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The Sun and the Moon (Gravity):

The Sun plays a big role in how the Moon orbits and how we see it.  One way is through the Sun's gravitational attraction. While the Sun is much farther than the Earth, it is MUCH bigger.

This affects the Moon's orbit in 2 ways.

1)  The Sun tugs on the Moon's elliptical orbit:  The Sun is always trying to pull the Moon closer, and it does this most effectively at Apogee. This has the effect of spinning the entire lunar orbit around once every 8.85 years!

2)    The Sun tugs on the Inclined Lunar Orbit:  The Sun's gravity also tugs on the inclination of the Moon's orbit.  This causes the line of nodes  to rotate along the plane of the orbit.  It's very similar to precession of the equinoxes on Earth, but in this case it goes in the opposite direction.


Since this motion is opposite (or retrograde) to the direction in which the Moon moves, we call this the Regression of the Nodes.  The nodes move around the entire Lunar orbit every 18.6 years.

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