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

2. Synchronicity
The Moon and Earth are affected by each other’s gravity, just like any two objects in the universe.
Earth’s gravity has caused the Moon’s orbit to “lock” into Earth’s revolutionary movement.

3. 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.
The period of the Moon’s revolution and its rotation are the same.

4. We say that the Moon has synchronous rotation.

6. Orbit Size and Shape
The Moon orbits at an average of 380,000 km from earth. It has an elliptical orbit.

7. What’s wrong with this picture?!??

8. The Moon is closest to Earth at its Perigee.
It is farthest from the Earth at its Apogee.

9. The apparent size of the Moon appears to vary from its average size by about 15%

10. The Lunar Tilt
The Earth and Moon do not orbit on the same orbital plane.
The Moon’s orbit is tilted 5° from Earth’s orbit.

11. This 5° tilt is why we don’t have eclipses each month.
Because of this orbital tilt the Moon is only occasionally on the ecliptic, and can be as much as 5° from it.

12. The Tilt of the Moon’s Orbit, Eclipse Seasons and Syzygy!
Due to this 5° tilt, the Moon-Earth-Sun system does not line up to have eclipses every month.
Rather, they only directly line up twice a year. This is known as syzygy.

13. Each year there are two periods, each two weeks long, when eclipses can occur. These are called Eclipse Seasons, and they are about Six Months apart.
During 2018 they are in January and July.

14. The Shadow of a Planet or a Moon
The shadow of a planet or moon has three parts:
The Umbra, Penumbra and Antumbra.

15. The Umbra, or true shadow, is where no light from the Sun reaches.
If an object is in the umbra, it will appear blacked out.
The Sun will also be completely blocked from its surface. (i.e. eclipsed.)

16. The Penumbra, is a partial shadow, where some - but not all - light reaches.
An object in the penumbra appears dimly lit.

17. The Sun will appears with a piece ‘bitten out.’
(A partial eclipse)

18. The Antumbra is another partial shadow, but it lies behind the Umbra.
An object in the antumbra appears to be dimly lit.
From the object the center of the Sun is blocked. (i.e. an annular eclipse.)

20. The Solar Eclipse

21. A solar eclipse occurs when the Moon casts its shadow on the Earth.
Because the Moon is so much smaller than the Earth, only a small part of the Earth will experience the eclipse.

23. The Annular Eclipse
Sometimes when a Solar Eclipse occurs, the Moon is at its apogee, or greatest distance.
The umbra is not long enough to reach the Earth so…
The Moon doesn’t cover all the Sun, just its center, leaving a bright ring around its shadow on the Sun.
This is an Annular Eclipse.

26. When Can Eclipses Occur?
Twice a year when there is Syzygy
when the moon is on the ecliptic
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.