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Can anyone explain why on the 14 Oct 2022 at 7.00 UTC at location (Lat 52.339 Long 0.419)does the waning gibbous moon (Alt 39º) appear to be illuminated from a different direction, i.e. slightly upwards, than from the sun which is only at Alt 5º? The azimuth difference is 149.83º.

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    $\begingroup$ Related, possible duplicates: astronomy.stackexchange.com/q/19985/16685 astronomy.stackexchange.com/q/48651/16685 astronomy.stackexchange.com/q/48998/16685 $\endgroup$
    – PM 2Ring
    Aug 26, 2022 at 11:26
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    $\begingroup$ What software are you viewing this future moon in? $\endgroup$
    – notovny
    Aug 26, 2022 at 12:43
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    $\begingroup$ It's called the "Parallactic Angle" (has nothing to do with "parallax"). Here's some details: researchgate.net/publication/… $\endgroup$ Aug 26, 2022 at 12:56
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    $\begingroup$ If you are using some software to view this future event, such as a planetarium program, check if the program can display the ecliptic. The Sun is on the ecliptic, and the Moon will be within 5 or 6 degrees of the ecliptic. The lit limb (I hesitate to use the term "crescent" since the gibbous moon is not a crescent) is pointed parallel to the ecliptic and towards the Sun. Essentially, the ecliptic is the direction that light travels from the Sun to the Moon. $\endgroup$
    – JohnHoltz
    Aug 27, 2022 at 3:18
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    $\begingroup$ I used aa.usno.navy.mil/calculated/… and I also used mooncalc.org/#/52.3394,0.42,14/2022.10.14/06:41/1/3 Isn't the ecliptic a curve? Light travels in a straight line. This is never a problem with anything else that is illuminated by the sun. If it were a football on the ground there would be a straight line from the football to the sun. $\endgroup$
    – Pies
    Aug 27, 2022 at 12:13

2 Answers 2

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The lit part of the moon points directly towards the sun along a great circle.

Great circles are "straight lines" on a sphere.

You may be aware that the most direct line between two points on a sphere is not (usually) parallel to the equator, but is a great circle. This means the when flying (for example) from London to Tokyo, the direct route takes a plane over Northern Siberia, and the initial direction of the flight is North-east (even though Tokyo is well to the south of London)

The same effect is occurring here, but on the sphere of the sky. The direct line from the moon to the sun points up in the sky and this is the side of the moon that is lit by the sun. It is an optical illusion that the direct line should have constantly decreasing altitude.

Since both the sun and moon are on (or near) the ecliptic, which is a great circle in the sky, the lit part of the moon should point in the direction of the ecliptic. The two images below confirm this.

enter image description here

This image shows the moon and the sun on the ecliptic. While it appears curved in this projection, it is in fact a straight line in the sky.

enter image description here

This shows the lit part of the moon points in the direction of the ecliptic, and so straight towards the sun.

Images from Stellarium

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There are no actual celestial spheres or curved ecliptic lines - they are just there for the convenience of plotting and predicting positions. The movement of the moon has nothing to do with the way it is illuminated. Only its position in the sky with respect to the sun affects this. You may observe the moon moving in a great circle but the light from the sun doesn't move in a circle - it travels in a straight line towards the moon. If you place a ball on top of a poll coincident with the moon and a bright light on top of a pole coincident with the sun then observation will show a straight line between the light and the ball. Its the same when the sun shines on any object, trees, houses, balls etc and the moon is no different. Is just a matter of scale. The image should be self explanatory.enter image description here

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    $\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Aug 31, 2022 at 2:04
  • $\begingroup$ The answer is confusing and the image is misleading. If you did it with real balls, you would see that the ball is lighted exactly in the same way as the moon. Just make sure to stand a bit far from the ball, say 10 meters away from the ball, to emulate the fact that the moon is actually quite far $\endgroup$
    – Prallax
    Sep 1, 2022 at 19:53
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    $\begingroup$ I agree that the balls have been added but as an artist if I were painting that scene I would paint the balls in exactly the same way as the photo image. As I said previously nothing is different, its just a matter of scale. Can you explain how parallax comes into it? $\endgroup$
    – Pies
    Sep 6, 2022 at 8:07

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