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I've got an argument with a mate.

He thinks that the moon shadow angle can change from night to night.

For example, one night, the near side of the moon is half black/white, black side on top, white on the bottom, and he believes that it can change, and another night, be, for example, black section on the left, white on the right, all observable from the same point on the planet.

Is it right?

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Yes. The moon is lit by the sun, so as its position relative to the sun changes, so does the angle from which it is lit.

I recommend reading the Wikipedia page on lunar phase for a good overview, some data, and some good references. This diagram from that page more or less sums it up — the bottom row shows how the moon looks from the earth over the course of a lunar month:

Diagram explaining phases of the moon. CC-BY-SA by Orion 8

Image is CC-BY-SA by Wikipedia user Orion 8.

The angle of the shadow depends on the time of the observation, and many lunar calendars seem to rotate the lunar images (as above). But this one gives a hint to the shifting relative sun position:

The lunar month

Image is of unknown origin, found here. Unclear rights; fair use.

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    $\begingroup$ Just a note that the direction that the phases change through the cycle is opposite in the southern hemisphere from this northern hemisphere frame of reference. While in the northern hemisphere the waxing crescent looks like ")" and the waning crescent like "(", in the southern hemisphere the waxing crescent looks like "(" and the waning crescent like ")". It took me a long time to be able to visualize why that is. $\endgroup$ – haresfur Aug 8 '18 at 2:02
  • $\begingroup$ Me too @haresfur. It's hard to realize that by turning around, you've managed to flip the axes of up and down. For those still struggling, this article and comparing locatiosn on mooncalc.org are hopefully pretty useful in visualizing what's going on. $\endgroup$ – JeopardyTempest Aug 8 '18 at 6:56
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The apparent angle of the moon is a function of the 28-day lunar orbit, the time of day/night one is looking at the moon, and the latitude from which it is viewed. So, for example, the current (14 May 16) view of the moon is that it is on its side, 'boat shaped' on the equator, but will appear progressively more sideways the further towards the poles that it is viewed.
There is a change in the shadow angle, but not obvious on a day to day basis. This is caused by the fact that the earth's spin axis is not perpendicular to the Earth-Moon orbital plane, so that is an annual wobble.

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  • $\begingroup$ It makes sense that the angle changes, but if you say, for example, that you stay at the equator at the same point, the ratio changes with the phases, but what about the angle? $\endgroup$ – Ant May 14 '16 at 14:49
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    $\begingroup$ Even at the equator the tilt angle of the Earth changes the apparent visual axis of the moon according to Earth's orbit around the sun. The moons visual axis would only remain stable if the Earths spin axis was at right angles to the plane of our orbit around the sun. Over the year its like viewing the moon from a wobbly platform,. $\endgroup$ – Gordon Stanger May 22 '16 at 6:04

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