<p>I'll assume you mean the <em>lit</em> side of the Moon (this wasn't clear from the original question, but I've edited it).</p>

<p>Suppose there's a crescent Moon visible in the sky during the day. Plot a straight line between the tips of the crescent, then plot a perpendicular straight line across its center across the sky. Intuitively, that line <em>should</em> pass through the Sun. You're saying it doesn't, and you're asking why.</p>

<p>In fact, that line does pass through the Sun, and what you're seeing is an optical illusion.</p>

<p>If you imagine the sky as a perfect hemisphere, with you at the center, the geometry will match up. But we we tend to imagine it as a flattened dome, probably because the horizon is <em>obviously</em> far away, but we have no reference points for the zenith so we imagine it being closer. This is (part of) the source of the "Moon illusion", where the Moon appears bigger when it's close to the horizon. In fact, it's very nearly the same angular size, but since we imagine it being farther away it must be "bigger".</p>

<p>The same mention distortion of the sky's geometry makes it look like the axis of the Moon's lit side doesn't point directly to the Sun.</p>

<p>Next time you see this, try holding up a straight rod intersecting both the Sun and the Moon. I think you'll find that the rod passes directly across the axis of the Moon's lit side.</p>

CC BY-SA 3.0
2016-01-17T20:05:51.883Z
2016-01-18T21:09:48.760Z
2016-01-18T21:09:48.760Z
Keith Thompson
<p>During different days when both the sun and the moon are seen I notice that the axis of the lit, illuminated, side of the moon does not pass through the sun there is some degree of diversion. Why?</p>

CC BY-SA 3.0
2016-01-17T19:22:43.987Z
What does "blit" mean? The Moon's orbit is inclined 5º wrt. the ecliptic.
pela
CC BY-SA 3.0
2016-01-17T20:06:11.683Z
@pela: I *think* I know what the OP is asking. See my answer.
Keith Thompson
CC BY-SA 3.0
2016-01-18T13:51:59.457Z
@KeithThompson: I guess you're right. Good answer!
pela
CC BY-SA 3.0
2016-01-18T21:09:30.270Z
I've edited the question, changing "blit" to "lit". nikolaos, if this isn't what you meant please edit the question again.
Keith Thompson
CC BY-SA 3.0
2016-01-19T19:25:18.970Z
You're drawing straight lines on a curved surface.
Howard Miller
<the-moon>
The relation of the lit side of the moon and the sun
nikolaos vougiouklis
# The relation of the lit side of the moon and the sun

(Asked by: username_0 on 2016-01-17)

During different days when both the sun and the moon are seen I notice that the axis of the lit, illuminated, side of the moon does not pass through the sun there is some degree of diversion. Why?

**Tags:** the-moon
> **Comments:**
> **username_1**: What does "blit" mean? The Moon's orbit is inclined 5º wrt. the ecliptic.  
> **username_2**: @pela: I *think* I know what the OP is asking. See my answer.  
> **username_1**: @KeithThompson: I guess you're right. Good answer!  
> **username_2**: I've edited the question, changing "blit" to "lit". nikolaos, if this isn't what you meant please edit the question again.  
> **username_3**: You're drawing straight lines on a curved surface.  

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## 1 answers

### Answer by username_2 on 2016-01-17. Score: 7

I'll assume you mean the *lit* side of the Moon (this wasn't clear from the original question, but I've edited it).

Suppose there's a crescent Moon visible in the sky during the day. Plot a straight line between the tips of the crescent, then plot a perpendicular straight line across its center across the sky. Intuitively, that line *should* pass through the Sun. You're saying it doesn't, and you're asking why.

In fact, that line does pass through the Sun, and what you're seeing is an optical illusion.

If you imagine the sky as a perfect hemisphere, with you at the center, the geometry will match up. But we we tend to imagine it as a flattened dome, probably because the horizon is *obviously* far away, but we have no reference points for the zenith so we imagine it being closer. This is (part of) the source of the "Moon illusion", where the Moon appears bigger when it's close to the horizon. In fact, it's very nearly the same angular size, but since we imagine it being farther away it must be "bigger".

The same mention distortion of the sky's geometry makes it look like the axis of the Moon's lit side doesn't point directly to the Sun.

Next time you see this, try holding up a straight rod intersecting both the Sun and the Moon. I think you'll find that the rod passes directly across the axis of the Moon's lit side.


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