Home > Ephemeris Program, Observing, The Moon > 11/12/2021 – Ephemeris – The Moon appears to wobble slowly over the month

11/12/2021 – Ephemeris – The Moon appears to wobble slowly over the month

November 12, 2021

This is Ephemeris for Friday, November 12th. Today the Sun will be up for 9 hours and 41 minutes, setting at 5:17, and it will rise tomorrow at 7:37. The Moon, 1 day past first quarter, will set at 1:27 tomorrow morning.

When I look at the Moon, I check the position of one particular feature. A small, nearly circular gray area near the right or western edge of the Moon’s disk. It’s called the Sea of Crises, or in Latin, Mare Crisium. The reason I check it is that over the month its distance from the edge, or what astronomers call the limb, changes. That sea is about as far as it can get from the limb now. The reason for this wobbling back and forth, called libration, is that the Moon rotates at a constant rate due to angular momentum, but its revolution around the earth is an ellipse, so it doesn’t move with a uniform speed in its orbit. It moves faster at its closest to the Earth, called perigee, and is slowest at apogee, its farthest from the Earth.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EST, UT – 5 hours). They may be different for your location.


Moon tonight libration

The Moon tonight (November 12, 2021) showing Mare Crisium (Sea of Crises) about as far from the limb (Moon’s edge) as it can get. Created using Stellarium.

Moon's libration animation

Simulation of the Moon’s phase and libration for October 2007 by Tomruen. Image is in the Public Domain.

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