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Thread: A question in relation to the positions of the Sun and Moon in the sky.

  1. #1 A question in relation to the positions of the Sun and Moon in the sky. 
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    The moon tonight at 15.40 is at half moon. We know that the bright side is the side which is illuminated by the Sun. If you look at the present half moon tonight however, the angle of the light horizon on the surface of the moon, is such that it locates the position of the Sun at an angle in the sky which appears to be about 30/40 degrees higher than the Sun's actual position in the other side of the sky at that time. Similarly, the moon appears to be in a higher position in the sky than we would expect it to be as determined by the position of the Sun.

    Could anyone explain this optical illusion? There is probably a reason to explain this phenemena, but as I'm neither an astronomer nor cosmologist, I cant figure it out


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    Brassica oleracea Strange's Avatar
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    There is a thread about this on an astronomy forum which has stretched over 7 years and more than 600 posts. I haven't followed it, but I am pretty sure the answer to the illusion (and it seems it is an illusion) is in there somewhere!
    Why does the moon's terminator not appear orthogonal to the direction of the sun?


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    ei incumbit probatio qui dicit, non qui negat
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    Take a right triangle. You might expect that the measure of the two acute angles is quite different from a right angle, generally speaking. Only in case one leg is much greater than the other, then you might expect that also the opposite angle approaches 90.
    Now, take the moon, the earth and the sun at the vertexes of a right triangle, with the moon in the right vertex, so that it is half moon. At first sight, you might expect that the angular distance between the moon and the sun as seen from the earth is quite smaller than 90. That happens if you forget that one leg of the triangle is much, much longer than the other. But the sun is 400 times more distant than the moon.
    Thus, at first sight you might expect to find the sun at approximately the same height than the half moon in the sky, with a sun-to-moon angular distance sensibly smaller than 90. But if you now remind that the moon-sun leg is much, much longer than the moon-earth leg, then you can realize that the opposite angle (i.e. the moon-earth-sun angle, that is the angular distance between the sun and the moon) can well approach 90 (its actual value with the moon at quadrature is 89 51'). So, no wonder if you see the sun low at the horizon when the half moon is high in the sky in front of you, since the former is much, much farther than the latter.
    The point is that we are mistakenly led to believe that the sun is approximately at the same distance than the moon, since their angular dimensions are similar.
    Last edited by Alpha; February 27th, 2014 at 07:31 AM.
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    Quote Originally Posted by imetheman View Post
    The moon tonight at 15.40 is at half moon. We know that the bright side is the side which is illuminated by the Sun. If you look at the present half moon tonight however, the angle of the light horizon on the surface of the moon, is such that it locates the position of the Sun at an angle in the sky which appears to be about 30/40 degrees higher than the Sun's actual position in the other side of the sky at that time. Similarly, the moon appears to be in a higher position in the sky than we would expect it to be as determined by the position of the Sun.

    Could anyone explain this optical illusion? There is probably a reason to explain this phenomena, but as I'm neither an astronomer nor cosmologist, I cant figure it out
    Light only takes a second to get to us from the Moon. So what you see on the Moon is up to date, and if you take the area of the Moon lit up by the Sun it is pointing directly at the Sun. Just force yourself to believe it and from there work out where the Sun is in relation to you. I like doing this when the Sun is out of sight.
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    Quote Originally Posted by Robittybob1 View Post
    Light only takes a second to get to us from the Moon. So what you see on the Moon is up to date, and if you take the area of the Moon lit up by the Sun it is pointing directly at the Sun. Just force yourself to believe it and from there work out where the Sun is in relation to you. I like doing this when the Sun is out of sight.
    One-second up to date? The sun displacement is imperceptible in one second.
    Mind that the light from the sun takes 8 minutes to travel to the moon, but it takes 8 minutes to travel to the earth too. Thus, the image of the sun we see from the earth and the area of the moon lit up by the sun are already synchronized (except for one irrelevant second).
    Furthermore, if the explanation was as you suggested, we should find the sun at sunrise higher than expected on the basis of the lit part of the moon, not lower as it actually appears to us.
    Last edited by Alpha; February 27th, 2014 at 02:28 PM.
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    Quote Originally Posted by Alpha View Post
    Quote Originally Posted by Robittybob1 View Post
    Light only takes a second to get to us from the Moon. So what you see on the Moon is up to date, and if you take the area of the Moon lit up by the Sun it is pointing directly at the Sun. Just force yourself to believe it and from there work out where the Sun is in relation to you. I like doing this when the Sun is out of sight.
    One-second up to date? The sun displacement is imperceptible in one second.
    Mind that the light from the sun takes 8 minutes to travel to the moon, but it takes 8 minutes to travel to the earth too. Thus, the image of the sun we see from the earth and the area of the moon lit up by the sun are already synchronized (except for one irrelevant second).
    Furthermore, if the explanation was as you suggested, we should find the sun at sunrise higher than expected on the basis of the lit part of the moon, not lower as it actually appears to us.
    You might be right, there is an error of 2 degrees (approx)
    There are 1440 minutes in a day and if the light from the Sun takes 8 minutes to get here the Earth has rotated 2 degrees in that time,
    so the Sun would have moved out of sight at sunset in that time, except we too are seeing "old" light from the Sun.
    The Sun apparently subtends 0.54 of a degree to our eyes. So I think it is safe to say if the observer on the moon can see the Sun we will also be able to see that part of the Moon in sunlight, but if it was based on real time position the Sun would have moved by 2 degrees.

    It is a bit confusing in fact. for we are seeing part of it with a 1 second delay and the other part with an 8 minute delay.
    But when in my previous post when I'd work out the position of Sun it would be a rough estimate of where the Sun was not worrying about a two degree discrepancy.
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    Optical illusions are tricky. We tend to see things as related to a curved surface in the sky. Almost like we were seeing them on the inside surface of a sphere.

    If you can see the big dipper and the north star (Polaris) you might be able to tell if your brain is doing it to you.
    People can have a hard time finding the Polaris even after they are told to line up the pointer stars in the dipper and extend a straight line from them to Polaris. They see Polaris as not being on what they think should be a straight line from the pointers.
    Sometimes people even need a ruler or other straight edge to get their line from the pointers to Polaris with.
    So if you can see the Big Dipper and Polaris and it does not look like Polaris and the pointers are in a straight line your brain is likely presenting you with an optical illusion by retranslating the sky as the inside surface a sphere.
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