Bob Moler's Ephemeris Blog

Ephemeris for Tuesday, December 11th. The Sun will rise at 8:10. It’ll be up for 8 hours and 52 minutes, setting at 5:02. The Moon, half way from new to first quarter, will set at 9:06 this evening.

Last Friday the Chinese launched their Chang’e 4 spacecraft to land on the far side of the Moon in January. That side of the Moon has been mistakenly called the dark side. It actually gets more sunlight than the near, Earth facing, side. This will be an extraordinary feat. So how can they tell what’s happening on the other side? In preparation for this landing attempt the Chinese put a communications relay satellite in a halo orbit about the Earth-Moon L2 Lagrange point 38 thousand miles (61 thousand km) beyond the Moon. It is a kind of a gravitational point of equilibrium behind the Moon. One of five, and they are very useful. The satellite will lazily orbit that point, in view of the lander and the Earth for continuous communication between the Earth and the lander.

The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Chinese mission to land a lander and rover on the far side of the Moon. Click on the image to enlarge.  It’s in Chinese except the numbers. Credit China Aerospace Science and Technology Corporation via Space.com.

How Queuiao, the lunar communications satellite orbiting L2 performs its relay function. Click on image to enlarge. Credit CNSA via gbtimes.com.

Note:  Next Monday’s program will be about Lagrangian points.