Crepuscular rays are parallel!

By Phil Plait | November 2, 2011 7:00 am

When you go outside at sunset, many times you’ll be greeted with spectacular rays of light and shadow stretching across the sky. These are called crepuscular rays, and are caused by clouds blocking the sunlight, their long shadows cast on haze and other particulates floating in our air.

Those rays fan out, spreading away at different angles… but that’s an illusion! The rays are parallel, and I offer this photograph as proof:

[Click to penumbrenate.]

That shot was taken on October 18, 2011, by an astronaut on board the International Space Station as it passed over India. Towering cumulonimbus clouds threw their long shadows back, away from the Sun. Note that the shadows from different clouds are parallel to each other! That’s because the Sun is very far away compared to the distance between the clouds.

Here’s a picture I found on Flickr showing what we see from the ground, though (it’s not of the same clouds, but just a typical display of crepuscular rays). The fanning out of the rays is actually an illusion, caused by perspective! It’s precisely the same thing that makes railroad tracks or long roads appear to converge in the distance. Things farther away look smaller, so the parallel rails of a railroad track appear to get closer together as you look farther away. For railroad tracks you look down to see this; for cloud shadows you look up! Other than that, they’re the same.

So why do the shadows in the first picture look parallel? It’s because the astronaut was looking straight down on the clouds and shadows, so his distance to any part of the shadow was roughly the same; the shadow near the cloud and way downstream (so to speak) were both about the same distance away from him. That negates the perspective effect, and the shadows are revealed for what they truly are: parallel.

Astronauts have said it for years, but it bears repeating: exploring space gives you perspective. And in this case, it’s literally true.

Image credit: NASA; Elsie, Esq.’s Flickr Stream


Related posts:

- Crepuscular rays
- My comic book premier
- A sun pillar gooses the sky
- Thus spoked the Dumbbell

CATEGORIZED UNDER: Pretty pictures, Science

Comments (29)

Links to this Post

  1. A Week Flying the ISS – Day 3 | December 13, 2011
  2. Word of the Week: Crepuscular « Patos Papa | October 18, 2012
  1. huh?

    they’re near parallel — but not parallel. sure, perspective intensifies their otherwise minuscule and imperceptible angle, but they’re not emanating from an infinite planar light source… they’re radiating from the sun.

    no?

  2. Tara Li

    Well, *ALMOST* perfectly parallel. The Sun is a very long way off, but it is not *INFINITELY* far away.

    If it were, lunar eclipses would not involve both a penumbra and an umbra.

  3. Robin

    Yup. Strictly speaking, they are not parallel. Sure, we can approximate the Sun as being infinitely far away and as being a flat disc, but it’s not the case in reality. In some applications, the Sun cannot be treated as being infinitely far away, and those rays cannot be treated as being collimated or the Sun as that flat disc. If you wanted to get even more anal about it, you’d have to also consider that our gravity well is roughly spherical as is our atmosphere. As such, both would effect the angle of any light “ray” with respect to a line between the center of the Sun and the center of Earth. Of course, none of that takes away from the pictures looking cool.

  4. All I see is the mothership. Yeah!

    Seriously, great description there. :)

  5. BJN

    I second Tara Li’s post. Also note that the shadows aren’t crepuscular rays, they’re the light beams within and between the shadows. Finally, I have to wonder if folks are so unfamiliar with perspective that they need to be reminded that the rays converge on a vanishing point that is a very distant sun. Train tracks ARE parallel, yet through the magic of perspective they appear to converge at some point on the horizon.

  6. Sir Eccles

    So, what you’re saying is that the earth is actually being faked on a sound stage in the deserts of Nevada?

  7. @Sir Eccles,

    But Nevada’s on the Earth so there must be a larger Earth which is on a larger sound stage in a larger desert in a larger Nevada, which in turn is in an even larger Earth…. It’s sound stages all the way up! ;-)

  8. CR

    Instead of sunbeams in the photo posted, I thought there’d be a pic of shadows converging at the horizon, as viewed from the ground. It’s a neat effect I’ve witnessed many times, but unfortunately have not photographed.
    Basically, at sunset (or sunrise, for that matter) on a partly cloudy day–one where sunbeams are visible like in the photo above–turn you back toward the sun and you may see long shadows from the clouds in the sky. (This is not the same as the ‘Venus’ Belt’ effect mentioned elsewhere in this blog, by the way, where one might observe one long shadow caused by the earth itself.) The effect almost looks like ‘reverse’ or ‘negative’ sunbeams, because they are shadow rays rather than light rays. They also indeed converge on the horizon, another ‘reversal’ from the sunbeams that appear to radiate outward from the sun.
    The effect is much more subtle than sunbeams, so it helps to know what you’re looking for/at… the ISS shot above really helps to give you an idea of what to look for if you imagine yourself on the ground just at the right side of those cloud towers.

    Oh, Sir Eccles & TechyDad: LOL!

  9. Curt Cameron

    To those saying that the rays are ALMOST parallel but not quite – you’re treating the Sun like a point source. Since the size of the Sun is actually much larger than the distance between the clouds that are casting the shadows, the penumbra effect is much much larger than any non-parallelness.

    The rays/shadows get fuzzier as they go farther from the cloud edges, and that fuzziness completely overwhelms any effect you could get from considering the Sun as a point source.

    Also, like CR mentions, anti-crepescular rays are the coolest. Imagine the sunbeams from a low setting Sun, extending from those clouds to the West, over your head, and continuing on towards the Eastern sky. You’d expect those rays to continue to diverge as they continued out to the East, right? But they don’t – they re-converge towards a point that’s exactly opposite from the Sun. ‘Cause they’re parallel.

  10. Gary Miles

    I saw a great example of these rays the other day driving west toward Wyoming at twilight. The sun had sunk just below the horizon. The sky was clear except some clouds that were also hanging below the western horizon, so a plethora of these crepuscular rays were emanating from what appeared to be a clear western horizon. Beautiful!

  11. Robin

    @ Curt Cameron (#9): No, I’m not treating the Sun as a point source at all. I’m treating it as a sphere with an 865,000 mile diameter. That sphere can be treated as a surface covered with an infinite number of point sources, all which radiate light over some solid angle. The Sun itself covers approximately 6×10^-5 steradians, and certainly all the light that falls within a cone with that solid angle does not have to be collimated. In fact, considering the Sun, it most certainly is not.

  12. I saw crepuscular rays this morning on the way to work.

    I just thought everyone needed to know that.

  13. John Weiss

    For many years I thought that crepuscular rays were something that romantic artists made up. When I honeymooned with my wife in Isla Muheres thirty years ago, we saw those radiating bands of light as the sun set. Astonishing!

  14. Crux Australis

    I have seen ‘sunbeams’ converging, rather than diverging, since the setting Sun and the clouds casting the shadows were behind me. I had to do a double take before I thought “Of course!”

  15. MadScientist

    Even at such a great distance the sun is an extended source and not a point source. As others have stated, the rays are not parallel. Even in the photograph the lines seem to diverge (but in the opposite sense of what I expect if there were no effect of perspective).

  16. Joseph G

    Am I the only one who expected “Crepuscular rays are Crepuscular”?

    I think I’ve spent too much time on icanhascheezburger :D

    @#7 TechyDad: But Nevada’s on the Earth so there must be a larger Earth which is on a larger sound stage in a larger desert in a larger Nevada, which in turn is in an even larger Earth…. It’s sound stages all the way up!

    I see what you did there.

  17. L Ron Hubbub

    That Flickr picture is what we Catholics of a certain age call a “catechism sky”. Illustrations like it were used in the old Baltimore Catechism to depict the presence of God in the heavens.

  18. David Vanderschel

    Many years ago, I was SCUBA diving in Cozumel and I witnessed, from the diving boat, what Curt Cameron (#9) refers to as “anti-crepescular rays”. The sun was setting behind a somewhat isolated cumulus cloud well out on the horizon. This led to a multitude of rays which seemed to radiate from the edges of the cloud. That part is normal. What was unusual in this case is that the rays were so well defined that they remained apparent as they passed over our heads. Because they really are parallel, they then seemed to converge back to a point on the eastern horizon. That appearance of reconverging on the opposite horizon is what I found so remarkable in this case. I didn’t have a camera. That was the only time I have witnessed the phenomenon.

  19. mfumbesi

    I learned something new Today. I’ve taken pictures of clouds like the last picture and always assumed the rays were not parallel. Talk about challenging misconceptions caused by a certain perspective, phew. Thanks for this.

  20. flip

    I’ve known about perspective illusions since high school, but did think to apply it to shadows of clouds though. This is a great example of why I love this blog. So much stuff I didn’t know I didn’t know.

  21. #5 BJN:
    “I have to wonder if folks are so unfamiliar with perspective that they need to be reminded…”

    Sadly, some people apparently are that unfamiliar with perspective! One of the favourite non-arguments of the “Apollo was faked” imbeciles is that the shadows in the photos appear non-parallel, which they claim “proves” that there were multiple light sources. Really!!! It doesn’t occur to them that (a) the non-parallel shadows are a perspective effect, like the convergence of railway tracks, and (b) multiple light sources would mean each object had multiple shadows. DUH!!!!

  22. Nigel Depledge

    Tara Li (2) said:

    Well, *ALMOST* perfectly parallel. The Sun is a very long way off, but it is not *INFINITELY* far away.

    If it were, lunar eclipses would not involve both a penumbra and an umbra.

    Your first sentence is right, but your second sentence is wrong.

    Lunar eclipses (and solar eclipses) have an umbra and a penumbra due to diffraction of the light around the object that casts the shadow. This is a larger effect than that the sun occupies about 0.5° of arc as seen from the Earth.

  23. Nigel Depledge

    Curt Cameron (9) said:

    To those saying that the rays are ALMOST parallel but not quite – you’re treating the Sun like a point source. Since the size of the Sun is actually much larger than the distance between the clouds that are casting the shadows, the penumbra effect is much much larger than any non-parallelness.

    The rays/shadows get fuzzier as they go farther from the cloud edges, and that fuzziness completely overwhelms any effect you could get from considering the Sun as a point source.

    Yes, and the fuzziness is due more to diffraction than it is due to the non-point-source nature of the sun.

  24. Anchor

    I’ve often seen PERFECTLY ‘parallel’ crepuscular rays from the ground, and there are many photos showing this. I’ve also seen ANTI-crepuscular rays, radially CONVERGING to a distant point opposite the setting or rising Sun. It is rather surprising that this elementary exercise in perspective should be so surprising to so many.

    To be sure, the scale of the locale involved on our tiny Earth (much less than the roughly 8000-mile diameter of our planet) compared to the distance to the Sun’s 93 million miles which provides the light source makes any assertion of such rays as EITHER radial OR parallel utterly moot. There is ALWAYS a point 90 degrees away from any light source in which the light rays (and shadows appropriately flung along them) will be EXACTLY parallel from the point of view of a given observer.

    Rudimentary geometry informs us how perspective behaves. Phil is right in that crepuscular rays can appear absolutely parallel (even though he doesn’t explicitly explain why). Others who nit pick about the source of light having a finite distance are also right – and they may also wish to consider the fact that that source happens to subtend a substantial half-degree of arc which further blurs their argument away from a clean point. (Clue: consider what the Earth subtends at a distance of 93 million miles…)

  25. Joseph G

    Do a GIS for “anti-crepuscular rays” if you’re in the mood for some eye candy :)

  26. Anchor
  27. Maybe we should call them micro-parallel. You know, like micro-gravity.

NEW ON DISCOVER
OPEN
CITIZEN SCIENCE
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT

See More

ADVERTISEMENT
Collapse bottom bar
+

Login to your Account

X
E-mail address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it e-mailed to you.

Not Registered Yet?

Register now for FREE. Registration only takes a few minutes to complete. Register now »