Archive for October 7, 2019

10/07/2019 – Ephemeris – Tides on and by the Moon

October 7, 2019 Comments off

Ephemeris for Monday, October 7th. Today the Sun will be up for 11 hours and 24 minutes, setting at 7:12, and it will rise tomorrow at 7:49. The Moon, 2 days past first quarter, will set at 2:21 tomorrow morning.

We always see the same face of the Moon turned toward the Earth. This does not mean that the Moon doesn’t rotate. It means that the Moon rotates on its axis in exactly the same time it takes to orbit the Earth. That is no coincidence. The effect of the Earth gravitation across the diameter of the Moon have essentially locked the Moon’s rotation to its revolution period. The crust of the far side of the Moon is thicker than the Earth facing side. The Moon is trying to do the same thing to the Earth. Its pull on the side of the Earth facing it is greater than the pull on the Earth’s opposite side. This stresses the Earth and raises tides in the ocean which actually slow the Earth’s rotation a tiny bit. As a consequence it pushes the Moon away by about 3.8 centimeters a year.

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


Asymmetry of the crust of the Moon. Credit Lunar and Planetary Institute and Center for Lunar Science and Exploration.

For more information on this illustration: 4th illustration.


Categories: Concepts, Ephemeris Program Tags: , ,