Squishy moonrise seen from space!

By Phil Plait | January 21, 2011 11:30 am

Check. This. Out: Moonrise as seen by astronaut Paolo Nespoli on board the International Space Station!

Holy wow! Click to spacestationate.

That is so cool. As the ISS races around the Earth at 8 km/sec (5 miles/sec), it sees up to 18 sunrises and sunsets each day, and the same number of moonrises and moonsets. Paolo had to snap quickly to get this sequence, which couldn’t have taken more than a minute to elapse.

But what’s with the squished Moon? Here’s a closeup of the Moon in the three pictures:

What causes this? It’s an atmospheric effect, due to the air surrounding the Earth acting like a lens, bending (or, if you want to impress your friends, refracting) light. You’ve probably seen how a spoon looks bent when it sits in a glass of water, right? Same thing. Light passing from the vacuum of space through our air gets bent a bit. The amount of bending depends on how much air the light is going through; the thicker the air the more it’s bent.

When an astronaut on the ISS sees the Moon near the Earth’s limb, as in these shots, light from the top part of the Moon is passing through less air than the bottom. So the light from the bottom gets bent more, in this case, up. This makes it look as if the bottom of the Moon is being squished up into the top, like a clay ball that’s been dropped on the ground. As the ISS orbits the Earth, and the Moon gets higher off the limb, the effect diminishes so in the two subsequent shots the Moon gets re-inflated.

You’ve probably seen this yourself, though not as dramatically. The next time you have a clear horizon view to a sunset (like maybe on the west coast, as the Sun sinks below the waters of the Pacific) you’ll see exactly this same effect. The Sun will look squashed. I’ve actually posted about this a couple of years ago, when a similar picture of the Moon from the ISS was released. It wasn’t as dramatic as this one, though!

Paolo Nespoli has a Flickr page where he posts amazing pictures he’s taken from space. You could have a much worse Friday than clicking through some of those shots and seeing how lovely our world is when seen from above.

Image credit: ESA/NASA. Tip o’ the spacesuit visor to Stuart at astronomyblog.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (45)

  1. Brian Davis

    “So the light from the bottom gets bent more, in this case, up.”

    Up? Wouldn’t this suggest that back projecting the ray describing the lower limb of the Moon would make the Moon look *bigger*?

  2. Well, there goes any productivity I planned for today.

    Trundles off to Flickr to click and stare

  3. Ciaran

    “Up? Wouldn’t this suggest that back projecting the ray describing the lower limb of the Moon would make the Moon look *bigger*?”

    The light gets bent down by the same angle as it exits the atmosphere so the rays from the top and bottom are still parallel exiting the atmosphere but closer together. Something like this picture: http://schools.ednet.ns.ca/avrsb/024/pwarren/Index/Science%208-2008/Unit%204%20Optics/Light-Refraction-Glass.gif

  4. James

    Paul, this is different from the apparent SIZE of the moon when it is near the horizon vs. being directly overhead? I seem to remember a post of yours about this a while back.

  5. It is getting wider but not so much taller, so maybe we need a different diagram, one looking down on the earth from above. Just a guess.

  6. Ryan

    Phil,

    Is this the same effect that causes people to perceive the moon as larger near the horizon as compared to overhead, or something else?

  7. UmTutSut

    Wonder if Paolo was listening to “Also Sprach Zarathustra…?”

  8. Dan

    @1 Brian Davis

    I think it was poor wording on Phil’s part. The light itself gets bent down, so that when we see it it looks like it’s coming from higher up than it actually is, thus creating the squish effect. So I think a better way to make the statement would be, “the light from the bottom gets bent more, causing the image to shift up.”

  9. Ryan and James, that is a totally unrelated effect known as the moon illusion.

    http://en.wikipedia.org/wiki/Moon_illusion

  10. Ryan

    Thanks, Larian. So essentially that effect is entirely a human perception problem, an optical illusion (what Neil Degrasse Tyson called a “brain failure”) as opposed to a real optical effect.

  11. Ian Menzies

    @5 Ryan

    No, the “Moon illusion” is apparently more psychological than anything. Check out Phil’s own blurb on the Moon illusion here: http://www.badastronomy.com/bad/misc/moonbig.html

  12. David

    I wonder if any astronauts have realised they are in the best position to make an alien invasion hoax. Cut out a paper flying saucer and attach it inside the window of the ISS and then take a picture so it looks like its hovering over earth and then tweet the image with the necessary #omg.

  13. Chris A.

    @John Claerbout (#5):

    “It is getting wider but not so much taller, so maybe we need a different diagram, one looking down on the earth from above.”

    Actually, I’m fairly sure the only reason it appears to be wider in the first shot, less so in the second, less still in the third, is because the photographer was zooming out a bit between shots. I don’t know of any refraction effects that would cause its apparent width to change significantly.

  14. pk_boomer

    @Ciaran #3

    No, I think the light from the bottom part of the moon is bent DOWN both when it enters the atmosphere, and when it leaves the atmosphere. Look at Phil’s diagram – the angle of incidence at these two locations is different.

  15. Paolo had to snap quickly to get this sequence, which couldn’t have taken more than a minute to elapse.

    Well, not having first-hand experience on-board the ISS, I can tell you my first-hand experience observing the ISS from good old terra firma…

    I forget the website (I’m sure Phil or one of his readers will post the URL), but I used last year to determine when the ISS would be visible shortly after dusk. (Dark sky, plus ISS still in sunlight, equals good viewing opportunity.) On one night, as the ISS passed overhead, it suddenly darkened and then disappeared in just a few short seconds. I realized afterward that this was probably “sunset on the ISS”.

    I would suspect that “moonset” would take about the same time.

    Perhaps we’ll see if our “six degrees of separation” from an ISS astronaut can come into play?

  16. Somebody send these guys some VIDEO cameras!

  17. Douglas Troy

    It’s so SQUISHY!!!!

  18. idahogie

    Light gets bent toward the normal as it enters the medium with higher refractive index (the atmosphere) and gets bent away from normal as it leaves the atmosphere. Given Phil’s simplified diagram above, that means that the beam would bend down on entry and then up on exit.

  19. pk_boomer

    Half right… it means the light gets bent down on entry (toward the normal), and DOWN again on exit (away from the normal). The normal vector is different on exit than it was on entry.

  20. TheBlackCat

    I was thinking the same thing, Major. I would love to see this as a video.

  21. idahogie

    Oops, pk – you are right. Which you already knew, of course.

  22. chris j.

    i shall call him squishy. and he shall be mine. and he shall be my squishy.

  23. Chris

    It’s amazing we haven’t seen that before.

  24. Messier Tidy Upper

    Great sequence of atmospheric (in both senses of the word! ;-) ) moon rise images.

    Love it – and that its taken by people orbiting our world and enjoying the view from space! :-)

    We take that for granted now, just fifty years or so ago that notionwould’ve been Science Fiction and science fiction seen as outlandish and weird and not to be aken too seriously by all too many folks. For hundreds and thousands of years before that the idea of people flying at all was but an idle dream.

    We live in a truly amazing & fortunate age and are highly temporally priviledged. It could be so much more amazing and we could’ve, perhaps really should have, gone so much further and done so much more but still we don’t appreciate that enough.

    Thankyou Science and all the giants who have come before and made what we take for granted reality.

    Special thanks to Paolo Nespoli and the Bad Astronomer here too.

    You could have a much worse Friday than clicking through some of those shots and seeing how lovely our world is when seen from above.

    Or in my case* a much worse Saturday. ;-)

    * Mine & also for other Aussies and Kiwis and those living in the more “advanced” timezones generally! ;-)

  25. Pete Jackson

    @15 Ken B – yes, that is a perceptive connection, comparing the Moon’s rising time to the time it takes the ISS to fade as the Sun sets on it.

    The ISS swoops 360 degrees around the Earth in 90 minutes, or 4 degrees a minute. Since the Moon and Sun’s diameters are both only one half degree, it will take only about 1/8 of a minute or about 8 seconds for the disk to cross the Earth’s horizon as seen from the ISS. Differential refraction will drag that out by a bit, however.

    Spectacular pics, Phil! Thanks for posting them.

  26. wagdog

    The ISS point of view is from outside of the atmosphere. The effect is different when viewing from within the atmosphere which is what the rest of the people on the planet experience. The reverse happens, and explains why the sunset that we actually witness would have occurred many seconds earlier were it not for this bending effect on light.
    http://www.youtube.com/watch?v=vvmq66op0G8

  27. Rick

    @Brian,

    I think the best way to explain the effect is to say that as the rays enter the ‘thick’ atmosphere down closer to the limb, they are bent ‘up’ — as they travel thru the atmosphere (higher index of refraction). They spend the most time in the medium and as a result receive the most ‘bending’. They exit again roughly parallel to their entry direction. This has the effect of shifting (translating) the ray “up” — and rays from the ‘bottom’ are effected the most.

    Rays from the top of the moon enter the atmosphere and are bent ‘down’ but spend less time in the medium so are translated only a small amount when the exit (again roughly parallel to their entry angle).

    You can use a trapezoid to model the atmosphere and do a ray trace (by hand) to see the squishing happening.
    :)

  28. Markle

    If you follow @Astro_Paulo on Twitter, you get a feed of these, plus, as the ESA Twitter noted, lessons in conversational Italian.

  29. Cumulative $900 billion for three photogpraphs and a case of radiation cataracts seems excessive. May the sun wake up and burn this scourge from our skies (sunspot 1149 can save us).

  30. kevbo

    it’s this refraction that makes the earth look round to those astronauts. It’s actually flat, you know. Just look outside.

  31. Elwood Herring

    This effect was mentioned in a recent episode of QI. Stephen Fry showed a speeded-up video of the moon setting and asked the panel to buzz when they thought the moon had actually disappeared below the horizon. Of course they all buzzed when the moon had visibly gone, but then he dropped his bombshell. No, Fry announced, by the time you see the bottom edge of the moon touch the horizon, it has already set! The atmosphere bends the light so you still see the moon above the horizon, by about the same amount as the moon’s apparent diameter.

    Do you get QI in the U.S.? If not, you don’t know what a great show you’re missing.

  32. Caleb Jones

    Reminds me of the time I wondered as a teenager why the moon was larger (or so I thought) near the horizon than when it was higher in the sky. A few hours on the internet (pre. google or wikipedia days) and the family encyclopedia (actual physical encyclopedia) yielded the following:

    What we perceive as the moon being “larger” when it is nearer the horizon is merely an optical illusion called ocular macropsia. It appears bigger only because it is nearer smaller objects we don’t normally compare it to (trees, homes, buildings, hills, mountains, etc.). It is the comparison that makes it appear larger. In reality, its angular size does not change.

    When it is higher in the sky, it is next to objects that are on much larger scales (constellations, stars, etc.) which we more frequently compare it to (which is why we perceive the moon in the sky as its “normal”-looking size). This is called ocular micropsia.

    Understanding this, it makes you appreciate just how big the sky is and how easily our eyes/minds can be deceived.

  33. Messier Tidy Upper

    @ ^ Caleb Jones : This is called ocular micropsia.

    Or, unless I’m mistaken, more usually known as the Large Moon Illusion. ;-)

    @31. Elwood Herring :

    .. Fry announced, by the time you see the bottom edge of the moon touch the horizon, it has already set! The atmosphere bends the light so you still see the moon above the horizon, by about the same amount as the moon’s apparent diameter.

    Yep – that same little known fact is true of the Sun and stars too I think. I knew the Moon was below the horizon at some point while still appearing above it – I didn’t realise it was already completely set when the bottom edge touched the horizon though.

    Just one question here – did Fry say (or does anyone know) at which point the Moon / Sun really *is* below the horizon while still appearing to be above it? When it is 1 degree above the horizon or 2 or 3 degrees above it? How big is the atmospheric delay gap exactly?

    Do you get QI in the U.S.? If not, you don’t know what a great show you’re missing.

    Seconded by me. :-)

    Stephen Fry has done a couple of other good shows &I’ve seen last year (?) – a documentary trip through the United States of America visiting some interesting places & people, forget the exact name of that & also another documentary series called (from memory) ‘Last Chance to See’ finding and discussing various animals that are on the brink of extinction, eg. the Blue whale, the Aye-aye (an odd type of lemur) and others. I’d recommend those too. :-)

  34. Messier Tidy Upper

    See :

    http://www.abc.net.au/tv/programs/lastchancetosee.htm

    For more about the “Last Chance To See’ series – note from there :

    … [Stephen Fry] teams up with naturalist Mark Carwardine in search of some of the most endangered animals on the planet. Last Chance to See originally started as a radio program and book from Carwardine and author Douglas Adams back in 1990. Twenty years on, and after the death of Douglas Adams, Stephen Fry and Carwardine retrace the vision of the original journey.

    Also, the doco where Fry explores the US of A was titled, simply enough, Stephen Fry In America’ too in case folks are wondering. ;-)

  35. Messier Tidy Upper

    @29. Uncle Al Says:

    Cumulative $900 billion for three photogpraphs and a case of radiation cataracts seems excessive. May the sun wake up and burn this scourge from our skies (sunspot 1149 can save us).

    Are you for real or just doing a bad Poe? :roll:

    The International Space Station may have its flaws and may not float everyone’s boat – I must admit it doesn’t overly float mine – but be fair. The ISS has achieved *a lot* more already than just taking these three photos and it isn’t even 100% finished yet!

    As for having it burnt out of the sky by a sunspot :

    I) It’ll take more than just a sunspot to do that methinks! Were you meaning a huge Solar flare, Coronal Mass Ejection or solar storm instead?

    II) The Sun isn’t “asleep” now & is building gradually towards its maxima, its been through those before when we’ve had space stations – specifically some of the Russian ones like Mir and the various Salyut‘s – orbiting and those “scourges” managed okay.

    III) Out of sheer morbid curiousity – you thinking of letting the astronauts get out first or are you happy to see them go with the ISS too? :-(

  36. Messier Tidy Upper

    @32. Caleb Jones :

    Reminds me of the time I wondered as a teenager why the moon was larger (or so I thought) near the horizon than when it was higher in the sky. A few hours on the internet (pre. google or wikipedia days) and the family encyclopedia (actual physical encyclopedia) yielded the following … What we perceive as the moon being “larger” when it is nearer the horizon is merely an optical illusion called ocular macropsia.

    BTW. There’s plenty more about that “Moon illusion” via this blog & the BA here :

    http://blogs.discovermagazine.com/badastronomy/2010/05/13/why-does-the-moon-look-so-huge-on-the-horizon/

    Plus here :

    http://blogs.discovermagazine.com/badastronomy/2007/06/30/big-moon-tonight/

    and also :

    http://blogs.discovermagazine.com/badastronomy/2010/06/07/very-large-moonset-but-not-why-you-think/

    there. If folks wish to read further. (I always do! ;-) ) :-)

  37. Chris (#23):

    It’s amazing we haven’t seen that before.

    I guess you don’t peruse the Atmospheric Optics website?

    http://www.atoptics.co.uk/atoptics/moonflat.htm

    A moonrise sequence from an earlier space station, Skylab.

    And http://www.atoptics.co.uk/atoptics/issmoon.htm

    Moonset from the ISS. On 16th April ’03 Don Pettit, the ISS science officer, imaged the full moon as it set behind the Earth’s limb.

  38. Messier Tidy Upper

    @29. Uncle Al : Also, you do realise that any solar flare or storm severe enough to down the International Space Station would also probably take out many of the vital satellites that provide us all with GPS, Global TV broadcasts, weather forecasting, etc .. Right? :roll:

    Plus such a cataclysmic solar storm would have effects here on Earth too. I think I recall reading a chapter about that in a certain book about Death Fom the Skies by an author whose name escapes me right now, think it started with ‘P’ and ended in ‘t’ or something! ;-)

  39. Nigel Depledge

    Elwood Herring (31) said:

    This effect was mentioned in a recent episode of QI. Stephen Fry showed a speeded-up video of the moon setting and asked the panel to buzz when they thought the moon had actually disappeared below the horizon. Of course they all buzzed when the moon had visibly gone, but then he dropped his bombshell. No, Fry announced, by the time you see the bottom edge of the moon touch the horizon, it has already set! The atmosphere bends the light so you still see the moon above the horizon, by about the same amount as the moon’s apparent diameter.

    I saw that episode and it was a sunset, not a moonset. But the principle is identical.

  40. Nigel Depledge

    Caleb Jones (32) said:

    What we perceive as the moon being “larger” when it is nearer the horizon is merely an optical illusion called ocular macropsia. It appears bigger only because it is nearer smaller objects we don’t normally compare it to (trees, homes, buildings, hills, mountains, etc.). It is the comparison that makes it appear larger. In reality, its angular size does not change.

    When it is higher in the sky, it is next to objects that are on much larger scales (constellations, stars, etc.) which we more frequently compare it to (which is why we perceive the moon in the sky as its “normal”-looking size). This is called ocular micropsia.

    This is a very nice explanation, but sadly it is wrong.

    If it were right, then we could make the apparent size of the moon vary by standing near to or away from nearby objects, and this does not happen.

    If you have a perfectly flat horizon with no trees, buildings or anything like that anywhere in your field of view (say, in the middle of a calm ocean), the moon illusion still happens. It still seems bigger near the horizon that it does near its zenith.

    The best explanation I have seen for this is along these lines:
    Our brain models the entire world from visual input. What we think we see is actually our brain’s model of our surroundings, with very little accurate detail (except directly where we look). Experiments can be done to show that if part of a scene changes while we are looking at a different part of the scene, we are unlikely to spot the change.

    The brain’s model of the sky is a shallow upturned bowl, with the zenith much closer than the parts of the sky near the horizon. If you consider that, on a cloudy day, the visible sky is indeed much closer overhead than near the horizon, you may see that this makes a kind of sense. Thus, the brain’s model tells us that objects in the sky overhead are closer to us than objects near the horizon.

    The real angular size of the moon does not change perceptibly from being overhead to being near the horizon. Thus, our brain models it as a large object very far away when it is near the horizon and as a smaller object rather closer when it is overhead. To compensate for the disparity between the actual angular size and the modelled “actual” size of the moon, our brain makes it appear to be larger when it is near the horizon (and thus a large object very far away) than when it is overhead (a smaller object a lot closer to us).

    The same effect occurs with the sun, but we almost never look at the sun when it is overhead (well, duh!) so it is noticed far less often.

  41. Nigel Depledge

    MTU (40) said:

    @29. Uncle Al : Also, you do realise that any solar flare or storm severe enough to down the International Space Station would also probably take out many of the vital satellites that provide us all with GPS, Global TV broadcasts, weather forecasting, etc .. Right?

    In fact, it would take out satellites first. I think the ISS is in a pretty low orbit as these things are reckoned. Geostationary satellies orbit outside the Van Allen belts, so have very little protection from our magnetic field. The ISS orbit is well inside both belts, so has nearly as much protection from charged particles as much of Earth’s surface.

    (BTW, I don’t mean that the Van Allen belts themselves protect Earth from the solar wind, but they are the destination to where our magnetic field diverts much of the radiation. Above the Van Allen belts, the magnetic field has yet to achieve much with incoming radiation, and below the belts, much of the radiation has already been trapped or diverted.)

    Plus such a cataclysmic solar storm would have effects here on Earth too. I think I recall reading a chapter about that in a certain book about Death Fom the Skies by an author whose name escapes me right now, think it started with ‘P’ and ended in ‘t’ or something!

    Yup. Any solar storm or CME of sufficient power to take out the ISS would also take out most of the power-distribution grids on Earth (or, at the very best, put them off-line for a few hours).

  42. Matt B.

    The moon looks awfully north-up in these pictures. How oblique is the ISS’s orbit?

  43. Steven Blount

    Great photo! And great explanation of the ‘squishing’ effect. Thanks for sharing.

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