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When one looks at the sun and the moon in the sky together, why is it that the illuminated crescent of the moon does NOT "point" at the sun?

(More correctly the perpendicular bisector of the straight line joining the end points of the moon's terminator does not point at the sun. Intuitively one feels it should - as that is where the light is coming from. It is always around 20 - 30 degrees "off target".)

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I have never seen that effect - I need to check again in case my memory is flaky, but I always showed people that if they follow that perpendicular line they will hit the sun... –  Rory Alsop Jan 21 '12 at 20:28
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5 Answers

You're probably perceiving it incorrectly. The sky is not a flat surface; it looks like the inside of a sphere, and we tend to perceive it as a flattened sphere, with the zenith "closer" than the horizon. This perception is reinforced by the appearance of the daytime sky, in which overhead clouds really are much closer than clouds near the horizon. That could throw off what you think you're seeing. Tracing a straight line across the sky can be difficult, especially near the horizon.

The perpendicular bisector should, and I believe does, point directly at the Sun.

Try holding a rigid rod, like a yard or meter stick, out at arm's length, intersecting both the Sun and the Moon, and observe the angle at which the rod crosses the moon. You could also use a length of string stretched taut between your hands.

Atmospheric refraction can shift the perceived position of the Sun or Moon by about one width (half a degree); that's not enough to explain what you're seeing.

If the Moon is very close to the horizon, refraction might distort its visible shape enough to explain the perceived anomaly, but that would be noticeable enough that I don't think it's what you're referring to.

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Have checked back on various sources and I think as @Keith has said, the perpendicular bisector does always point directly at the sun, no matter what you are perceiving.

Have a look at this University of Nebraska page to see why this always has to be the case.

Minnaert's "The Nature of Light and Colour in the Open Air" also discusses these and explains why it is a perception problem. Use a straight edge or a taut piece of string to prove to yourself that it is actually true.

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Moon's illuminated side always faces Sun. The boundary between the illuminated half and the unilluminated half, the terminator, is always perpendicular to the line from Moon's center to Sun's center. The line joining the terminator's endpoints (the cusps) always bisects the lunar disk, the projection of the side facing Earth. By simple geometry, the perpendicular bisector of the line joining the cusps must point toward Sun. Some thought should confirm that this must be true even when Moon's orbit carries it above or below the ecliptic.

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I suggest you look at how strong this effect is as a function of the lunar phase. This "illusion" should disappear when the moon is a thin crescent. For the reason, I agree with what is written above.

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I don't see how the phase of the Moon would affect this, unless perhaps it's easier to judge the direction when it's a thin crescent. –  Keith Thompson Sep 12 '12 at 18:54
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I'm not 100% clear on the question, but I'm going to go ahead and take a stab at it, as I THINK I know what you mean.

There are certain phases of the moon that we see, and some of those happen to have the moon rise during the day. When the moon is in the sky during the day, the reason we see it is because of what is called "Earthshine." This occurs when the earth actually reflects light back up and hits the moon. The Earth reflects about 4 times as bright (i believe) as our moon does during the night. With such bright light hitting the moon during the day, we can see the moon as a "flat" object rather than the crescent shaped.

If I'm wrong on what your question was, maybe that will provide at least a little bit of insight and maybe help out.

So recap:

Sun hits Earth "Earthshine" occurs Earth reflects light back to the moon Moon is able to be seen as full because of the amount of shine reflecting back

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-1: What is this? This is irrelevant wrong obfuscation. –  Ron Maimon Jun 26 '12 at 21:41
    
Yes, -1 if I had the rep for just being flat out wrong. –  ThePopMachine Jun 27 '12 at 1:06
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protected by Qmechanic Jun 25 '13 at 8:51

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