Astronomy

Do stars twinkle when seen from Mars' surface?

Do stars twinkle when seen from Mars' surface?

Mars' average atmospheric pressure is 0.006 atm (0.088 psi). Is that enough to make fixed stars on Mars' night sky twinkle? Do we know an air pressure or density limit for that?


Following howstuffworks and my own metereological intuition, the key effect for stars to twinkle seems less the pressure or size of the atmosphere but rather if different layers (usually of different temperature) exist, and if the gradiant between the layers is steep enough.

Light passing through zones of differently dense air on its way to the observer lead to small (time-dependent) distortions occuring for instance over warm asphalt of a road. Here, the light travels horizontally through the atmosphere, rather than vertically like in the case of twinkling stars. I like to imagine the parcles of hot, less dense air rising from the ground as the bubbles in a lava lamp. In the case of a hot road, a few dozends of meters (on Earth's surface) is enough to cause the flickering of e.g. cars at a distance. The visible image then becomes somewhat blurred, and since the air is constantly (and irregularly) moving upward over the hot street, it looks as if the air is flickering.

Applied to your question: I know that dust devils are likely occuring on Mars, which need atmospheric instabilities in order to exist, so I would assume that there are indeed atmospheric conditions where one could observe twinkling stars on Mars.


Why Do Stars Twinkle?

All things considered, our atmosphere is pretty great. This blanket of nitrogen, oxygen and other gases keeps the world's temperature nice and habitable while protecting us from harmful UV radiation — to say nothing of the space debris it vaporizes. Oh yeah, and without all that oxygen in our atmosphere, animal life couldn't survive on planet earth. Not a bad resume.

But despite its many good qualities, the atmosphere can be a nuisance to astronomy buffs. That's because it distorts light. At night, the atmosphere makes some heavenly bodies appear to flicker and shimmer. The technical term for this phenomenon is "astronomical scintillation." You probably know it by a different name: twinkling.

Like an onion, the atmosphere is made up of layers. At the bottom is the troposphere, which starts right here at ground level on the planet's surface. Standing about 5 to 9 miles (8 to 14.5 kilometers) tall, it's where most of Earth's weather events take place. The other layers are — in ascending order — the stratosphere, mesosphere, thermosphere and exosphere. (There's also a region called the ionosphere, which encompasses parts of the mesosphere and thermosphere.)

These layers have different temperatures. In addition, the air's density varies from level to level. When starlight enters our atmosphere, it runs into pockets of cool and warm air. The pockets act as big lenses, causing the light to change direction — or "refract" — as it passes through them. Yet the lenses are not fixed in place they move around and change shape. As they shift, so does starlight refraction. That's why the stars appear to twinkle.

Scintillation affects planets, too. Mercury, Venus, Mars and other planets in our solar system do twinkle when viewed from Earth on a clear night. (So does our moon.) However, the planets twinkle to a barely noticeable degree.

Distance is the main reason stars twinkle more conspicuously than the planets in our solar system. Because the former are so far away, each star looks like a single pinpoint of light. It's a different story for Earth's moon and our neighboring planets. Being a lot closer, they're less affected by the atmosphere. Planets and moons appear as tiny disks up in the sky. The light they emanate comes not from a single point but from many individual points all clustered together. These rarely scintillate in unison, which is why planets and moons don't twinkle as dramatically as the stars.

Twinkling only can happen when an atmosphere is present. It's for this reason that photos taken by the Hubble Telescope look so clear there aren't any atmospheric air pockets to refract the starlight. Earthbound astronomers use telescopes with adaptive optics systems to compensate for twinkling, making the stars look more stable.

Our closest planetary neighbor is Venus, which is 25 million miles, or 41 million kilometers, away from us at the nearest point in its orbit. On the other hand, you'd need to travel more than four light-years to reach the closest foreign star system (that of Alpha Centauri). That's a long way to go. Just one light-year is equal to 5,878,625,373,183.6 miles or 9,460,730,472,580.8 kilometers.


Why Don't Planets Twinkle Too?

Unlike stars, planets don't twinkle. Stars are so distant that they appear as pinpoints of light in the night sky, even when viewed through a telescope. Because all the light is coming from a single point, its path is highly susceptible to atmospheric interference (i.e. their light is easily diffracted).

The much closer planets appear instead as tiny disks in the sky (a distinction more easily discerned with a telescope than with the naked eye). Their apparent sizes are usually larger than the pockets of air that would distort their light, so the diffractions cancel out and the effects of astronomical scintillation are negligible.

Star wheels will help you find your way among the twinkling constellations, and you can trace the appearance of the planets along the ecliptic with a Skygazer's Almanac.


Do stars twinkle when seen from Mars' surface? - Astronomy

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WHY DO STARS TWINKLE?
Twinkle, twinkle, little star,
How I wonder .

The scientific name for the twinkling of stars is stellar scintillation (or astronomical scintillation). Stars twinkle when we see them from the Earth's surface because we are viewing them through thick layers of turbulent (moving) air in the Earth's atmosphere.

Stars (except for the Sun) appear as tiny dots in the sky as their light travels through the many layers of the Earth's atmosphere, the light of the star is bent (refracted) many times and in random directions (light is bent when it hits a change in density - like a pocket of cold air or hot air). This random refraction results in the star winking out (it looks as though the star moves a bit, and our eye interprets this as twinkling).

Stars closer to the horizon appear to twinkle more than stars that are overhead - this is because the light of stars near the horizon has to travel through more air than the light of stars overhead and so is subject to more refraction. Also, planets do not usually twinkle, because they are so close to us they appear big enough that the twinkling is not noticeable (except when the air is extremely turbulent).

Stars would not appear to twinkle if we viewed them from outer space (or from a planet/moon that didn't have an atmosphere).


Do planets twinkle in the sky?

Read, more on it here. People also ask, do stars twinkle in the sky?

Stars do not really twinkle, they just appear to twinkle when seen from the surface of Earth. The stars twinkle in the night sky because of the effects of our atmosphere. When starlight enters our atmosphere it is affected by winds in the atmosphere and by areas with different temperatures and densities.

Beside above, what is the bright star by the moon right now? Venus &mdash the third-brightest object visible from the Earth, after the sun and moon &mdash is currently appearing as &ldquothe evening star.&rdquo It is approaching its greatest evening elongation of the year, the point at which, from an Earthly vantage, it appears farthest from the sun.

Then, why does Mars twinkle?

Although Mars, which shines by relected sunlight, is intrinsically dimmer than a brilliant star like Spica, the planet is much brighter in the night sky. Neither stars nor planets twinkle as seen from outer space, but the view from Earth's surface is a different matter.

Because planets do not have nuclear fusion, they do not produce their own light. Instead, they shine with light reflected from a star. When we see planets in the night sky, such as Venus, the so-called "Evening Star," we're seeing reflected sunlight.


9 stargazing tips to get you watching the stars from home

Being stuck at home could provide the perfect opportunity to get to know the night sky, and you don’t even need a telescope.

Published: 15th July, 2020 at 11:54

A clear night sky filled with stars is a wonderful sight. Looking up to the heavens can make us feel warm and fuzzy, even if it’s freezing cold outside.

But stargazing isn’t something that is reserved for astronomers with big fancy telescopes. Stargazing for beginners is something you can do from home with only a warm jacket, an idea of where to look and plenty of time to sit back and enjoy the view.

If you have a pair of binoculars, great, but don’t worry, these tips will help you go star spotting with nothing but the naked eye.

So, fancy heading outside to see what the night sky has to offer? These stargazing tips from our friends at BBC Sky at Night Magazine will help you get the most of your night under the stars.

Dress for the occasion

Before you even look at the sky, take a look at yourself in the mirror. Are you dressed properly? You’re going to be outside for at least an hour, hopefully longer, and it can get cold even on summer nights.

So dress appropriately, with a warm jacket, thick socks, and gloves, scarf and a hat. Basically, you want to look like one of the rosy-cheeked children playing happily in the winter snow from a vintage Ladybird book.

Prepare your site

Find a spot in your garden where you can see as much of the night sky as possible, away from other houses, tall buildings, and trees. If you don’t have a garden, you can also stargaze from a balcony or front step, or through a skylight.

If you’re going to be outside for a while, take a chair – a reclining chair will allow you to look upwards without straining your neck. Turn off all your outdoor lights and, if you can, get your neighbours to do the same.

If you don’t have somewhere to stargaze, or if there’s a lot of light pollution, then you might need to find another site (lockdown restrictions permitting!). Head to a dark spot in the countryside, or just walk around the corner to your local park or playing field.

Read more about astronomy:

Adjust your eyes

Once you’re comfy, let your eyes adjust to the darkness. Astronomers call this process ‘dark adaptation’, and it takes about half an hour.

Don’t browse on your phone while you wait (its bright screen will ruin your night vision). If you need some light, use a red light torch, or a red bicycle light if you don’t have one.

See the stars

Once your eyes have adjusted, you won’t believe how many more stars you can see. You might be able to notice subtle differences in the stars’ colours, which depend on their temperature: the hottest stars are more of a bluish colour, while cooler stars have a yellow, orange or red tint.

Look for patterns

The stars can be joined up to form patterns. You might recognise one straight away: the saucepan-shaped Plough, which is visible all year round. The Plough is an ‘asterism’ – a star pattern that’s not one of the official constellations. It forms part of the constellation Ursa Major (the Great Bear).

Another asterism to look out for in June is the Summer Triangle.

Very few of the 88 constellations look like the person, animal or object they represent, so you’ll need to use some imagination!

If you return to your observing site in another season, you’ll notice that the constellations you can see has changed. This is because the Earth is in a different position in space as it orbits the Sun, and it’s why it takes a year to properly get to know the sky, not just one night.

Spot a planet

If you see a bright ‘star’ that isn’t twinkling, it’s almost certainly a planet.

Why do stars twinkle, and planets don’t? A star is so far away that its pinprick of light arrives in a very narrow beam, which is easily distorted as it passes through our atmosphere (we see this as twinkling).

Planets are much closer, and their light (reflected from the Sun) comes to us in a thicker beam, which isn’t so easily distorted. There are five planets that can be seen with the naked eye: Mercury, Venus, Mars, Saturn and Jupiter. For tips on how to spot them, visit BBC Sky at Night Magazine.

Spot a meteor

As you’re gazing skywards, you might be lucky enough to see a meteor dash across the sky: a shooting star!

These are tiny grains of space dust burning up in the atmosphere. Larger, brighter meteors, called fireballs, can survive their trip through the atmosphere and drop meteorites on the ground.

Your best chance of seeing a shooting star is during a meteor shower, when the Earth passes through the dusty trail left by a comet. One of the best annual showers is the Perseids, which peaks this year around 11-13 August.

Marvel at the Moon

Go outside on a cloudless night and reacquaint yourself with our closest celestial neighbour. Impact craters appear as bright patches on the Moon’s surface, while the dark regions, known as lunar maria, are vast plains of solidified lava.

The Moon’s features cast more impressive shadows, and are hence easier to pick out, when they’re close to the ‘terminator’ – the dividing line between the Moon’s dark and illuminated portions. So a crescent or a gibbous moon can be just as interesting as a full moon, especially when seen through binoculars.

Read more about the Moon:

Download an app

Stargazing apps can be a great way to familiarise yourself with the positions of the stars and planets. You’ll need to make sure you turn your screen brightness right down to keep your night vision, or turn on the app’s red screen filter if it has one. Some good, free-to-download apps include Stellarium Mobile Free, SkyView Lite and Star Walk 2 Free.

Looking for stargazing tips? Check out our complete astronomy for beginners UK guide.


Watch Capella flashing red and green

This evening, check out one of the flashiest stars in the sky. It’s so bright that every year in northern autumn, we get questions from people in the Northern Hemisphere who see a bright star twinkling with red and green flashes. It’s found low in the northeastern sky at nightfall or early evening as seen from mid-northern locations. That star is likely Capella. If you could travel to it in space, you’d find that Capella is really two golden stars, both with roughly the same surface temperature as our local star, the sun … but both larger and brighter than our sun.

Capella is in the constellation Auriga the Charioteer, but since antiquity it has carried the name Goat Star. You might pick it out just by gazing northeastward from a Northern Hemisphere latitude during the evening hours in October. Capella climbs upward through the night, and, this month, soars high overhead in the wee hours before dawn.

Scott MacNeill at Frosty Drew Observatory in Rhode Island wrote in October 2016: “I noticed how fabulous Capella looked just hanging in the northeast sky. So I directed my telescope to Capella and captured this shot.” Thanks, Scott!

So here is a golden point of light that flashes red and green when it’s low in the sky. Why does it do that?

The reality is that every star in the sky undergoes the same process as Capella, to produce its colorful twinkling. That is, every star’s light must shine through Earth’s atmosphere before reaching our eyes. But not every star flashes as noticeably as Capella. The flashes are happening because Capella is low in the sky in the evening at this time of year. And, when you look at an object low in the sky, you’re looking through more atmosphere than when the same object is overhead.

The atmosphere splits or “refracts” the star’s light, just as a prism splits sunlight.

So that’s where Capella’s red and green flashes are coming from … not from the star itself … but from the refraction of its light by our atmosphere. When you see Capella higher in the sky, you’ll find that these glints of red and green will disappear.

By the way, why are these flashes of color so noticeable with Capella? The reason is simply that it’s a bright star. It’s the sixth brightest star in Earth’s sky, not including our sun.

Capella is a bright star, what astronomers call a 1st magnitude star. It’s one of the brightest stars in our sky. If you’re in the Northern Hemisphere, and you happen to look in the northeast one evening, you might notice Capella as a bright, flashy star near the northeastern horizon.

Bottom line: If you’re in Earth’s Northern Hemisphere, a bright star twinkling with red and green flashes, low in the northeastern sky on October evenings, is probably Capella.


SEEING

TWINKLE, TWINKLE LITTLE STAR.
HOW THE STARS DO SPARKLE.

These words bring sighs to the romantic but, to the astronomer, they can mean a lost night of useful work. Contrary to popular belief, crisp, clear Winter nights, with the stars twinkling like Christmas lights, are the worst possible for serious observing.

Twinkling is caused by masses of air of different temperatures passing between the observer and the object being observed. They have the same effect as poor quality window glass when trying to look through it. The image waves around and the atmosphere acts like an out of control lens that keeps changing the focus.

To the unaided eye, stars twinkle or change brightness irratically. Through a telescope, twinkling stars dance around like a drop of water in a hot frying pan. Planetary detail is smeared and fuzzy and little of interest can be seen on such a night.

This phenomonon is called " SEEING" and varies considerably from place to place and from time to time. In general, the higher up in altitude, the better the seeing because there is less atmosphere to see through. This is one reason professional observatories are usually located high up on mountain tops. However, some sea level locations (southern Florida for example) can be nearly as good at certain times and some locations such as the Midwestern U.S. are nearly always bad.

In order to discuss seeing intelligently and to help in site selection of large observatories, seeing must be quantified. It is also important to know what the seeing is when evaluating a telescope . Many new scopes and their manufacturers get a bad rap for "lousy optics" when, in fact, the best optics on Earth could not produce a better view because of poor seeing.

To quantify seeing, one could simply judge the amount and amplitude of twinkling but it would be to hard to get a good handle on the range or value. Much better is the system developed by William H. Pickering of Harvard at the turn of the century. The popular Pickering 1 to 10 scale is in common use by professionals and amateurs alike. The Pickering scale is based on what a highly magnified star looks like when carefully focused, in a small telescope.

A star at high magnification, under perfect seeing (P-10) looks like a bull's eye. A small central disk surrounded by one or more concentric rings. At P-1, it is just an amorphous blob. The central disk is known as the Airy disk and it's size in inversely proportional to the size of the telescope objective. That is why a large telescope can see more detail under perfect conditions than a small one. Because of physical limits the Airy disk is the smallest detail that can be seen at maximum magnification and the smaller it is, the less it intrudes on the detail. Makes little difference when looking at a star which can never be resolved because of distance but when looking at the surface of Mars or the Moon, every feature is just a lot of Airy disks all jumbled together and the larger they are, the fuzzier the image.

One fact little understood by purchasers of new telescopes is that the effects of poor seeing increase dramatically as the size of the telescope is increased. This is simply because a small telescope has to look through a much smaller column of air than a large one. A fairly good night with a small scope might be not worth taking out a large one. Pickering established his system using a 5" diameter telescope and his scale would have to be fudged when used with a scope of larger or smaller aperture.

The small scope in the foreground was designed specifically to evaluate seeing at my site and is the same size as the one used by Pickering. The larger one in the background is the 16".

For more details on the "Pickering 5". [Telescopes]

The "bull's eye" or diffraction pattern as it is known starts to appear at about P-4 in the 5" and is distinct but unstable at P-7 (a very good night for the Midwest). At P-7, the 16" is about the same as the 5" at P-4. I have yet to see anything better than P-7 here which points out the fact that the 16" has yet to be used to it's fullest capability.

It should be pointed out that I am referring here to the ability to resolve detail and not just the ability to see dim objects. The large scope always prevails in the latter but when viewing the surface of Mars or the Moon, for example, no more detail can be seen on a poor night with a larger scope.

By now it should be clear why, in spite of it's modest size, the Hubble Space Telescope has produced photographs that exceed the reach of even the largest Earth based instruments. Being above the atmosphere, it is immune to the effects of seeing and its resolution is only limited by its size.

For these and other reasons, it is very difficult to photograph the diffraction pattern but there are more pragmatic ways of demonstrating the effects of seeing. Because seeing not only varies from location to location and from night to night but also changes drastically from moment to moment, particularly on poor nights. A P-3 night can have instants of P-6 and a patient observer can often snatch good views if persistant enough and does not blink at the right moment.

Because of the fast and continuous frame capture of video, it is very easy to demonstrate these moments just by attaching a video camera to a telescope and pointing it at the moon.

The following images were captured only seconds apart and by stepping through the tape, a moment of good seeing was found.

We now come to the details of Pickering scale as described in a S&T article from April, 1995.

P-1 Star image is usually about twice the diameter of the third diffraction ring (if the ring could be seen.

P-2 Image occasionally twice the diamteter of the third ring.

P-3 Image about the same diameter as the third ring and brighter at the center.

P-4 The central disk often visible arcs of diffraction rings sometimes seen.

P-5 Disk always visible arcs frequently seen.

P-6 Disk always visible short arcs constantly seen.

P-7 Disk sometimes sharply defined rings seen as long arcs or complete circles.

P-8 Disk always sharply defined rings as long arcs or complete but in motion.

P-9 Inner ring stationary. Outer rings momentarily stationary.

P-10 Complete diffraction pattern is stationary.

The following is a summary of the seeing conditions at my location about 50 miles NW of Chicago since Oct 1997.

The 71 clear nights out of 213 tell another story about the effects of El Nino on observational astronomy. It is however, only part of the story. Most of those 71 nights were only clear enough, long enough to make the evaluation. The actual number of useful clear nights during the period was a small fraction of that number.

I do not have a feel for what is "normal" for this area but it is not hard to understand why no new observatories have been built in the Midwest in the past several decades and the older ones are either decomissioned or virtual museums.

If anyone has or wants to gather info on their site, I would be be glad to post it here for comparison.


Astronomy in the Harry Potter Series

Within the Harry Potter stories are many scenes that can be analyzed astronomically. As both an astronomer and a teacher, these interest me, and so I will present such analysis here. My goal is not to criticize Rowling when she is inaccurate in her astronomy (or, for that matter, to praise her when she gets it right), because she is clearly not concerned with making her tales scientifically accurate—she’s writing a story, not an astronomy textbook.

“Mars is bright tonight,” Ronan repeated . . . . “Unusually bright.” (PS15)

As seen from Earth, Mars varies greatly in its apparent brightness, due to both the changing distance between the two planets, and the changing amounts of sunlit surface Mars presents to us. Roughly every 26 months, the Earth is closest to the red planet, and Mars looks brightest at these times Mars is said to be in opposition.

However, according to the Lexicon Timeline, Ronan’s remark about Mars’s unusual brightness was made in late May of 1992, about 7.5 months before its January 1993 opposition. On that night in the Forbidden Forest, Mars was in fact only at about 10% of its maximum brightness [1] —hardly “unusually bright.”

[In Diagon Alley, Harry] was sorely tempted, too, by the perfect, moving model of the galaxy in a large glass ball, which would have meant he never had to take another Astronomy lesson. (PA4)

A perfect model of our Milky Way Galaxy—the enormous conglomeration of hundreds of billions of stars to which our Sun belongs—would be very cool indeed, but as Professor Sinistra’s class appears to focus only on our solar system (just the Sun, its planets, and their moons), it is hard to understand why this galactic model would be so helpful to Harry. If the glass ball containing the scaled-down Milky Way was three feet in diameter, our solar system would be an invisible speck within it, about a millionth of an inch in size. Perhaps, when Rowling was describing this item, she confused the terms “galaxy” and “solar system”—a common error.

“Did you check the lunar chart and realize that I was always ill at the full moon?” (PA17)

The average length of time from one full moon to the next is 29.53 days. Three dates when we know Lupin was ill are November 5, December 25, and June 6. However, since those dates are not multiples of 29.53 days apart, they cannot all have been full moons. [2]

“the hippogriff Buckbeak, hereafter called the condemned, shall be executed on the sixth of June at sundown” (PA21)

The time of that fateful sunset can be estimated by using the events in the story. At 11:55 PM that night, Harry and Hermione went back in time by three hours. They heard their earlier selves crossing the entrance hall with Ron and leaving for Hagrid’s cabin, walking slowly under the Invisibility Cloak (PA21). If it took the Trio fifteen minutes to cross the grounds, then they arrived at Hagrid’s at 9:10 PM. According to Lupin, they were in Hagrid’s hut for twenty minutes (PA17), and therefore left around 9:30 PM. The execution party arrived at the same time they read the official notice, signed it, walked outside, and saw Buckbeak gone then the executioner threw his axe (PA21). If all of that took five minutes, then the axe was thrown at approximately 9:35 PM. At that moment, “the very last rays of the setting sun were casting a bloody light over the long-shadowed grounds,” (PA17) so it was just before sundown. [3]

At first glance, this might seem like an unusually late sunset, but we must remember that (1) Britain is on Summer Time in June (what Americans call Daylight Saving Time), (2) the date is just before the summer solstice, the longest day of the year, and (3) Hogwarts is quite far to the north, in Scotland (according to this web site referenced by the Lexicon).

Mainland Scotland lies roughly at latitude 55-58º N and longitude 2-6º W. On June 6 in that region, sunset ranges between 9:40 PM and 10:20 PM British Summer Time [4] , so the timing roughly agrees with Rowling’s chronology (the agreement gets worse the farther Hogwarts is to the north and west).

“Jupiter’s biggest moon is Ganymede, not Callisto,” [Hermione] said. . . . , “and it’s Io that’s got the volcanoes. . . . Europa’s covered in ice, not mice . . . .” (OP14)

This information about Jupiter’s four largest moons is indeed correct.

A twinkling red star winked at [Harry] from overhead. . . . “Mars, bringer of battle, shines brightly above us.” (OP27)

Mars does look like a red star to the naked eye, but since it is a planet, it does not appear to twinkle as the stars do. Also, the Lexicon Timeline estimates that Firenze’s first Divination class happened in early March of 1996, which was midway between Mars’s 1995 and 1997 oppositions. At that time, Mars was shining with only about 10% of its maximum brightness. [5]

Harry’s O.W.L. Astronomy practical (OP31)

In the discussion that follows, I will assume that this exam began at 11 PM British Summer Time [6] on June 24, 1996, [7] and lasted for 90 minutes. [8] I will also (somewhat arbitrarily) place Hogwarts about midway between Edinburgh and Aberdeen, at latitude 56.5º N, longitude 2.5º W. Using these parameters and the online interactive planetarium Your Sky (implemented by John Walker), I have generated maps showing which stars would have been visible in the sky at the beginning ( Figure 1.) and end ( Figure 2.) of the exam (the moon and the planets are not drawn, as they change their positions from year to year).

Figure 1.: Beginning of O.W.L.s – 11 PM

Figure 2.: End of O.W.L.s – 12:30 AM

. . . a perfect night for stargazing, cloudless and still. The grounds were bathed in silvery moonlight . . .

There are two reasons why this night would in fact be less than perfect for stargazing. First, if the grounds were bathed in moonlight, then the sky would have been quite washed out, making it difficult to see faint stars. Astronomers usually try to observe when the moon is below the horizon, or barring that, when the moon is not very full.

Second, the summer sun would have set around 10 PM, [9] only an hour before the beginning of the exam. At 11 PM, the sky would have still been aglow with twilight, and only the moon, the planets, and a handful of the brightest stars could have been seen with the naked eye. Indeed, at that latitude near the date of the summer solstice, the sun never gets lower than ten degrees below the horizon, so the sky is never truly dark.

As Harry completed the constellation Orion on his chart . . .

Orion is a constellation best seen in December. In June, the stars of Orion are almost directly behind the Sun as seen from the Earth. Consequently, at that time of year it is impossible to view Orion—not only in Scotland, but almost anywhere on the planet.

Harry put his eye back to his telescope and refocused it, now examining Venus.

Harry seems to have observed Venus a bit more than an hour into the exam—that is, just after midnight. Since Venus is usually only visible either shortly after sunset or just before sunrise (due to its proximity to the Sun), my initial reaction was that Rowling was describing an impossible scenario. However, in a letter to the amateur astronomy magazine Sky & Telescope, [10] astronomer Kevin Krisciunas pointed out that this is indeed possible in certain years.

Since Krisciunas made his calculations for a slightly different location in the UK than what I have assumed, I have done my own investigation (although the results are not much different). It turns out that in 1991, Venus was visible in the western sky at 12:05 BST on June 25, albeit just a fraction of a degree above the horizon. (Here [ Figure 3.] is a simulated view using Your Sky.) As seen from Earth, Venus’s motion in the sky repeats itself every eight years, so this late Venus could also be seen from Hogwarts on the same date in 1999 or 2007. Depending on Hogwarts’s exact location, it might have also been just barely possible to see Venus at that same time and date in 1994 and 2002. (It was not possible in 1996, however.) Thus, the situation that Rowling describes is indeed astronomically possible: Venus can sometimes be seen at midnight.

Figure 3.: Venus during the O.W.L.s

. . . Harry . . . noticed that he had mislabelled Venus as Mars.

It’s a good thing that Harry noticed, because this is quite a mistake to make: Venus and Mars are very different in color, as well as brightness. Venus appears bright white in the sky, while Mars is reddish and somewhat dimmer.

. . . [Harry] saw, for mere seconds, a vision of the main street in Hogsmeade, still dark, because it was so much farther north.

(DH24)

Harry has this vision at Shell Cottage, which is located “on the outskirts of Tinworth” (DH23), and is therefore (according to Bagshot’s A History of Magic) somewhere in Cornwall (DH16), along the extreme southwestern coast of England. According to DH24, the sun rose at Shell Cottage just before Harry spoke to Griphook and Ollivander, and it rose at Hogwarts (in Scotland) just after these interviews. Perhaps a half-hour passed between the two sunrises, and Rowling explicitly states that it rose later at Hogwarts because it was farther north than Shell Cottage. Does this work out astronomically?

To answer this question, we need to know the date when Harry’s vision took place. Fortunately, Rowling has given us enough clues to determine that the sunrise in question happened between March 14 and April 1. [11] During this period, which is roughly within a week of the spring equinox, two locations that lie on the same line of longitude will experience sunrise at more or less the same time. At the latitude of the U.K., the more northern location will see sunrise only a few minutes later during the week before March 21, and a few minutes earlier during the week after. Thus, it seems that Rowling has made an error—the fact that Hogwarts is farther north than Shell Cottage cannot explain why its sunrise is a half-hour later.

The time of sunrise also depends on longitude. Close to an equinox, when sunrise is insensitive to latitude, it happens four minutes later for every degree of longitude farther west. Therefore if Shell Cottage was significantly east of Hogsmeade (by about 7.5°), then the two sunrises could have occurred as Rowling relates, though for a different reason. However, since Shell Cottage is supposedly in Cornwall (longitude 4.5-6° W), it is in fact farther west than much of mainland Scotland (longitude 2-6° W). So unfortunately, we cannot reconcile Rowling’s sunrises with either astronomy or geography.

It was a relief when six o’clock arrived and they could slip out of their sleeping bags, dress in the semidarkness, then creep out into the garden. . . . The dawn was chilly, but there was little wind now that it was May. Harry looked up at the stars still glimmering palely in the dark sky. . . .

(DH26)

Is it possible for the stars to still be visible at shortly past 6 AM BST on a May morning in the U.K.? Alas, no: sunrise in that part of the world is always before 6 AM at that time of year. [12]

Notes

[1] This was determined with the Freeman edition of the astronomical software Starry Night Backyard, ©2000 SPACE.com Canada Inc.

[2] November 5: First Hogsmeade weekend is Sunday(?), October 31 (PA8) Gryffindor vs. Hufflepuff match is next Saturday, November 6 Snape substitutes for Lupin the day before (PA9).

December 25: “I’m afraid the poor fellow is ill again . . . . Most unfortunate that it should happen on Christmas Day” (PA11).

June 6: “the hippogriff Buckbeak … shall be executed on the sixth of June” (PA21) Lupin transformed that evening.

See also Troels Forchhammer’s essay “Mapping the Harry Potter Timeline,” where he does basically the same analysis.

[3] See also Hollydaze’s “Time Line: The End of PA.”

[4] This was determined with the US Naval Observatory’s online sunrise/sunset calculator..

[5] Starry Night Backyard, Freeman edition, ©2000 SPACE.com Canada Inc.

[6] OP31 gives the start time as 11 PM, and all of Britain is on Summer Time in June.

[7] The Lexicon Timeline guesses that O.W.L.s began on either June 8 or 15, which would place the Astronomy practical (nine days later) on June 17 or 24. I will adopt the latter date for the following reason. In Half-Blood Prince, we find that “Harry remained within the confines of the Burrow’s garden over the next few weeks,” and then he left the Burrow for Diagon Alley a few days after his July 31 birthday (HBP6) thus he arrived at the Burrow around July 10. On that day, Dumbledore asked him, “I gather that you have been taking the Daily Prophet over the last two weeks? … Then you will have seen that there have been not so much leaks as floods concerning your adventure in the Hall of Prophecy?” (HBP4) This adventure therefore took place around June 26, and the Astronomy practical was two days before that.

As a further check, in HBP1 Fudge visited the Muggle Prime Minister on a day “in the middle of July.” During that visit, he said, “I was sacked three days ago! The whole Wizarding community has been screaming for my resignation for a fortnight”—presumably since Fudgefirst announced the return of Voldemort two days after the Astronomy practical. Thus, if the practical was on June 24, Fudge’s visit would have been roughly two weeks and five days later on July 13, which is indeed near mid-July.

[8] “Half an hour passed, then an hour,” then Harry sees Umbridge and her crew cross the grounds and enter Hagrid’s cabin. Shortly after that, the examiner calls “twenty minutes to go.” (OP31)

[9] This was determined with the US Naval Observatory’s online sunrise/sunset calculator.

[10] Sky & Telescope, “Rowling Gets It Right,” p. 12 of December 2003 issue (Vol. 106, No. 6), Sky Publishing Corporation, Cambridge, MA.

[11] The relevant timeline is as follows: on the night Ron managed to tune in Potterwatch (DH22), the Trio were captured by the Snatchers and brought to Malfoy Manor (DH23). The sunrise in question was the following morning.

We are told that the Potterwatch broadcast took place on an evening in March, so the sunrise could have been no later than April 1. We also know that Draco was on Easter holidays at the time. The earliest possible date of Easter is March 22 (see the U.S. Naval Observatory’s article “ The Date of Easter”). According to Diana Summers’s essay “British Schooling in the 1970s,” Easter holidays always begin two Fridays before Easter Sunday, so they never start earlier than March 13. Assuming that Hogwarts is on the same schedule, the sunrise in question could therefore have been no earlier than March 14.

[12] This was determined with the US Naval Observatory’s online sunrise/sunset calculator.


Do stars twinkle when seen from Mars' surface? - Astronomy

Do constellations looks the same from space?

We get this questions in a variety of flavours. Two recent examples:

Can the constellation Orion be seen from the surface of Mars? And if so, where in the sky would it appear?

If an astronaut was traveling through the solar system and could look out a window, would stars be more or less visible? Would the constellations be visible as constellations? That is, would the stars still be in pretty much the same relationship as seen from Earth?

The answer is always the same. Although the constellations are not usually stars which are physically associated with each other, you have to go a very significant distance from Earth before you would be able to see them appear as different shapes. Everywhere within the solar system the constellations would look just the same. If you could travel significant fractions of the distance to the nearest stars (many light years) then you would start to see some changes, but such travel is (unfortunately) way beyond our capabilities at present.

This page updated on June 27, 2015

About the Author

Karen Masters

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.


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