The lines in the sky are stars

By Phil Plait | June 24, 2010 7:00 am

Gifted astrophotographer StĂ©phane Guisard — whose Easter Island picture garnered him the #3 spot in my Top Ten Astronomy Pictures of 2009 — has done it again. He just published this amazing picture of star trails, but it’s not like one you’ve ever seen:


[Click to see a bigger, cleaner pic, and yes, you really want to!]

This astonishing picture shows the entire sky from horizon to horizon with the help of a wide angle lens (to help orient you, south is to the left, north to the right, west at the bottom, and east is at the top). It was taken on a volcano called Chimborazo, which is in Ecuador. The volcano has a latitude of 1.5° south, so it sits almost exactly on the Equator [Update: StĂ©phane sent me a note that he has been to this volcano before, and has an amazing Milky Way picture taken from it.] Guisard started the exposure about an hour after sunset, once the sky got dark, and ended 10 hours later, about an hour before sunrise. Because of this, it shows roughly 90% of the entire visible sky!

How can this be?

If you’ve ever been to this blog before, you know I’ll be happy to explain. But it takes a minute, so I’ve split the rest of this post up into two sections: you can read about the guts of how this picture works just below, or you can skip to the part where I describe what’s in it (stars and so on). Enjoy.

1) How this can be:

[First, a note: when I say "entire sky", I mean the whole thing, like you were floating out in deep space and could see in every direction with nothing blocking your way. It would feel like you were in the center of a sphere with the stars surrounding you no matter which way you look. On Earth at any one moment, the most you can see is 1/2 the sky, because the Earth itself blocks your view.]

OK, we need a a little geometry lesson. Imagine you are standing on the north pole. As the Earth spins under you, the stars appear to make circles centered directly over your head. Polaris would make a tiny little circle in 24 hours (it’s not exactly on the north celestial pole, the point in the sky directly over the Earth’s north pole, but it’s close), and stars farther from the pole would make bigger circles. At the horizon, the circles would be biggest.

But that’s all you could see. The Earth itself blocks your view of the southern sky, so you can only see half the entire sky. The stars all make circles that are parallel with the horizon, so they never rise nor set. It doesn’t matter what time of night you go out; you see the same stars, just in different positions in the sky. You’ll never see Alpha Centauri or the Southern Cross.

The same is true if you were to stand on the south pole, except this time you can only see the southern sky. The Earth would forever block your view of the Big Dipper, Polaris, and other exclusively northern sights.

Things change if you’re on the Equator. Facing north, you would just see Polaris on the horizon — and actually it would be a bit above the horizon, due to, of all things, our atmosphere. The air of the Earth acts like a lens, bending the path of the light from stars near the horizon. Because of this, Polaris would actually be about two degrees (about 4 times the size of the full Moon) above the horizon. If the Earth had no atmosphere, Polaris would be exactly on the horizon as seen from the Equator.

Turning round and facing south, you’d see the south pole of the sky (marked by the much fainter star Sigma Octans), which would likewise be on the horizon. Facing east, you’d see stars just now rising, and facing west those that are just now setting. If we had no Sun, over the course of 12 hours you’d see every single star in the sky as the Earth rotates beneath you. That’s because any star just setting in the west as you start your observation will be just rising in the east 12 hours later.

However, we do have the Sun (yay!) and so you can’t observe for 12 straight hours, only realistically about 10. So you don’t see the whole sky over the course of the night, you can only see about 10/12 = 5/6 = 80%. If you can push the observing for another half hour you can get up to about 87% of the sky. [Update: StĂ©phane corrected my math here, and he’s right. My apologies for any confusion!] Astronomers divide the sky up into 24 hours — in this case, each "hour" is the distance a star on the Equator of the sky will travel in that time. It’s equivalent to 15 degrees (360 degrees / 24 hours = 15 degrees per hour). The part of the sky not seen in a picture like the one above is shaped like a watermelon slice, with the narrow points at each pole and the widest part at the Equator, near the Sun’s position. It’s one hour wide on each side of the Sun, so altogether that slice is two hours wide. That means the picture has the entire sky (24 hours) minus those two hours, for a total of 22 hours. The amount of sky seen is therefore 22/24 = 11/12 = 92%.

Plus, the Earth’s air give you a bonus: because it bends the light of stars low to the horizon, you can actually see stars that are below your physical horizon! In a sense, the air is acting like a periscope, allowing you see around a corner.

So all this together means that Guisard’s remarkable all-night photo from the Earth’s Equator shows over 90% of the entire sky that is possible to see from the ground.


2) So what’s the deal with this picture, anyway?

He used a fish-eye, a wide angle lens, to capture the entire 360° view of the sky. That’s why star trails near the poles are distorted. The bright trails near the southern pole are from Crux, the Southern Cross. You can also see a bright meteor that blazed its way near the pole, too.

Other stars are identifiable if you know your way around — though without labels they’re hard to find and even harder to describe. Let me try a few (though no promises on my accuracy!):

The reddish-orange streak to the right of center and extending almost the whole picture vertically is from Arcturus. I suspect the red streak just to its left that only extends a third of the way up from the photo’s bottom is Mars. I think the whitish streak to the upper right is Capella, and the bright blue one just to the right of that is Vega.

Interestingly, Orion straddles the Equator, so the three stars marking the belt would all blur together in a picture like this. Plus, this picture was taken when Orion was near the Sun, so it’s only seen briefly in this picture, making a short trail: orange-hued Betelgeuse is the very short streak at the bottom of the image, just to the right of center. I suspect the bright streak at the very top is Sirius, the brightest star in the night sky, and the bright streak near the bottom just to the left of center is Fomalhaut. The famous star Alpha Centauri is the long yellow streak to the left of center starting at the top of the image. Where that streak ends, at about the center of the picture, you can see an orange streak continuing just to its left; that’s Beta Centauri.

I’ll leave the rest to you to discover. And I’ll note that I spent some time a couple of years ago in the Galapagos, at pretty much the exact same latitude this picture was taken. The southern skies are breathtaking, and seeing Alpha Centauri for the first time was incredible. I hope sometime to repeat the experience. And if I do, I may just try to get some star trail pictures for myself.

Picture courtesy Stéphane Guisard, who actually alerted me to it.

P.S. The title of this post is a pun on the title of a classic science fiction story by one of my favorite authors of all time. And that’s all I’ll say about that.

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (24)

  1. I think you misidentified the bright streak at the top center. You said it is Sirius, but I believe it might be Venus. Either way, it is an amazing photo. It’s going to be my new background.

  2. Um, the view is 360° around the horizon, but the lens only sees 180° of sky; if it did see 360°, you’d see the camera that took the shot
    Very cool startrail photo though.

  3. Tom K

    “Interestingly, Orion straddles the Equator, so the three stars marking the belt would all blur together in a picture like this….”

    Looks to me like the three stars of the belt are pretty evident, right at the bottom center of the picture. You can even make out the dagger of Orion (containing the Orion Nebula) to the left of them. And I think that the short, bright streak to the left is Sirius, not Fomalhaut – the three stars of the belt point right to Sirius. And to the right of center is the red streak of Betelgeuse. Looks like Rigel juust makes it above the horizon.

    Mentally rotate this map 90 degrees clockwise to match the orientation of the all-sky picture.

  4. Messier Tidy Upper

    Wow! Now that’s a “star trails” astrophograph with a difference! :-)

    Great idea and well captured – thanks StĂ©phane Guisard & the BA. :-)


    An excercise for y’all here if y’wish – Can you spot the top ten brightest (apparent magnitude*) stars in this photo? For those that may not already know (apologies if this is boring for those who do!) these stars are :

    1. Sirius – Or “the Dogstar”, a nearby white-hot A1 star with a white dwarf companion known as “the Pup” it is twice the Suns mass and twenty-six times as bright which combined with its next-door neighbour status of just 8.7 light years off gives it the “brightest star” honour although many stars are considerably brighter yet rarer.

    2. Canopus – A white F0 or A9-type supergiant is a staggeringly superluminous (15,000 times our Sun) and distant 310 ly) star. Canopus seen in ancient times in Egypt but not Athens helped prove the Earth is round and will in the distant future become southern pole star.

    3. Alpha Centauri – Our nearest neighbour and a triple star system where the main pair A &B appears like two headlights in a telescope. Its components are G2 & K0 for that brighter pair then a distant – perhaps even unrelated M5 for the red dwarf Proxima or Alpha Centauri C.

    4. Arcturus – A thick disk star that is moving away from us and an orange giant that is the brightest Northern hemisphere star. A type K2 orange giant it lies about 36 light years away

    5. Vega – A blue-white hot type A0 star which has a ring of gas and dust that maybe forming planets about it. We also know it is a star we see pole-on to us.

    6. Capella – A triple star system consisting of two very close yellow giants and a pair of distant red dwarfs. We see the combined light of the twin giants which are sun-like in temperature, 14 and 11 larger in radius and just 600 million years old.

    7. Rigel – A distant B8 supergiant and one of the most intrinsically bright stars in our galaxy. Rigel is also a triple star with the main star Rigel A accompanied by a distant pair of B9 blue dwarfs. Rigel A is the brightest star within 1,000 ly of us with 66,000 times our Sun’s luminosity. It emits more light in a minute than the Sun does in a month! Rigel A boasts 17 times our Sun’s mass and is 70 times the solar radius. It will certainly go supernova in a few million years time.

    8. Procyon – The “little Dogstar” is a nearby F5-type star, mag. +0.38, located 11.5 ly away. It has a white dwarf companion of only 11th magnitude orbiting it once every 41 years. Some astronomers believe it is evolving off the main-sequence into gianthood and classify it as a sub-giant whilst others do not. Procyon A radiates as much light in a day as our Sun does in a week being 7 times as luminous despite being only one and half solar masses – showing a little mass goes a long way! A recent study found Procyon is 1.7 billion years old

    9. Achernar – A bright B3-type giant located 140 light years from us, this star with the “Pointers” makes a handy signpost to the South Celestial pole. A hot young and blue-white star it is an impressive 400 times brighter than our Sun and attains magnitude 0.46 in our skies. It also spins extremely rapidly and has consequently become flattened as its equator bulges outwards from its poles.

    10. Betelgeuse – A red supergiant star of M2 Iab type located about 400 light years away & large enough to swallow all our planets out to Mars if placed in our solar system’s centre. It is an irregular or semi-irregular variable ranging from 0.0 to 0.9 magnitude and about 15,000 times as luminous as our Sun. It’s companion star actually orbits inside the red-hot vacuum of its outer layers. It will almost certainly go supernova leaving behind a neutron star or black hole.

    * Or the brightest stars as we seen them in our sky not in terms of intrinsic real or Absolute Magnitude brightness.

  5. I guessed right away what the image was. I wouldn’t, however, have guessed that it covers 90% of the sky.

    How long before someone suggests doing it again at the winter solstice, so you can get more than 10 hours of night? :-)

  6. Pierre

    Ken B @ 5 : At the equator, you always get a perfect 12 hours of darkness and
    12 hours of daylight no matter what time of the year, whether it’s during a solstice
    or an equinox or any other moment. It’s only at higher latitudes that the mix
    changes, but then the higher you go, the less of the other hemisphere you can
    see. So there’s no place or time on the planet where you could get it all: the
    boundary between day and night will always interfere a little (like it did for
    the missing 10% in the picture above).

  7. Nemesis


    “Um, the view is 360° around the horizon, but the lens only sees 180° of sky; if it did see 360°, you’d see the camera that took the shot”

    The BA explained the 360 degree thing, I guess you missed that part. Here it is:

    “However, we do have the Sun (yay!) and so you can’t observe for 12 straight hours, only realistically about 10. So you don’t see the whole sky over the course of the night, you can only see about 10/12 = 5/6 = 80%. If you can push the observing for another half hour you can get up to about 87% of the sky. Plus, the Earth’s air give you a bonus: because it bends the light of stars low to the horizon, you can actually see stars that are below your physical horizon! In a sense, the air is acting like a periscope, allowing you see around a corner.

    So all this together means that Guisard’s remarkable all-night photo from the Earth’s Equator shows almost 90% of the sky that is possible to see from the ground. “

  8. Justin (1): I think you’re right about Sirius; it’s at the bottom, not the top. I don’t have time right now to look it all up, but you may be right about Venus as well.

  9. Nemesis: I didn’t miss that. it’s not what I’m referring to. Besides, he doesn’t capture the entire sky, “only” 92% (an amazing feat!) Whats that, close to 331 degrees? Just arguing semantics anyway- the lens he used sees a 180 “cone” of the sky.

    Ken B: the thing about being on the equator is the nights aren’t longer during winter solstice. The Sun is up pretty much the same length every day. Only when you move away from the equator does the days become shorter in the Winter (and reverse for the Summer). Here’s a nifty weather animation of 12 hours on our hemisphere:
    note that the map cuts off at the equator at the bottom; I live in New Hampshire. My friend lives in Miami. My nights are quite a bit shorter than his at this time of the year because I’m closer to the pole that’s tipped towards the sun than he is in Miami. On the equator, there’s no difference, and there isn’t seasons there as those of us in temperate climates know them.

  10. The star trail pic is amazing but I was perusing Guisard’s other pictures and a 3D timelapse of Paranal Observatory in Chile totally jumped out at me. It is very cool. The Moai pictures are awesome too.

    BA mentioning Alpha Centauri reminded me, embarrassingly, that for at least half my life I never realised that one of the pointers we in Oz can see almost every night is Alpha Centauri. It was always just one of the pointers. I attempted to take a few photos of the Southern Cross and The Pointers a week or so ago and managed to snag the Coal Sack too (blogged it). It made me realise, if it wasn’t obvious already, just how brilliant a photographer StĂ©phane Guisard is.

  11. MT-LA

    This is a two-fer! Not only is this an amazing photo, but it provides a nice little illusion: if you scroll the image up and down on your computer screen, it looks like the trails are separating and converging as they move.
    Phil – calm down on the quality of posts…you’re making the not-so-BA-bloggers look bad.

  12. Arthur Maruyama

    Incidental to your description of this wonderful picture, an idea occured to me: how large a circle does Polaris draw in the sky? A glance at Wikipedia (and a bit of calculation) tells me that Polaris is almost 0.74 degrees from the celestial North Pole, so Polaris’ circle in the sky is about 3 times larger in diameter than the apparent size of the Moon–isn’t that odd? If you had asked me before I looked up the details I would have guessed that Polaris’ circle was MUCH smaller than the Moon.

  13. complex field

    Dave Poole….

  14. It looks like the magnetic flux lines across a wafer in a plasma etch chamber. (Sorry, I’m just wired that way).

    – Jack

  15. magetoo

    Sean Walker:

    Um, the view is 360° around the horizon, but the lens only sees 180° of sky; if it did see 360°, you’d see the camera that took the shot.

    The context of that “360” is pretty clear from the post…

    But lets talk about numbers anyway: it seems that it should be able to see more than 180 degrees, given the effects of the atmosphere that is mentioned. I still can’t see any obvious trails that look like they would be permanently above the horizon – so is Polaris visible here or is it too faint?

    Pierre (+ Sean):

    At the equator, you always get a perfect 12 hours of darkness and
    12 hours of daylight no matter what time of the year

    Wow. Guys, it’s time to recalibrate your humor detectors. (It’s even mentioned in the linked page, so I’m not sure who you think you are lecturing.)

  16. “My God, it’s full of stars!”

  17. Awesome picture. At the University of Nebraska we’ve made an interactive tool (in Flash) to explore just this phenomenon:

    – set the latitude to 0 degrees
    – add a bunch of stars (shift-click on a sphere or press the “add star randomly” button)
    – select the “long star trails” option
    – press “start animation”

    Drag the dot around on the map to see how the star trails vary with latitude. You can also show
    which parts of the sky are not visible for a location by selecting the “show never rise region”

    Simulations are great but pictures like this really bring the concept home.

  18. Levi in NY

    Chimborazo is shorter than Everest and some other mountains, but due to the shape of the Earth it is the farthest point on the Earth’s surface from its center.

  19. NelC

    The lines lights in the sky are stars

    Nightfall by Isaac Asimov?

  20. Loess

    @# 19

    I was thinking it was a Gurren Lagan reference. Unless Gurren Lagan was quoting Asimov…

  21. Patricia

    @19 NelC, Fredric Brown’s 1952 book, The Lights in the Sky Are Stars, “tells the story of an aging astronaut who is trying to get his beloved space program back on track after Congress has cut off the funds for it.”
    Props to BA for the punny post title!

  22. Messier Tidy Upper

    @ 19. NelC Says:

    The lines lights in the sky are stars. Nightfall by Isaac Asimov?

    See : for more.

    Or, better still, read it for yourselves in (among other places) the eponymous anthology (Grafton Books, 1991) if you haven’t already. Awesome short story which is one of Isaac Asimov’s best ever – which is *very* high praise indeed when you realise how many great things he’s written. :-)

  23. Buzz Parsec

    Sean Walker and Pierre… I’m pretty sure (judging by the smiley) that Ken knew that and was pulling everyone’s leg ;-)

    Kind of like sending a spaceship to the Sun… “But it will burn up!” “No, we have thought of that. We are going to go at night.”

    P.S. Beautiful picture.

  24. Brian Too

    It suddenly occurred to me that this photo makes time visible, in a sense. The stars leave linear streaks on the image, with sections of that streak showing the stars (or other objects) spectra and brightness at that point in time.

    Perhaps not the best trick with most stars, but some stars are variable. Meteors and other “nearby” objects might be more interesting overall. I wonder if you could do something interesting with such a technique…

    Of course long duration exposures are commonplace, but usually the goal is to keep the subject still during the imaging process, so as to increase the brightness of the image without smearing.

    One really interesting photo technique I read about was to greatly reduce the light coming into a lens using filters. They used it in metro street scenes. With exposure times measured in many hours, the effect was to “erase” any transient objects. People, cars, anything moving didn’t leave enough of an image to register. You could take a picture of a city on the busiest day imaginable and all the people would vanish in the final image.


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