1. Z.W. Wolf

    Z.W. Wolf Senior Member

    I do understand what you mean by the anti-solar point.

    [​IMG]


    But doesn't a shadow cast by an object below your elevation also aim toward the anti-solar point?

    Are the shadows in this photo aiming toward the anti-solar point? I really don't know.

    [​IMG]
     
  2. Trailblazer

    Trailblazer Moderator Staff Member

    But if you turned round 180 degrees after taking that photo you would see crepuscular rays. The shadows are parallel, passing overhead, so they appear widest overhead and converge at both horizons.

    There is no difference between crepuscular and anticrepuscular rays, any more than there is between the train tracks stretching out in front of you and those behind you. It's just rarer to be able to see them all the way to the horizon opposite the sun.

    The shadows will only appear parallel on a photo if the camera is looking either straight up or straight down. The photo from space has the camera pointing at an angle, so the shadows appear to converge. I think that's what you are saying, but calling them "anticrepuscular rays" is rather confusing because that term applies to viewing the same shadows from the ground so that they appear to converge opposite the sun.
     
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  3. Mick West

    Mick West Administrator Staff Member

    You seem to be overthinking it.
     
  4. Henk001

    Henk001 Active Member

    We are getting in a bit of a semantic debate here. I think that the pictures that Wolf showed are very illustrative of what may be (a part of) the explanation for the fact that the shadows are not exactly parallel.
     
    Last edited: Nov 28, 2016
  5. Z.W. Wolf

    Z.W. Wolf Senior Member

    Yes, everything is defined by observer position.

    My point about cropping the photo is that it doesn't change the nature of the photo. The camera was in the position it was in. If you go back to my post I'll think you'll see that. But that was a side note.

    I don't think that last bit is right. The camera pointing at an angle doesn't make the shadows appear to converge or diverge. It's the observer's position that does so.

    The important part about the angle of the camera is that it can be difficult to tell from a photo of relatively ambiguous objects like clouds if the camera was angled or not. An angled shot can look like a straight down shot.

    And in the case of the photo Abishua was asking about... He was assuming the cloud shadows would be parallel in the photo because he was assuming it was taken looking straight down. I say the cloud shadows are not exactly parallel as he expected, because the camera was not pointing straight down. That's the simplest form I can put that in.

    I then showed a cropped portion of a photo taken from a larger photo to prove that the illusion of the straight down angle can be just that; a seductive illusion.

    (Incidentally, when I first linked to that photo, I also assumed that the camera was pointing straight down and the shadows were parallel. It wasn't until Abishua pointed out that they aren't that we took a second look, so kudos to him.)






    Of little consequence but interesting to think about...


    If you do have to be on the ground to see a crepuscular ray, exactly at what point above the ground does a crepuscular ray become not a crepuscular ray?


    After some thought here's my answer:

    The anti-solar point is defined by the observer and is a point on the celestial sphere opposite to the sun from the observer's viewpoint. You don't have to be on the ground for that to be true, and the anti-solar point doesn't have to be in the sky from your viewpoint.

    Crepuscular and anti-crepuscular rays are created by the contrast between the brighter areas where the sunlight is being scattered (Rayleigh scattering) and the darker areas of the shadows. The fundamental issue in our discussion is that they are undistorted. In the case of anti-crepuscular rays, they aim directly at the anti-solar point on the celestial sphere as defined by our viewpoint.

    But if we are above the clouds and seeing shadows cast by the clouds on the earth's surface, the shadows will be distorted and will not point directly to the anti-solar point. It's not the same situation.

    This is the question. In this photo below, are we seeing shadows and rays in the atmosphere, or are we seeing the shadows of clouds cast on the earth's surface? If the former I say that these are still anti-crepuscular rays and they are aimed at the anti-solar point on the celestial sphere as defined by the observer's position, even though that position is in space. That's kind of a mind-bender but it would have to be true.

    If they are shadows cast by the clouds on the earth's surface then they are mostly aimed at the anti-solar point but are distorted and are not anti-crepuscular rays.


    Regardless, the observer's position determines whether they appear to converge or diverge. Whether they are aimed directly at the anti-solar point or not won't change that.
    [​IMG]
     
    Last edited: Nov 28, 2016
  6. Trailblazer

    Trailblazer Moderator Staff Member

    Yes, they are shadows in the atmosphere. But there is no distinction between crepuscular rays and anticrepuscular rays. They are just shadows.

    upload_2016-11-28_11-44-9.

    A person on the ground where the red dot is would see (what we call) crepuscular rays if they looked towards the sun (red arrow).

    upload_2016-11-28_11-46-47.

    If they turned around (purple arrow) they would see (what we call) anticrepuscular rays.




    We think of crepuscular rays "diverging" and anticrepuscular rays "converging" because the light is travelling from the sun, but in fact if you just think of them as lines then they are still meeting in the distance at the vanishing point in exactly the same way.
     
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  7. Z.W. Wolf

    Z.W. Wolf Senior Member

    Yes, this is all obvious.

    They are crepuscular rays as defined by the viewpoint of the camera on the ISS that took this specific photo.

    Yes, other observers in other places would see them differently, and each observer would also have a unique anti-solar point on the celestial sphere as defined by their viewpoint. The viewpoint of each observer defines these things for that observer. Nothing you're saying is in opposition to what I said.
     
  8. Mick West

    Mick West Administrator Staff Member

    But you kind of forcing a new usage of the term there, and it's not helping the clarity of the explanation, and will almost certainly confuse people. Saying they "are" anti-crepuscular or crepuscular rays implies that they will stay that way, and from the POV of the ISS that can change from second to second.

    They are just parallel rays and shadows.
     
  9. Leifer

    Leifer Senior Member

    A quick drawing I did (already explained by others).
    If you were in this photo, and turn your head to the right or left, the vertical yellow lines would look even more incorrect.
    rays_perspective.

    Below, is one video/test some people are using to claim that ALL the sun's rays should be seen as vertical.
    The problem with this test is that it only uses a distance of about 8 feet, when in reality, the sun's rays are seen at a distance of miles.
    This test is scaled incorrectly to actually notice the effects of perspective.