Astronomy

Why can I see the dark side of the moon?

Why can I see the dark side of the moon?



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Last weekend (29/30 April) the sky in my area (Central Europe) was pretty clear and the Moon was nicely visible. Funnily, one could not only see the sunlit area of the Moon, but also the dark side* could be easily recognised throughout the whole evening from blue hour until dark night. I attached two photos of the Moon that should illustrate the effect. The pictures (sorry for the poor quality) needed to get a little overexposed to capture the illuminated dark side, but with plain eye the effect could be well seen. We were speculating about the reasons why the non-sunlit area of the Moon could be seen.

  • diffraction of sunlight through the atmosphere can be neglected as the Moon doesn't have an atmosphere.
  • reflection of light that in turn was reflected from earth (Sun -> Earth -> Moon -> observer) sounded a little unlikely to me
  • a psychological effect that kind of mentally completes the shape of the Moon, because we know it should be circular, sounded ok to me. But could be debunked as the dark side stays visible if one covers the bright side on the photo?

So I guess the sunlight has to reach the dark side of the Moon somehow differently.

What could be provide enough light to make the dark side visible?

* not the far side of the Moon, but the non-sunlit area of the Moon


You can generally see the unlit side of the moon when a considerable amount of sunlight is reflected off the earth. This reflected sunlight illuminates the unlit side of the moon. This is referred to as earthshine, and a decent explanation can be found at timeanddate.com. I seem to recall reading somewhere (but now can't find a reference) that it is more prominent when it is very cloudy on earth near where the moon is overhead.


A full Earth on the Moon is much brighter than a full Moon is on Earth, because the Earth is significantly bigger.

Btw, "dark side of the Moon" is not really a technical term (it's rather a song of Pink Floyd). On the Moon, there is (as on Earth) day and night (though the day is as long as month on Earth) and no spot remains dark for longer (except for the pole caps as on Earth). There is, of course, the far side of the Moon, most of which is invisible from Earth (because the Moon is tidally locked to spin with the same rate as it rotates around Earth).


As the older answers stated, Earthshine is what you're seeing.

Compared to the Moon as seen from Earth, Earth as seen from the Moon has almost four times the apparent diameter, and close to 14 times the apparent surface area. If Earth's reflectivity were the same as the Moon's, that would make Earthlight on the Moon 14 times brighter than moonlight on the Earth.

But Earth is shinier than the Moon. The Moon's "albedo" (the proportion of light it reflects instead of absorbing) averages 0.12, about the same as a well-traveled asphalt road; that means it reflects back only 1/8 of the light that hits it. Earth's albedo averages 0.37, three times greater -- and if there's a lot of cloud cover, it can be even higher (since clouds are white).

So, think about how well you can see the Earth's landscape under a nearly-full Moon. If you were standing on the Moon's night side under a nearly-full Earth, there would be nearly 50 times as much light. No wonder we can see it from Earth!


What’s on the far side of the moon?

The far side looks a lot like the near side. Image via NASA’s Scientific Visualization Studio.

Looking up at the silvery orb of the moon, you might recognize familiar shadows and shapes on its face from one night to the next. You see the same view of the moon our early ancestors did as it lighted their way after sundown.

Only one side of the spherical moon is ever visible from Earth – it wasn’t until 1959 when the Soviet Spacecraft Luna 3 orbited the moon and sent pictures home that human beings were able to see the “far side” of the moon for the first time.

Comparison of humanity’s first glimpse of the lunar far side and the same view thanks to LRO data 50 years later. Image via NASA’s Goddard Space Flight Center Scientific Visualization Studio

A phenomenon called tidal locking is responsible for the consistent view. The Earth and its moon are in close proximity and thus exert significant gravitational forces on each other. These tidal forces slow the rotations of both bodies. They locked the moon’s rotation in sync with its orbital period relatively soon after it formed – as a product of a collision between a Mars-sized object and the proto-Earth, 100 million years after the solar system coalesced.


The moon’s orbital period and rotational period are the same length of time.

Now the moon takes one trip around the Earth in the same amount of time it takes to make one rotation around its own axis: about 28 days. From Earth, we always see the same face of the moon from the moon, the Earth stands still in the sky.

The near side of the moon is well studied because we can see it. The astronauts landed on the near side of the moon so they could communicate with NASA here on Earth. All of the samples from the Apollo missions are from the near side.

Buzz Aldrin descends from the lunar module to the surface of the moon on July 20, 1969. Image via NASA.

Although the far side of the moon isn’t visible from our vantage point, and with all due respect to Pink Floyd, it is not accurate to call it the dark side of the moon. All sides of the moon experience night and day just like we do here on Earth. All sides have equal amounts of day and night over the course of a single month. A lunar day lasts about two Earth weeks.

With modern satellites, astronomers have completely mapped the lunar surface. A Chinese mission, Chang’e 4, is currently exploring the Aitken Basin on the far side of the moon — the first such mission ever landed there. Researchers hope Chang’e 4 will help answer questions about the crater’s surface features and test whether things can grow in lunar soil. A privately funded Israeli mission, Beresheet, started as a mission to compete for the Google Lunar X Prize. Despite crashing during an attempted landing earlier this month, the Beresheet team still won the Moon Shot Award.

Being shielded from civilization means the far side of the moon is “radio dark.” There, researchers can measure weak signals from the universe that would otherwise be drowned out. Chang’e 4, for instance, will be able to observe low-frequency radio light coming from the sun or beyond that’s impossible to detect here on the Earth due to human activity, such as TV and radio broadcasts and other forms of communication signals. Low-frequency radio peers back in time to the very first stars and the very first black holes, giving astronomers a greater understanding of how the structures of the universe began forming.

Arrows indicate position of Chang’e 4 lander on the floor of the moon’s Von Kármán crater. The sharp crater behind and to the left of the landing site is 12,800 feet (3,900 meters) across and 1,970 feet (600 meters) deep. Image via NASA/GSFC/Arizona State University.

Rover missions also investigate all sides of the moon as space scientists prepare for future human missions, looking to the moon’s resources to help humanity get to Mars. For instance, water – discovered by NASA’s LCROSS satellite beneath the moon’s north and south poles in 2009 – can be broken up into hydrogen and oxygen and used for fuel and breathing.

Researchers are getting closer to exploring the moon’s polar craters, some of which have never seen the light of day – literally. They are deep and in just the right place to never have the sun shine onto the crater floor. There are certainly dark parts of the moon, but the whole far side isn’t one of them.

Wayne Schlingman, Director of the Arne Slettebak Planetarium, The Ohio State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.


First quarter moon on February 12

Image above – first quarter moon – via Suzanne Murphy in Wisconsin. View full-sized image.

On February 12, 2019, the moon is at or near its first quarter phase, which means the portion of the moon we see from Earth is 50 percent illuminated by sunshine and 50 percent engulfed in the moon’s own shadow. It means that – for all of Earth – the moon rises around midday and sets around midnight.

The illuminated side of the February 12 first quarter moon will be pointing right at the red planet Mars. With binoculars, Mars and Uranus can be seen to occupy a single field of view. See the chart below.

On February 12, 2019, the lit side of the moon points right at Mars and Uranus. Despite the moonlit glare, binoculars may enable you to see the Mars/Uranus conjunction in a single binocular field. Click here for a sky chart via Sky & Telescope, keeping in mind that Uranus will be dimmer than Mars and the star Omicron Piscium (not shown). Fortunately, Uranus appears as a dim “star” quite close to Mars, whereas the star Omicron Piscium is noticeably farther distant from the red planet.

For the whole Earth, the moon reaches its exact first quarter phase on February 12, 2019, at 22:26 UTC translate UTC to your time. At U.S. time zones, that is 5:26 p.m. EDT, 4:26 p.m. CDT, 3:26 p.m. MDT, 2:26 p.m. PDT, 1:26 p.m. Alaskan Time and 12:26 p.m. Hawaiian Time.

It’s good to remember that half the moon is always illuminated in space. In other words, the moon has a day side and a night side, just as Earth does. At first quarter moon, we’re seeing about equal portions of the moon’s day side and night sides. Because a first quarter moon is a waxing moon, we’re bound to see more of its day side each evening for another week or so, culminating with full moon on February 19.

The part of the moon that isn’t in sunlight is often called the moon’s dark side. Just realize that all of the moon undergoes day and night, just as Earth does. Any given lunar location experiences night for about two weeks, followed by about two weeks of daylight. So there’s a permanent far side of the moon, but no permanent dark side.

The moon does rotate on its axis. But billions of years of Earth’s strong gravitational pull have slowed it down such that today the moon takes as long to rotate as it does to orbit once around Earth. Astronomers would say that the moon is tidally locked with Earth. For that reason, one side of the moon always faces Earth.

Incidentally, the moon’s gravitational effects on Earth are much smaller, but – given billions of years of time – the Earth will slow down and keep one face always toward the moon.

Bottom line: The moon reaches its exact first quarter phase on February 12, 2019, at 22:26 UTC translate UTC to your time. For the whole Earth, a first quarter moon rises around midday and sets around midnight.


What's on the far side of the Moon?

Comparison of humanity’s first glimpse of the lunar far side and the same view thanks to LRO data 50 years later. Credit: NASA's Goddard Space Flight Center Scientific Visualization Studio, CC BY

Looking up at the silvery orb of the Moon, you might recognize familiar shadows and shapes on its face from one night to the next. You see the same view of the Moon our early ancestors did as it lighted their way after sundown.

Only one side of the spherical Moon is ever visible from Earth – it wasn't until 1959 when the Soviet Spacecraft Luna 3 orbited the Moon and sent pictures home that human beings were able to see the "far side" of the Moon for the first time.

A phenomenon called tidal locking is responsible for the consistent view. The Earth and its Moon are in close proximity and thus exert significant gravitational forces on each other. These tidal forces slow the rotations of both bodies. They locked the Moon's rotation in sync with its orbital period relatively soon after it formed – as a product of a collision between a Mars-sized object and the proto-Earth, 100 million years after the solar system coalesced.

Now the Moon takes one trip around the Earth in the same amount of time it takes to make one rotation around its own axis: about 28 days. From Earth, we always see the same face of the Moon from the Moon, the Earth stands still in the sky.

The near side of the Moon is well studied because we can see it. The astronauts landed on the near side of the Moon so they could communicate with NASA here on Earth. All of the samples from the Apollo missions are from the near side.

Although the far side of the Moon isn't visible from our vantage point, and with all due respect to Pink Floyd, it is not accurate to call it the dark side of the Moon. All sides of the moon experience night and day just like we do here on Earth. All sides have equal amounts of day and night over the course of a single month. A lunar day lasts about two Earth weeks.

With modern satellites, astronomers have completely mapped the lunar surface. A Chinese mission, Chang'e 4, is currently exploring the Aitken Basin on the far side of the Moon—the first such mission ever landed there. Researchers hope Chang'e 4 will help answer questions about the crater's surface features and test whether things can grow in lunar soil. A privately funded Israeli mission, Beresheet, started as a mission to compete for the Google Lunar X Prize. Despite crashing during an attempted landing earlier this month, the Beresheet team still won the Moon Shot Award.

Being shielded from civilization means the far side of the moon is "radio dark." There, researchers can measure weak signals from the universe that would otherwise be drowned out. Chang'e 4, for instance, will be able to observe low-frequency radio light coming from the Sun or beyond that's impossible to detect here on the Earth due to human activity, such as TV and radio broadcasts and other forms of communication signals. Low-frequency radio peers back in time to the very first stars and the very first black holes, giving astronomers a greater understanding of how the structures of the universe began forming.

Arrows indicate position of Chang'e 4 lander on the floor of the Moon’s Von Kármán crater. The sharp crater behind and to the left of the landing site is 12,800 feet across and 1,970 feet deep. Credit: NASA/GSFC/Arizona State University, CC BY

Rover missions also investigate all sides of the Moon as space scientists prepare for future human missions, looking to the Moon's resources to help humanity get to Mars. For instance, water – discovered by NASA's LCROSS satellite beneath the Moon's north and south poles in 2009 – can be broken up into hydrogen and oxygen and used for fuel and breathing.

Researchers are getting closer to exploring the Moon's polar craters, some of which have never seen the light of day – literally. They are deep and in just the right place to never have the Sun shine onto the crater floor. There are certainly dark parts of the Moon, but the whole far side isn't one of them.

This article is republished from The Conversation under a Creative Commons license. Read the original article.


47 Minutes of Sinister Silence: The Solo Astronaut who Orbited the Far Side of the Moon

Michael Collins is an unsung hero of space. When people talk about the historic Apollo 11 mission and the Moon landing, one of the most significant events in the history of space exploration, they usually mention Neil Armstrong and Buzz Aldrin. Those two famous astronauts landed the lunar module and were the first humans to walk and drive around on the surface of the Earth’s only natural satellite. Sadly, people rarely talk about Michael Collins, the member of the Apollo 11 crew who earned the infamous nickname “the forgotten astronaut,” because his role in the mission wasn’t sensationalized and his face was rarely seen on the covers of magazines.

However, Collins’ role in the Apollo 11 mission was of vital importance. He was in charge of piloting the Columbia command module, the “mothership” from which the lunar lander embarked on its descent to the surface of the Moon. His training was unusually intense and quite different than that of Armstrong and Aldrin: He was trained to pilot the spacecraft on his own in extreme conditions and to manually maneuver the docking of the lunar module in the event of an emergency.

He became so skilled in operating the command module that he actually compiled a 117-page-long book that contained a list of customized emergency procedures. Also, he designed the iconic Apollo 11 mission patch that features a bald eagle flying above the Moon and carrying an olive branch.

Armstrong and Aldrin spent a little over 22 hours on the surface of the Moon. During that time, Collins orbited the Moon alone in the command module and waited for them to return. This meant that he passed over the dark side of the Moon several times: during these passes he experienced a new sensation: an eerie kind of loneliness.

The entire body of the Moon stood between the Earth and Collins in the command module for 47 minutes. Therefore, he was unable to communicate neither with the Apollo 11 Mission Control back on Earth, nor with the other two astronauts who were on the surface.

Far side of the Moon, photographed by Apollo 16

In a press release in which they informed the public of the time of Collins’ whereabouts, NASA’s Mission Control stated the following: “Not since Adam has any human known such solitude as Mike Collins is experiencing during this 47 minutes of each lunar revolution when he’s behind the Moon with no one to talk to except his tape recorder aboard Columbia.”

Collins suiting up for the Apollo 11 flight

Upon returning to Earth, Collins stated that he didn’t give in to fear while facing the lonely eternal night of the dark side of the Moon. Still, he was obviously aware of the vast and intimidating unknown which surrounded him: “I don’t mean to deny a feeling of solitude. It is there, reinforced by the fact that radio contact with the Earth abruptly cuts off at the instant I disappear behind the moon, I am alone now, truly alone, and absolutely isolated from any known life. I am it. If a count were taken, the score would be three billion plus two over on the other side of the moon, and one plus God knows what on this side.”

Collins (in the middle) with Armstrong and Aldrin in the Command Module

In 2009, which marked the 40th anniversary of the Moon Landing, Collins admitted that his biggest fear during the entire mission arose from a different concern: he was afraid that something might go wrong with the lunar module and that Aldrin and Armstrong might perish on the surface of the Moon or during their ascent back to Columbia.

In that case, he would have been forced to finish the mission on his own and return home alone. In his opinion, such an outcome would have marked him for life: he would remain infamous as the one lucky astronaut who could return home without actually landing on the Moon.

Thankfully, Armstrong and Aldrin diligently performed their tasks on the lunar surface and then successfully docked with the command module. Collins piloted Columbia flawlessly and ensured the men’s safe return to Earth. Although Armstrong and Aldrin generated significantly more media attention, all three astronauts deserve to be remembered as a team of space-exploring virtuosos who accomplished one of the hardest missions in the history of humanity.


Re: Why don't we see the dark side of the moon?

It's not a silly question! But what I would suggest you do is a google search on some of the other moons in the solar system. You can find data tables that list the orbital and rotational periods of moons.

For example, if you do a google search for : moon orbital rotational period jupiter


Scroll down to the section titled: Orbital parameters. All of the moons that have S as the rotation period, that means that the orbital and rotational periods are the Same. So that would be the answer to your question:

From the link I gave you, Io, Europa, Ganymede, Callisto, Metis, Adrastea, and Amalthea are all gravitationally locked, just like our moon. If you could stand on the surface of Jupiter, you would see the same face of all those moons all the time. That's a lot of moons. Isn't it? And that's just Jupiter.

So, this isn't a wierd coincidence at all. It's the natural end state.

I should have done a google search at first, indeed.(sorry)

One more question, if 'tidal locking' is common, why isn't Earth for example than tidally locked with the Sun? I assume distance has to do with it.

I have a hard time in believing in such coincidences when tidal effects and resonances are so common.


6. The moon is a perfect sphere

You might be surprised to learn the moon is not perfectly round, but rather a little bit lemon-shaped. When the moon first formed, it was made of molten rock. Outside gravitational forces and areas of uneven density in the moon’s core as it cooled caused the satellite to take on a slightly irregular shape, called an oblate spheroid.

The moons of other planets in our solar system have been observed to have the same not-quite-round shape, like Jupiter’s icy moon Europa.


Chinese Probe Uncovers New Mystery on the Dark Side of the Moon

The "dark" side of the moon isn't really darker than the "light" side of the moon. But that far side does appear to get colder at night.

Earth's moon is tidally locked to the planet, meaning that the same side of the moon faces us at all times. But the moon is still spinning in order to constantly point one face at us, so it experiences days and nights from the varying sunlight. These periods are about two Earth-weeks long. Data from the Apollo missions had already revealed that the moon's sunlit surface can climb to 260 degrees Fahrenheit (127 degrees Celsius) during the day, and drop to minus 280 F (minus 173 C) at night. But all of that data comes from the side of the moon that faces Earth. The new Chinese mission that landed on the "dark" (read: far) side of the moon on Jan. 3 has recorded even colder temperatures during the long lunar night.

The Chinese lander Chang'e 4 and its rover, Yutu 2 (Jade Rabbit 2), woke from dormant, power-saving modes at the end of January and beamed back data suggesting that temperatures there had plummeted to minus 310 F (minus 190 C), according to an Agence France-Presse report. [Photos from the Moon's Far Side]

The difference between Chang'e 4's reading and the Apollo missions' is "probably due to the difference in lunar soil composition between the two sides of the moon. We still need more careful analysis," Zhang He, executive director of the Chang'e 4 probe project, told Xinhua.

In other words, something about the lunar dirt where Chang'e 4 sits is probably causing the soil to retain less heat overnight than the Apollo landing sites did. But researchers still aren't sure what that something is.

Chang'e 4 and Yutu 2 are the first probes to explore the far side of our nearest neighbor, so the data they return will be one of a kind. It may be a long time before researchers have a firm answer to the temperature-difference question.


A Bit of Moon History

This was not always the case. The moon was probably formed about 4.5 billion years ago, when a planet the size of Mars crashed into the young Earth and was smashed apart. The rubble went into orbit around the Earth and collected together into the moon, which with a diameter of 3,476 km (2159.89 miles) is just over a quarter the size of the Earth. (See our article on How was the Moon Formed)

At this time the moon was much closer to the Earth and both spun much faster, but the effects of the tides caused them to slow down and move further apart. Today they are 384,400 km (about 238,855 miles) apart, a distance that increases by 3.8 cm (1.5 in) a year). If there were no moon the Earth would now be spinning about three times faster, giving us 1,095 days in a year, each of them just eight hours long.

Seeing In The Dark

It could be said that the far side of the moon is ‘dark’ in the sense of invisible and unknown, except that this is no longer true either. Spacecraft have now photographed the far side of moon extensively, starting with the Soviet probe Luna 3 in 1959. In addition the moon wobbles slightly as it spins, making a few degrees of the far side visible from Earth.

The same ‘braking’ effect acted on the moon, slowing its rotation until it reached what is called the ‘tidal locking point’, when its rotation became synchronous (perfectly coinciding) with its orbit, so that the same side always faces Earth.


Soul-eating aliens?

Writers and film-makers have been inspired by the far side of the Moon, too, though none seem to have produced anything with the longevity of Pink Floyd's masterpiece.

A 1990 horror film entitled The Dark Side of the Moon tells the story of a group of astronauts who discover an abandoned shuttle there. They later discover it went missing in the Bermuda Triangle and is home to an alien which feeds on human souls.

We haven't had the chance to watch, but its average score of 2.6 on Rotten Tomatoes seems about right.

In literature, the website Goodreads returns more than 200 results for books entitled Dark Side of the Moon, including a 1976 tale from children's author William Corlett about the journey of a lonely, troubled astronaut.

So whatever the Changɾ-4 probe tells us about rock formations and craters, it seems certain that the hidden face of the Moon will continue to exert a gravitational pull on imaginations here on planet Earth.

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Watch the video: Why we dont see the dark side of the Moon (August 2022).