# Eclipses/Phases of the Moon.

**URL:** <https://boards.straightdope.com/t/eclipses-phases-of-the-moon/31334>\
**Category:** Factual Questions\
**Created:** [August 30, 2000, 2:31pm UTC](https://boards.straightdope.com/t/eclipses-phases-of-the-moon/31334 "2000-08-30T14:31:20Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\
**Post date:** [September 1, 2000, 12:32pm UTC](https://boards.straightdope.com/t/eclipses-phases-of-the-moon/31334/21 "2000-09-01T12:32:48Z")

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> [@](#):
>
> \*Originally posted by VarlosZ \*  
> \*\*Now, I’m going to hijack my own thread. The moon’s “day” (rotational period, whatever) is one lunar year long; that’s why we only see the one side of the moon from earth. This happens to all satelites eventually, the tighter the satelite’s revolution, the faster it happens (Mercury is well on its way, e.g.). This phenomon has a name, but I forget it. Anyone know what causes this? \*\*

…as long as you define “lunar year” to mean one orbit around the Earth (i.e., not one orbit of the Earth/moon system around the sun)

The phenomenon is called synchronous rotation…  
[http://www.seds.org/billa/tnp/help.html#syncrot](http://www.seds.org/billa/tnp/help.html#syncrot)  
Good moon link…  
[http://www.seds.org/billa/tnp/luna.html](http://www.seds.org/billa/tnp/luna.html)

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**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\
**Post date:** [September 1, 2000, 12:36pm UTC](https://boards.straightdope.com/t/eclipses-phases-of-the-moon/31334/22 "2000-09-01T12:36:51Z")

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> [@](#):
>
> \*Originally posted by CurtC \*  
> \*\*Lib, there is a very small crescent around the edge of a new moon, and a very thin shadow around the edge of a full moon. But you can’t see either one. With a new moon, the moon is so near the sun that you can’t see the thin crescent - it’s not bright enough to show through the atmosphere that’s being lit up by the sun.  
> \*\*

Being an astronomy geek, I often try to see how thin a cresent I can find (i.e., how close to “new” I can see). But between working late, sleeping late, and living in an area without a clear horizon, I’ve only managed something like 1 day, 18 hr past new. Not too good (but not awful). In Astronomy magazine, someone claimed to see a 14 hour old crescent. You’d need a clear horizon and perhaps a pair of binoculars for that!

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**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:** [September 1, 2000, 4:53pm UTC](https://boards.straightdope.com/t/eclipses-phases-of-the-moon/31334/23 "2000-09-01T16:53:50Z")

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Mercury probably won’t become tidally locked in the same sense as the Moon, actuall, because it’s already in what’s called a “harmonic lock”. Its rotational period is exactly two-thirds the length of its orbital period, so that every time it reaches perihelion (closest point to the Sun), it’s pointed with its long axis (it’s slightly elongated) either towards or away from the Sun (which is equivalent, either way). This is a stable situation, so if it ever slowed down or sped up a little bit, it’d tend to return to that state.

Venus, on the other hand, is on its way to becomming tidally locked. Its rotational period is very close to its orbital period, but not exactly the same yet.

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