# Moon phases and how they look **URL:** \ **Category:** Factual Questions\ **Created:** [August 4, 2015, 12:19pm UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961 "2015-08-04T12:19:45Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 4, 2015, 8:14pm UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961/21 "2015-08-04T20:14:22Z")
That guy who claimed the earth was concave (and we live inside) cited this phenomenon as part of his evidence. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 4, 2015, 9:01pm UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961/22 "2015-08-04T21:01:03Z")
The sky is curved: It’s spherical. And the perpendicular to the Moon’s terminator, if extended in a “straight line” (or rather, a geodesic, the closest thing there is to a straight line) on that spherical surface, does indeed meet the Sun. It just doesn’t look like a straight line to us, because we mistakenly think of the sky as being less curved than it actually is. It’s sort of like how airplane paths look curved on most flat maps, but they really are the shortest path on a globe. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [August 4, 2015, 9:24pm UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961/23 "2015-08-04T21:24:47Z")
This is a video about the moon terminator illusion. This is the illusion that when both the sun and the moon are visible it does not look like the bright side is facing the sun. I don’t know if it the the one Saint Cad saw. [![](https://img.youtube.com/vi/Y2gTSjoEExc/maxresdefault.jpg "The Moon Terminator Illusion") ](https://www.youtube.com/watch?v=Y2gTSjoEExc) ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [August 4, 2015, 9:37pm UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961/24 "2015-08-04T21:37:40Z")
To most people the sky does not seem to be a hemisphere, but more of a flattened bowl. In other words, the zenith seems to be closer than the horizon. This is perhaps caused, or at least exacerbated, by the fact that clouds usually really _are_ closer when they’re at the zenith than when they’re near the horizon. This is also the only convincing explanation for the [Moon Illusion](https://en.wikipedia.org/wiki/Moon_illusion) that I’ve heard. And yes I know that The Master [addressed this](http://www.straightdope.com/columns/read/831/why-does-the-moon-appear-bigger-near-the-horizon) and gives some credence to the relative size theory, but that doesn’t explain why the illusion persists when the moon is over the ocean with no terrestrial objects in view. –Mark ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [August 4, 2015, 10:19pm UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961/25 "2015-08-04T22:19:34Z")
[quote=“gazpacho, post:23, topic:726961”] This is a video about the moon terminator illusion. This is the illusion that when both the sun and the moon are visible it does not look like the bright side is facing the sun. I don’t know if it the the one Saint Cad saw. [/QUOTE] Yep. I was sure it was vsauce but still couldn’t find it. Thanks. ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [August 5, 2015, 1:17am UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961/26 "2015-08-05T01:17:15Z")
So does the squirrel go “around” the tree? (Oh, never mind… And, yes, I’ve seen the Cheshire Cat moon, here in southern California, including a lovely one over Disneyland once.) ---
**Author:** ![Bones\_Daley](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@Bones\_Daley](https://boards.straightdope.com/u/Bones_Daley)\ **Post date:** [August 5, 2015, 10:14am UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961/27 "2015-08-05T10:14:00Z")
> [@LSLGuy](#): > > The reason it makes no sense to you is that the dark part of the Moon is NOT caused by a shadow of the Earth. It’s caused by the angle you’re looking at the Moon versus the illumination coming from the Sun. > > At any moment exactly one half = one hemisphere of the Moon is totally illuminated by the Sun. And the other half is totally in shadow. Because the Moon has no atmosphere, there is no twilight. On any given spot on the Moon at any given moment it’s either 100% day or 100% night. > > So you can simulate the Moon by taking any spherical ball and painting one half one color and the other half another color. Say we use black & white. > > Once you’ve done that, you can look at your model from various angles. What you’ll see is that IF you look at it directly towards one colored side or the other you’ll see all white or all black. And if you look at it angled 90 degrees to those choices you’ll two hemi-circles, one of each color separated by a straight line. > > At any other angle you hold the model, you’ll see the model is partly one color and partly the other, and the dividing line will appear to be a curve. Because it really is a curve in 3-space. It’s just that in the two special cases in the previous paragraph, the curved line in 3-space becomes a straight line in the plane of your line of sight. Excellent explanation, thanks. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 5, 2015, 9:27pm UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961/28 "2015-08-05T21:27:15Z")
It should be noted, by the way, that sometimes the Earth’s shadow _does_ fall on the Moon and darken part of it. But this is a lunar eclipse, and happens only about once a year, not the standard monthly phases. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [August 5, 2015, 10:05pm UTC](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961/29 "2015-08-05T22:05:05Z")
And the Earth’s shadow during a lunar eclipse _is_ always convex. That’s one of the things that Aristotle used in his argument that the Earth was a sphere, since otherwise, he reasoned, the Earth’s shadow would be different shapes during different eclipses. –Mark [Previous page](https://boards.straightdope.com/t/moon-phases-and-how-they-look/726961.md?page=1)