Are the Planets Ever Crescent?

When we see Jupiter, Venus, Mars etc are they ever crescent like the moon is? If so, how long are the cycles? - Question posed by @KnikiTea.

The Moon's phases as seen from Earth

Do We Ever See the Planets as Crescents?

We've all seen the Moon going through it's phases: these are the times when different portions of the Moon's face as observed by us are lit up by the Sun.

You can read this post to find out all about the phases of the Moon, and these all apply to any bodies in space, so it's relevant reading for this question.

The important thing here, though, is that the Moon is crescent (that's the toenail shape, or when less than half of the face that we're seeing is lit up) when it's in the part of its orbit around us that brings it closer to the Sun than we are, and gibbous (that's when the toenail is the dark bit) when it's further away from the Sun.

This has an important effect on the answer to KinkiTea's question- only planets that are closer to the Sun than we are will ever be seen as a crescent. This means that Mercury and Venus can both be seen as crescents (indeed, when I set my telescope up at #CampEd12 we saw a great view of Venus's toenail clipping), but all of the other planets cannot be seen from Earth as crescent: Mars and the others wobble between being mostly full and full. If you ever see a photograph of Jupiter, Saturn or the rest as a crescent, it has been taken from a space vehicle such as Cassini that has traveled past its orbit.

How Long are the Cycles?

The Moon's phase cycle lasts about a month because it takes about a month to orbit the Earth once. However, things are a little trickier for Venus and Mercury because, rather than orbiting Earth, we're all orbiting the Sun.

Venus takes 224.7 Earth days to orbit the Sun once, and if we were stationary in our orbit, that's how long it would take Venus to cycle through its phases once. The fact that we're moving too means that the cycle takes a little longer: imagine Earth and Venus are in the starting blocks next to each other in a race around the Sun. Venus gets back to the starting line before Earth does, and keeps going. Eventually Venus will lap us, and it's at this point that the cycles start again. The time between successive 'lapping points'* is 584 days.

Mercury whizzes around the Sun in 88 days, but for the same reasons discussed above, it overtakes the Earth every 116 days, which is how long its phase cycle lasts.

* I made that phrase up, but I think it works.


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