Moon and Planets 
There’s a nice conjunction in the western sky the next couple of evenings — a meeting of the Moon and the planets Venus and Mercury. They’re quite low in the sky as the Sun sets, though, so the viewing window is short.
Mercury and Venus are the closest planets to the Sun, so they never roam far from the Sun as seen from Earth. At best, they’re visible for a little while after sunset or before sunrise.
Venus’s orbit is synched up with Earth’s in such a way that it follows a series of set patterns across the sky. It traces out five loops every eight years. Each cycle is the same, with Venus appearing at the same point in the sky every eight years.
There’s some speculation that Venus and Earth are in resonance — like guitar strings that vibrate together to produce a musical chord.
In the case of planets, the resonance is a result of their mutual gravitational attraction — they essentially synchronize to form a repeating pattern. Venus, for example, completes 13 orbits around the Sun for every eight orbits that Earth makes.
The Venus-Earth resonance isn’t quite perfect, though — it’s off by a day or so. That means the patterns could simply be a coincidence — one that makes it easy to predict Venus’s motions across the sky.
Look for Venus shining as the brilliant “evening star” to the right of the crescent Moon this evening. Much fainter Mercury stands above them, completing the pretty evening triangle. More about this lineup tomorrow.
Script by Damond Benningfield, Copyright 2013