Lunar Motions Rotation, Orbit, Eclipse “Seasons” And the Annular Eclipse

Synchronicity The Period of the Moon’s revolution and its rotation are the same. We say that the Moon has synchronous revolution & rotation As a result one side of the Moon is always facing the Earth (the near-side) and one away (the far–side) even though the Moon does rotate.

Orbit Size and Shape The Moon orbits at an average of 380,000 km from earth. Its orbit is elliptical. The nearest point is called Perigee. The Farthest point is called Apogee. The apparent size of the Moon appears to vary from its average size by about 15% The Moon’s orbit is tilted 5° from Earth’s Orbit This 5° tilt is why we don’t have eclipses each month. Because of it the Moon is usually not on the ecliptic, and can be as much as 5° from it

The Tilt of the Moon’s Orbit and Eclipse Seasons Due to the 5° tilt of the moon’s orbit… the Moon-Earth-Sun do not line up the right-way to have Eclipses every month. They only line-up right twice a year. (Syzygy) So each year there are two periods, each two weeks long, when the eclipses occur These are called Eclipse Seasons, they are Six Months apart During 2014 they are in April and October.

The Shadow of a Planet or a Moon The shadow of a planet or moon has two parts The Umbra and the Penumbra The Umbra, or true shadow, where no light from the Sun reaches If an object is in the umbra It will look dark From it the Sun will be totally blocked (eclipsed)

The Penumbra, or partial shadow, some but not all the light reaches it. The object appears dimly lit From the object the Sun appears If to the side …With a piece ‘bitten out’ (A partial eclipse) If behind…The center of the Sun is blocked (An annular eclipse)

Solar Eclipse Is when the Moon casts its shadow on the Earth. Because the Moon is smaller than the Earth only a part of the Earth experiences an eclipse. A Total Eclipse is visible only to a narrow line of places on Earth where the umbra falls In a Total Eclipse the Sun is completely blocked by the Moon and the Corona and Chromosphere becomes visible. Places where the penumbra falls see a Partial Eclipse

An Annular Eclipse Sometimes when a Solar Eclipse should happen the Moon is at its Farthest point. The Umbra is not long enough to reach the Earth so… The Moon doesn’t cover all the Sun, only the center leaving a bright ring around it. This is an Annular Eclipse

When Can Eclipses Happen? It is when the moon is on the ecliptic that an eclipse can happen A Solar Eclipse can only happen during a New Moon A Lunar Eclipse can only happen during a Full Moon Lunar and Solar Eclipses therefore happen about two weeks apart.