolution. As we watch the planets from our vantage point on the moving Earth, it is a little like watching a car race while you are competing in it. Sometimes opponents’ cars pass you, but at other times you pass them, making them appear to move backward for a while with respect to you.

Figure 2.13 shows the motion of Earth and a planet farther from the Sun—in this case, Mars . Earth travels around the Sun in the same direction as the other planet and in nearly the same plane, but its orbital speed is faster. As a result, it overtakes the planet periodically, like a faster race car on the inside track. The figure shows where we see the planet in the sky at different times. The path of the planet among the stars is illustrated in the star field on the right side of the figure.

Link to Learning

The planetary configurations simulator from Foothill AstroSims allows you to see the usual prograde and occasional retrograde motion of other planets. You can switch back and forth between viewing motion from Earth and Mars (as well as other planets).

Normally, planets move eastward in the sky over the weeks and months as they orbit the Sun, but from positions B to D in Figure 2.13, as Earth passes the planets in our example, it appears to drift backward, moving west in the sky. Even though it is actually moving to the east, the faster-moving Earth has overtaken it and seems, from our perspective, to be leaving it behind. As Earth rounds its orbit toward position E, the planet again takes up its apparent eastward motion in the sky. The temporary apparent westward motion of a planet as Earth swings between it and the Sun is called retrograde motion . Such backward motion is much easier for us to understand today, now that we know Earth is one of the moving planets and not the unmoving center of all creation. But Ptolemy was faced with the far more complex problem of explaining such motion while assuming a stationary Earth.

Furthermore, because the Greeks believed that celestial motions had to be circles, Ptolemy had to construct his model using circles alone. To do it, he needed dozens of circles, some moving around other circles, in a complex structure that makes a modern viewer dizzy. But we must not let our modern judgment cloud our admiration for Ptolemy’s achievement. In his day, a complex universe centered on Earth was perfectly reasonable and, in its own way, quite beautiful. However, as Alfonso X, the King of Castile, was reported to have said after having the Ptolemaic system of planet motions explained to him, “If the Lord Almighty had consulted me before embarking upon Creation, I should have recommended something simpler.”

Ptolemy solved the problem of explaining the observed motions of planets by having each planet revolve in a small orbit called an epicycle . The center of the epicycle then revolved about Earth on a circle called a deferent (Figure 2.14). When the planet is at position x in Figure 2.14 on the epicycle orbit, it is moving in the same direction as the center of the epicycle from Earth, the planet appears to be moving eastward. When the planet is at y, however, its motion is in the direction opposite to the motion of the epicycle’s center around Earth. By choosing the right combination of speeds and distances, Ptolemy succeeded in having the planet moving westward at the correct speed and for the correct interval of time, thus replicating retrograde motion with his model.

Link to Learning

Use the Ptolemaic System simulator from Foothill AstroSims to explore how Ptolemy's system of deferents and epicycles explained the apparent motion of the planets.

However, we shall see in Orbits and Gravity that the planets, like Earth, travel about the Sun in orbits that are ellipses, not circles. Their actual behavior cannot be represented accurately by a scheme of uniform circular motions. In order to match the observed motions of the planets, Ptolemy had to center the deferent circles, not on Earth, but at points some distance from Earth. In addition, he introduced uniform circular motion around yet another axis, called the equant point. All of these considerably complicated his scheme.

It is a tribute to the genius of Ptolemy as a mathematician that he was able to develop such a complex system to account successfully for the observations of planets. It may be that Ptolemy did not intend for his cosmological model to describe reality, but merely to serve as a mathematical representation that allowed him to predict the positions of the planets at any time. Whatever his thinking, his model, with some modifications, was eventually accepted as authoritative in the Muslim world and (later) in Christian Europe.

The Inner Planets:

The four inner planets are called terrestrial planets because their surfaces are solid (and, as the name implies, somewhat similar to Earth — although the term can be misleading because each of the four has vastly different environments). They’re made up mostly of heavy metals such as iron and nickel, and have either no moons or few moons. Below are brief descriptions of each of these planets based on this information from NASA.

Mercury: Mercury is the smallest planet in our Solar System and also the closest. It rotates slowly (59 Earth days) relative to the time it takes to rotate around the sun (88 days). The planet has no moons, but has a tenuous atmosphere (exosphere) containing oxygen, sodium, hydrogen, helium and potassium. The NASA MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft is currently orbiting the planet.

The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute

Venus: Venus was once considered a twin planet to Earth, until astronomers discovered its surface is at a lead-melting temperature of 900 degrees Fahrenheit (480 degrees Celsius). The planet is also a slow rotator, with a 243-day long Venusian day and an orbit around the sun at 225 days. Its atmosphere is thick and contains carbon dioxide and nitrogen. The planet has no rings or moons and is currently being visited by the European Space Agency’s Venus Express spacecraft.

Earth: Earth is the only planet with life as we know it, but astronomers have found some nearly Earth-sized planets outside of our solar system in what could be habitable regions of their respective stars. It contains an atmosphere of nitrogen and oxygen, and has one moon and no rings. Many spacecraft circle our planet to provide telecommunications, weather information and other services.

Mars: Mars is a planet under intense study because it shows signs of liquid water flowing on its surface in the ancient past. Today, however, its atmosphere is a wispy mix of carbon dioxide, nitrogen and argon. It has two tiny moons (Phobos and Deimos) and no rings. A Mars day is slightly longer than 24 Earth hours and it takes the planet about 687 Earth days to circle the Sun. There’s a small fleet of orbiters and rovers at Mars right now, including the large NASA Curiosity rover that landed in 2012.

The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute

The Science: Orbital Mechanics

Kepler&rsquos Laws of Planetary Motion

While Copernicus rightly observed that the planets revolve around the Sun, it was Kepler who correctly defined their orbits. At the age of 27, Kepler became the assistant of a wealthy astronomer, Tycho Brahe, who asked him to define the orbit of Mars. Brahe had collected a lifetime of astronomical observations, which, on his death, passed into Kepler&rsquos hands. (Brahe, who had his own Earth-centered model of the Universe, withheld the bulk of his observations from Kepler at least in part because he did not want Kepler to use them to prove Copernican theory correct.) Using these observations, Kepler found that the orbits of the planets followed three laws.

Like many philosophers of his era, Kepler had a mystical belief that the circle was the Universe&rsquos perfect shape, and that as a manifestation of Divine order, the planets&rsquo orbits must be circular. For many years, he struggled to make Brahe&rsquos observations of the motions of Mars match up with a circular orbit.

Eventually, however, Kepler noticed that an imaginary line drawn from a planet to the Sun swept out an equal area of space in equal times, regardless of where the planet was in its orbit. If you draw a triangle out from the Sun to a planet&rsquos position at one point in time and its position at a fixed time later&mdashsay, 5 hours, or 2 days&mdashthe area of that triangle is always the same, anywhere in the orbit. For all these triangles to have the same area, the planet must move more quickly when it is near the Sun, but more slowly when it is farthest from the Sun.

This discovery (which became Kepler&rsquos second law of orbital motion) led to the realization of what became Kepler&rsquos first law: that the planets move in an ellipse (a squashed circle) with the Sun at one focus point, offset from the center.

Kepler&rsquos third law shows that there is a precise mathematical relationship between a planet&rsquos distance from the Sun and the amount of time it takes revolve around the Sun. It was this law that inspired Newton, who came up with three laws of his own to explain why the planets move as they do.

Newton&rsquos Laws of Motion

If Kepler&rsquos laws define the motion of the planets, Newton&rsquos laws define motion. Thinking on Kepler&rsquos laws, Newton realized that all motion, whether it was the orbit of the Moon around the Earth or an apple falling from a tree, followed the same basic principles. &ldquoTo the same natural effects,&rdquo he wrote, &ldquowe must, as far as possible, assign the same causes.&rdquo Previous Aristotelian thinking, physicist Stephen Hawking has written, assigned different causes to different types of motion. By unifying all motion, Newton shifted the scientific perspective to a search for large, unifying patterns in nature. Newton outlined his laws in Philosophiae Naturalis Principia Mathematica (&ldquoMathematical Principles of Natural Philosophy,&rdquo) published in 1687.

Law I. Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed theron.

In essence, a moving object won&rsquot change speed or direction, nor will a still object start moving, unless some outside force acts on it. The law is regularly summed up in one word: inertia.

Law II. The alteration of motion is ever proportional to the motive force impressed and is made in the direction of the right line in which that force is impressed.

Newton&rsquos second law is most recognizable in its mathematical form, the iconic equation: F=ma. The strength of the force (F) is defined by how much it changes the motion (acceleration, a) of an object with some mass (m).

Law III. To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

As Newton himself described: &ldquoIf you press a stone with your finger, the finger is also pressed by the stone.&rdquo


Within the pages of Principia, Newton also presented his law of universal gravitation as a case study of his laws of motion. All matter exerts a force, which he called gravity, that pulls all other matter towards its center. The strength of the force depends on the mass of the object: the Sun has more gravity than Earth, which in turn has more gravity than an apple. Also, the force weakens with distance. Objects far from the Sun won&rsquot be influenced by its gravity.

Newton&rsquos laws of motion and gravity explained Earth&rsquos annual journey around the Sun. Earth would move straight forward through the universe, but the Sun exerts a constant pull on our planet. This force bends Earth&rsquos path toward the Sun, pulling the planet into an elliptical (almost circular) orbit. His theories also made it possible to explain and predict the tides. The rise and fall of ocean water levels are created by the gravitational pull of the Moon as it orbits Earth.

Einstein and Relativity

The ideas outlined in Newton&rsquos laws of motion and universal gravitation stood unchallenged for nearly 220 years until Albert Einstein presented his theory of special relativity in 1905. Newton&rsquos theory depended on the assumption that mass, time, and distance are constant regardless of where you measure them.

The theory of relativity treats time, space, and mass as fluid things, defined by an observer&rsquos frame of reference. All of us moving through the universe on the Earth are in a single frame of reference, but an astronaut in a fast-moving spaceship would be in a different reference frame.

Within a single frame of reference, the laws of classical physics, including Newton&rsquos laws, hold true. But Newton&rsquos laws can&rsquot explain the differences in motion, mass, distance, and time that result when objects are observed from two very different frames of reference. To describe motion in these situations, scientists must rely on Einstein&rsquos theory of relativity.

At slow speeds and at large scales, however, the differences in time, length, and mass predicted by relativity are small enough that they appear to be constant, and Newton&rsquos laws still work. In general, few things are moving at speeds fast enough for us to notice relativity. For large, slow-moving satellites, Newton&rsquos laws still define orbits. We can still use them to launch Earth-observing satellites and predict their motion. We can use them to reach the Moon, Mars, and other places beyond Earth. For this reason, many scientists see Einstein&rsquos laws of general and special relativity not as a replacement of Newton&rsquos laws of motion and universal gravitation, but as the full culmination of his idea.

Ancient geometry: cosmological and metaphysical

The Pythagoreans used geometrical figures to illustrate their slogan that all is number—thus their “triangular numbers” ( n(n−1) /2 ), “square numbers” (n 2 ), and “altar numbers” (n 3 ), some of which are shown in the figure . This principle found a sophisticated application in Plato’s creation story, the Timaeus, which presents the smallest particles, or “elements,” of matter as regular geometrical figures. Since the ancients recognized four or five elements at most, Plato sought a small set of uniquely defined geometrical objects to serve as elementary constituents. He found them in the only three-dimensional structures whose faces are equal regular polygons that meet one another at equal solid angles: the tetrahedron, or pyramid (with 4 triangular faces) the cube (with 6 square faces) the octahedron (with 8 equilateral triangular faces) the dodecahedron (with 12 pentagonal faces) and the icosahedron (with 20 equilateral triangular faces). (See animation .)

The cosmology of the Timaeus had a consequence of the first importance for the development of mathematical astronomy. It guided Johannes Kepler (1571–1630) to his discovery of the laws of planetary motion. Kepler deployed the five regular Platonic solids not as indicators of the nature and number of the elements but as a model of the structure of the heavens. In 1596 he published Prodromus Dissertationum Mathematicarum Continens Mysterium Cosmographicum (“Cosmographic Mystery”), in which each of the known six planets revolved around the Sun on spheres separated by the five Platonic solids, as shown in the photograph. Although Tycho Brahe (1546–1601), the world’s greatest observational astronomer before the invention of the telescope, rejected the Copernican model of the solar system, he invited Kepler to assist him at his new observatory outside of Prague. In trying to resolve discrepancies between his original theory and Brahe’s observations, Kepler made the capital discovery that the planets move in ellipses around the Sun as a focus.

The History of How We Discovered All the Planets in the Solar System

Satellites such as the Kepler have been working overtime to uncover hundreds of new planets in our galaxy. But how did we first discover the planets in our local volume of space? Here are the stories of how astronomers living hundreds of years ago discovered each planet in our solar system.

Photo of the 18th Century Jantar Mantar observatory in Jaipur, India, by Norman Koren .

The innermost planet in our solar system, Mercury orbits our sun between just under 70 million and 46 million kilometers. Ancient astronomers knew of the planet's speed around the sun: Assyrian astronomers associated it with gods such as Nabu, the scribe and messenger to the gods, while the Greeks named the body Mercury, the messenger of the gods. The association is apt: the planet has a short year of 88 days in all.

In 1631, astronomer Pierre Gassendi first observed Mercury making a transit across the sun, and just a couple of years later, another astronomer, Giovanni Zupi discovered phases, indicating that the planet orbited the sun. Other astronomers followed, making incremental discoveries along the way: Italian Astronomer Giovanni Schiaparelli observed the planet, and concluded that Mercury was tidally locked with the sun.

More discoveries came during the modern era of space exploration: much more about the planet has been found recently. Soviet scientists first used radar to study the planet in the early 1960s, while scientists at Puerto Rico's Arecibo Observatory Radio Telescope discovered that the planet in fact rotated once every 59 days, rather than 88 as previously thought. In 1974, Mariner 10 first visits the planet, making several passes, mapping the surface, and in 2008, the MESSENGER satellite marked a return to the planet, where it's currently in orbit.

The second planet in the solar system, Venus is the brightest of the planets as observed Earth. As a result, it's been studied since ancient times, with the first records coming from the Babylonians, who named the planet Ishtar. The Romans viewed Venus as the goddess of beauty, while the Mayans believed that the planet was the brother of the Sun. In 1610, Galileo Galilei observed phases of Venus, confirming that the planet did indeed orbit the sun. Due to the planet's thick atmosphere, observation of the surface wasn't possible until the 1960s, but many believed that Venus harbored life, due to the planet's similar size to Earth.

In 1958, radar imagery found that the planet's surface was hot – inhospitably so. Mankind was about to get a closer look. The first attempt, the Soviet Union's Venera 1, launched in 1961, failed, but Mariner 2, launched by the United States, succeeded in a flyby confirming the planet's temperature and that it lacked a magnetic field. A new Soviet mission, Venera 4, successfully reached Venus and sent back information about its atmosphere before it burned up during entry. Several additional probe followed: Mariner 5, Venera 5 and 6, before Venera 7 successfully landed, becoming the first manmade object to land on another planet while Venera 8, landed two years later. Both were destroyed by the planet's heat and pressure, but the Soviet Union continued to send probes: 9 through 12 took pictures and gathered information on the planet's geology. NASA also continued to send probes: Pioneer 12 orbited the planet for 14 years, mapping the surface, while Pioneer 13 sent several probes down to the surface.

Earth has been continually observed by humanity as long as we've been around. But, while we knew we stood on solid ground, the true nature of our home took a little while longer to figure out. For many centuries, humanity believed that Earth wasn't an object such as those that they observed above them: everything was thought to revolve around us. As early as Aristotle, philosophers had determined that Earth was spherical by observing its shadow against the Moon.

Mikołaj Kopernik – known as Nicholas Copernicus posited a Heliocentric view of the solar system as early as 1514. Titled De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), the book was first published in 1543, challenged the conventional wisdom of the day. The theory was controversial, but was followed up by Johannes Kepler with his three volume work, Epitome astronomiae Copernicanae (Epitome of Copernican Astronomy). Kepler had devised three laws of planetary motion: "Planets move around the Sun in ellipses, with the Sun at one focus", "the line connecting the Sun to a planet sweeps equal areas in equal times" and "the square of the orbital period of a planet is proportional to the cube (3rd power) of the mean distance from the Sun in (or in other words—of the "semi-major axis" of the ellipse, half the sum of smallest and greatest distance from the Sun)". These laws helped define the motion of the planets and allowed us the first real idea that our solar system was vastly different than previously thought. Kepler's theories weren't popular at first, but eventually caught on in Europe. At the same time that Copernicus was publishing his views, Ferdinand Magellan's expedition was able to circumnavigate the globe in 1519.

However, it wasn't until October 24th, 1946 when we first got a look at our home world when the first picture of Earth was taken from a modified V-2 Rocket fired from the White Sands Missile Range in New Mexico.

The blood-red fourth planet of our solar system has long been associated with the Roman God of war, bearing the same name: Mars. Where many people thought that Venus might very well enjoy an earth-like atmosphere, there were similar thoughts about Mars. Most notoriously, astronomer Giovanni Schiaparelli examined the planet through a telescope in 1877, describing a number of features, which he described as Canali. The mistranslated word was understood to mean that he had discovered Canals on Mars, and by extension, people assumed that they were artificial. Twenty years later, another astronomer, Camille Flammarion, further identified the features as artificial, and the general public generally assumed that the planet could support life. Undoubtedly, the public's perception led to the rise of a number of Mars-oriented SF novels, such as The War of the Worlds, by H.G. Wells, and Edgar Rice Burrough's John Carter series.

Advances in telescopic technology that came later allowed for new observations of the planet. Astronomers were able to measure the temperature of the planet, determine its atmospheric content and it's mass. Throughout the 1960s, the Soviet Union attempted to send eight probes towards Mars, each resulting in failure, although several additional orbiters that followed in the 1970s were successful in orbiting the planet. NASA would have poor luck with the Mariner 3 mission, but Mariner 4, launched in 1964, successfully flew by the planet, taking readings as it did so, revealing a dead world. Mariner 9 would later orbit the planet, further adding to our knowledge of the planet. Where those missions were the scouts, the Viking missions represented the initial invasion: On July 20th, 1976, the probe touched down on the planet for an unprecedented mission that would last for until 1982. Viking 2 followed shortly thereafter, landing in September 1976, remaining in operation until 1980.

Despite the mission's success, it wasn't until 1997 that the Mars Pathfinder mission successfully landed on Mars, the first mobile rover to be landed on a planetary body. A follow up mission, the Mars Climate Orbiter, failed due to human error, and several additional Mars Probes failed as well. It wasn't until 2004 when NASA launched the Mars Exploration Rover Mission with the Spirit and Opportunity rovers that a successful landing was made. The rovers outdid everyone's expectations, and it wasn't until 2011 that their missions were closed down. In 2012, NASA successfully landed the Curiosity Rover, which landed on August 6th, 2012, where it has begun to perform its duties.

The largest planet in our solar system, Jupiter has long been watched since ancient times. It helped guide the Chinese 12 year cycle, and the planet was named for the king of the Roman gods. It also provided a big target for early astronomers. Galileo was the first to observe Jupiter's four major moons, now known as the Galilean Moons: Io, Europa, Ganymede and Callisto, named for Zeus's lovers. Astronomer Robert Hooke first discovered a major storm system on the gas planet, and it was confirmed by Giovanni Cassini in 1665, believed to be the first sightings of Jupiter's Great Red Spot, which was later formally recorded in 1831. Without an underlying land mass, the storms of Jupiter are free to rage on, and the feature has remained on the planet since. Astronomers Giovanni Borelli and Cassini, using orbital tables and mathematics, discovered something odd: Jupiter, when in opposition to Earth, appeared to be seventeen minutes behind their calculations, lending the indications that light was not an instantaneous phenomenon.

As observations advanced in the 1900s, other discoveries were made: While using a radio telescope to study the Crab Nebula between 1954 and 1955, astronomer Bernard Berke was hampered by interference from one part of the sky, and eventually found that Jupiter was emitting the waves as part of the planet's radiation. In 1973, the Pioneer missions became the first probes to fly past the planet, taking a number of close-up pictures. In 1977, two space probe missions were launched from Earth: Voyagers 1 and 2, designed to explore the outer planets of the Solar system. They first reached Jupiter two years later: Voyager 1 arrived in March 1979, and Voyager 2 arrived on July 1979. Both uncovered a wealth of new information about the planet and its moons before they left, uncovering a small ring system and a number of additional moons. Other robotic missions have since followed: the Ulysses mission arrived in 1992, the Galileo probes orbited the planet in 1995, Cassini flew past in 2000, and New Horizons passed by in 2007. In 1994, scientists observed an astonishing event: a planetary impact, when the Shoemaker-Levy comet crashed into the Jupiter's southern horizon, leaving enormous impact scars in the planet's atmosphere. Currently, there are efforts underway to examine Jupiter's moons, thought to be the next best candidates for life.

Our system's sixth planet from the Sun is possibly one of the most striking, and is the last classically recognized planet: the Romans named the planet for their God of Agriculture. However, it wasn't until Galileo turned his attention to the planet in 1610 before the planet's dominant feature was uncovered when. While he studied the planet's features, he believed heɽ uncovered several orbiting moons. However, it wasn't until 1655, when Christiaan Huygens, with a more powerful telescope, discovered that the feature was actually a ring that encircled the entire planet. Shortly thereafter, he uncovered the Saturn's first moon, Titan. During his own observations, Giovanni Cassini, in 1671, uncovered four additional moons: Iapetus, Rhea, Tethys and Dione and a gap in the planet's rings, leading him to believe that the ringswere made up of smaller particles. In 1789, German astronomer William Herschel noted two additional moons: Mimas and Enceladus, and over the next hundred years, two other satellites were found: Hyperion, in 1848, and Phoebe, in 1899.

As NASA began to explore the outer planets, Saturn would first be visited by the Pioneer 11 mission in September 1979, taking a number of pictures. The twin Voyager probes would come next, in 1980 and 1981, taking high resolution pictures. The planet became a divergent point for the pair: Voyager 1 used Saturn to arc out of the Solar System, while Voyager 2 was directed to Uranus. The planet wouldn't be visited again until 2004 with the Cassini mission, which orbited the planet and studied its moons, where it remains today.

The seventh planet, Uranus, is hard to detect without the aid of a telescope, and thus, the planet doesn't have the same long history as its other neighbors. Watching the skies in December 1690, astronomer John Flamsteed first noted the planet, but identified it as a star, which he named 34 Tauri. It wasn't until March 13th, 1781 that Herschel first believed that the star that he was studying was a comet. It wasn't until he began to study the object's orbit when he found that it was nearly circular, leading him to believe that it was in fact a planet. Herschel named the planet Georgium Sidus, in honor of King George III, but the eventually the planet was named Uranus, after Chronos. It's discovery was sensational, the furthest known object in the solar system. In the 19th century, astronomers noted something odd about the planet's orbit: it didn't quite follow mathematical theories, and it deviated from its course. It was clearly being influenced by something further out in the Solar system.

The planet's most unusual feature is its orientation: rather than rotating like the other planets in the system, Uranus rotates on its side, with its rings and moons orbiting in bulls-eye pattern. The underlying reasons for this are unknown, and have at times been attributed to a planetary collision. However, in 2009, members of the Paris Observatory theorized that a moon in the planetary disk, formed while the planet was in its infant stages, could have made the planet wobble. In 1986, the Voyager 2 probe passed by Uranus, examining the planet's atmosphere and discovering a number of additional moons and the planet's ring system. It was the first and only probe to reach the planet, and at this point, no further missions are planned.

The Planets in our Solar System:

Having covered the basics of definition and classification, let’s get talking about those celestial bodies in our Solar System that are still classified as planets (sorry Pluto!). Here is a brief look at the eight planets in our Solar System. Included are quick facts and links so you can find out more about each planet.

Mercury is the closest planet to our Sun, at just 58 million km (36 million miles) or 0.39 Astronomical Unit (AU) out. But despite its reputation for being sun-baked and molten, it is not the hottest planet in our Solar System (scroll down to find out who that dubious honor goes go!)

Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury is also the smallest planet in our Solar System, and is also smaller than its largest moon (Ganymede, which orbits Jupiter). And being equivalent in size to 0.38 Earths, it is just slightly larger than the Earth’s own Moon. But this may have something to do with its incredible density, being composed primarily of rock and iron ore. Here are the planetary facts:

  • Diameter: 4,879 km (3,032 miles)
  • Mass: 3.3011 x 10 23 kg ( 0.055 Earths)
  • Length of Year (Orbit): 87.97 Earth days
  • Length of Day: 59 Earth days.
  • Mercury is a rocky planet, one of the four “terrestrial planets” in our Solar System. Mercury has a solid, cratered surface, and looks much like Earth’s moon.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mercury.
  • Mercury does not have any moons.
  • Temperatures on Mercury range between -173 to 427 degrees Celcius (-279 to 801 degrees Fahrenheit)
  • Just two spacecraft have visited Mercury: Mariner 10 in 1974-75 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011 and ended its mission by impacting the surface of Mercury on April 30, 2015. MESSENGER has changed our understanding of this planet, and scientists are still studying the data.
  • Find more details about Mercury at this article on Universe Today, and this page from NASA.

Venus is the second closest planet to our Sun, orbiting at an average distance of 108 million km (67 million miles) or 0.72 AU. Venus is often called Earth’s “sister planet,” as it is just a little smaller than Earth. Venus is 81.5% as massive as Earth, and has 90% of its surface area and 86.6% of its volume. The surface gravity, which is 8.87 m/s², is equivalent to 0.904 g – roughly 90% of the Earth standard.

A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL

And due to its thick atmosphere and proximity to the Sun, it is the Solar Systems hottest planet, with temperatures reaching up to a scorching 735 K (462 °C). To put that in perspective, that’s over four and a half times the amount of heat needed to evaporate water, and about twice as much needed to turn tin into molten metal ( 231.9 °C )!

  • Diameter: 7,521 miles (12,104 km)
  • Mass: 4.867 x 10 24 kg (0.815 Earth mass)
  • Length of Year (Orbit): 225 days
  • Length of day: 243 Earth days
  • Surface temperature: 462 degrees C (864 degrees F)
  • Venus’ thick and toxic atmosphere is made up mostly of carbon dioxide (CO2) and nitrogen (N2), with clouds of sulfuric acid (H2SO4) droplets.
  • Venus has no moons.
  • Venus spins backwards (retrograde rotation), compared to the other planets. This means that the sun rises in the west and sets in the east on Venus.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Venus.
  • Venus is also known and the “morning star” or “evening star” because it is often brighter than any other object in the sky and is usually seen either at dawn or at dusk. Since it is so bright, it has often been mistaken for a UFO!
  • More than 40 spacecraft have explored Venus. The Magellan mission in the early 1990s mapped 98 percent of the planet’s surface. Find out more about all the missions here.
  • Find out more about Venus on this article from Universe Today, and this page from NASA.

Our home, and the only planet in our Solar System (that we know of) that actively supports life. Our planet is the third from the our Sun, orbiting it at an average distance of 150 million km (93 million miles) from the Sun, or one AU. Given the fact that Earth is where we originated, and has all the necessary prerequisites for supporting life, it should come as no surprise that it is the metric on which all others planets are judged.

Earth, pictured by the crew of the Apollo 17 mission. Credit: NASA

Whether it is gravity (g), distance (measured in AUs), diameter, mass, density or volume, the units are either expressed in terms of Earth’s own values (with Earth having a value of 1) or in terms of equivalencies – i.e. 0.89 times the size of Earth. Here’s a rundown of the kinds of

  • Diameter: 12,760 km (7,926 miles)
  • Mass: 5.97 x 10 24 kg
  • Length of Year (Orbit): 365 days
  • Length of day: 24 hours (more precisely, 23 hours, 56 minutes and 4 seconds.)
  • Surface temperature: Average is about 14 C, (57 F), with ranges from -88 to 58 (min/max) C (-126 to 136 F).
  • Earth is another terrestrial planet with an ever-changing surface, and 70 percent of the Earth’s surface is covered in oceans.
  • Earth has one moon.
  • Earth’s atmosphere is 78% nitrogen, 21% oxygen, and 1% various other gases.
  • Earth is the only world known to harbor life.
  • Find out more about Earth at a series of articles found here on Universe Today, and on this webpage from NASA.

Mars is the fourth planet from the sun at a distance of about 228 million km (142 million miles) or 1.52 AU. It is also known as “the Red Planet” because of its reddish hue, which is due to the prevalence of iron oxide on its surface. In many ways, Mars is similar to Earth, which can be seen from its similar rotational period and tilt, which in turn produce seasonal cycles that are comparable to our own.

Global image of the planet Mars. Credit: NASA

The same holds true for surface features. Like Earth, Mars has many familiar surface features, which include volcanoes, valleys, deserts, and polar ice caps. But beyond these, Mars and Earth have little in common. The Martian atmosphere is too thin and the planet too far from our Sun to sustain warm temperatures, which average 210 K (-63 ºC) and fluctuate considerably.

  • Diameter: 6,787 km, (4,217 miles)
  • Mass: 6.4171 x 10 23 kg ( 0.107 Earths)
  • Length of Year (Orbit): 687 Earth days.
  • Length of day: 24 hours 37 minutes.
  • Surface temperature: Average is about -55 C (-67 F), with ranges of -153 to +20 °C (-225 to +70 °F)
  • Mars is the fourth terrestrial planet in our Solar System. Its rocky surface has been altered by volcanoes, impacts, and atmospheric effects such as dust storms.
  • Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), nitrogen (N2) and argon (Ar).If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mars.
  • Mars has two small moons, Phobos and Deimos.
  • Mars is known as the Red Planet because iron minerals in the Martian soil oxidize, or rust, causing the soil to look red.
  • More than 40 spacecraft have been launched to Mars. You can find out more about missions to Mars here.Find out more about Mars at this series of articles on Universe Today, and at this NASA webpage.

Jupiter is the fifth planet from the Sun, at a distance of about 778 million km (484 million miles) or 5.2 AU. Jupiter is also the most massive planet in our Solar System, being 317 times the mass of Earth, and two and half times larger than all the other planets combined. It is a gas giant, meaning that it is primarily composed of hydrogen and helium, with swirling clouds and other trace gases.

Io and Jupiter as seen by New Horizons during its 2008 flyby. (Credit: NASA/Johns Hopkins University APL/SWRI).

Jupiter’s atmosphere is the most intense in the Solar System. In fact, the combination of incredibly high pressure and coriolis forces produces the most violent storms ever witnessed. Wind speeds of 100 m/s (360 km/h) are common and can reach as high as 620 km/h (385 mph). In addition, Jupiter experiences auroras that are both more intense than Earth’s, and which never stop.

  • Diameter: 428,400 km (88,730 miles)
  • Mass: 1.8986 × 10 27 kg ( 317.8 Earths)
  • Length of Year (Orbit): 11.9 Earth years
  • Length of day: 9.8 Earth hours
  • Temperature: -148 C, (-234 F)
  • Jupiter has 67 known moons, with an additional 17 moons awaiting confirmation of their discovery – for a total of 67 moons. Jupiter is almost like a mini solar system!
  • Jupiter has a faint ring system, discovered in 1979 by the Voyager 1 mission.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 115 kg (253) pounds on Jupiter.
  • Jupiter’s Great Red Spot is a gigantic storm (bigger than Earth) that has been raging for hundreds of years. However, it appears to be shrinking in recent years.
  • Many missions have visited Jupiter and its system of moons, with the latest being the Juno mission will arrive at Jupiter in 2016. You can find out more about missions to Jupiter here.
  • Find out more about Jupiter at this series of articles on Universe Today and on this webpage from NASA.

Saturn is the sixth planet from the Sun at a distance of about 1.4 billion km (886 million miles) or 9.5 AU. Like Jupiter, it is a gas giant, with layers of gaseous material surrounding a solid core. Saturn is most famous and most easily recognized for its spectacular ring system, which is made of seven rings with several gaps and divisions between them.

  • Diameter: 120,500 km (74,900 miles)
  • Mass: 5.6836 x 10 26 k g ( 95.159 Earths )
  • Length of Year (Orbit): 29.5 Earth years
  • Length of day: 10.7 Earth hours
  • Temperature: -178 C (-288 F)
  • Saturn’s atmosphere is made up mostly of hydrogen (H2) and helium (He).
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh about 48 kg (107 pounds) on Saturn
  • Saturn has 53 known moons with an additional 9 moons awaiting confirmation.
  • Five missions have gone to Saturn. Since 2004, Cassini has been exploring Saturn, its moons and rings. You can out more about missions to Saturn here.
  • Find out more about Saturn at this series of articles on Universe Today and at this webpage from NASA.

Uranus is the seventh planet from the sun at a distance of about 2.9 billion km (1.8 billion miles) or 19.19 AU. Though it is classified as a “gas giant”, it is often referred to as an “ice giant” as well, owing to the presence of ammonia, methane, water and hydrocarbons in ice form. The presence of methane ice is also what gives it its bluish appearance.

Uranus is also the coldest planet in our Solar System, making the term “ice” seem very appropriate! What’s more, its system of moons experience a very odd seasonal cycle, owing to the fact that they orbit Neptune’s equator, and Neptune orbits with its north pole facing directly towards the Sun. This causes all of its moons to experience 42 year periods of day and night.

  • Diameter: 51,120 km (31,763 miles)
  • Mass:
  • Length of Year (Orbit): 84 Earth years
  • Length of day: 18 Earth hours
  • Temperature: -216 C (-357 F)
  • Most of the planet’s mass is made up of a hot dense fluid of “icy” materials – water (H2O), methane (CH4). and ammonia (NH3) – above a small rocky core.
  • Uranus has an atmosphere which is mostly made up of hydrogen (H2) and helium (He), with a small amount of methane (CH4). The methane gives Uranus a blue-green tint.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Uranus.
  • Uranus has 27 moons.
  • Uranus has faint rings the inner rings are narrow and dark and the outer rings are brightly colored.
  • Voyager 2 is the only spacecraft to have visited Uranus. Find out more about this mission here.
  • You can find out more about Uranus at this series of articles on Universe Today and this webpage from NASA.

Neptune is the eighth and farthest planet from the Sun, at a distance of about 4.5 billion km (2.8 billion miles) or 30.07 AU. Like Jupiter, Saturn and Uranus, it is technically a gas giant, though it is more properly classified as an “ice giant” with Uranus.

Neptune photographed by the Voyager 2 space probe. Credit: NASA/JPL

Due to its extreme distance from our Sun, Neptune cannot be seen with the naked eye, and only one mission has ever flown close enough to get detailed images of it. Nevertheless, what we know about it indicates that it is similar in many respects to Uranus, consisting of gases, ices, methane ice (which gives its color), and has a series of moons and faint rings.

  • Diameter: 49,530 km (30,775 miles)
  • Mass: 1.0243 x 10 26 kg ( 17 Earths)
  • Length of Year (Orbit): 165 Earth years
  • Length of day: 16 Earth hours
  • Temperature: -214 C (-353 F)
  • Neptune is mostly made of a very thick, very hot combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core.
  • Neptune’s atmosphere is made up mostly of hydrogen (H2), helium (He) and methane (CH4).
  • Neptune has 13 confirmed moons and 1 more awaiting official confirmation.
  • Neptune has six rings.
  • If you weigh 45 kg (100 pounds) on Earth, you would weigh 52 kg (114 pounds) on Neptune.
    Neptune was the first planet to be predicted to exist by using math.
  • Voyager 2 is the only spacecraft to have visited Neptune. You can find out more about this mission here.
  • Find out more about Neptune at this series of articles on Universe Today and this NASA webpage. We have written many articles about the planets for Universe Today. Here are some facts about planets, and here’s an article about the names of the planets.If you’d like more info on the Solar System planets, dwarf planets, asteroids and more, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.Venus is the second planet from the Sun, and it is the hottest planet in the Solar System due to its thick, toxic atmosphere which has been described as having a “runaway greenhouse effect” on the planet.

Now you know! And if you find yourself unable to remember all the planets in their proper order, just repeat the words, “My Very Educated Mother Just Served Us Noodles.” Of course, the Pie, Ham, Muffins and Eggs are optional, as are any additional courses that might be added in the coming years!

We have many great articles on the Solar System and the planets here at Universe Today. Here is a rundown of the Inner Planets, the Outer Planets, a description of Terrestrial Planets, the Dwarf Planets, and Why Pluto is no Longer a Planet?.