Cycles of the Moon

1. Cycles of the Moon

2. Essential Questions • Why does the Moon go through phases? • What are the types and causes of a lunar eclipse? • What are the types and causes of a solar eclipse?

3. How does the Moon move? First, two definitions: • Rotation-spin about an internal axis. (ex. Earth’s daily rotation) • Revolution-spin about an external axis. (Earth’s yearly revolution)

4. The motion of the Moon If you watch the Moon night after night you notice three things: • It moves eastward against the constellations. • The markings on its face don’t change. • Different parts of it are illuminated each night. (Phases)

5. Revolution and Rotation • The first motion, eastward against the background stars, is explained by the Moon’s orbit around the Earth. • We call that a… • Revolution! Good job! • What about the surface features not changing? Does the Moon rotate?

6. Modeling the Moon • A volunteer with a textbook to model the Moon please… • A volunteer to play the Earth please… • “Moon guy” orbit the Earth keeping one side of the book facing “Earth guy” all the time. • Did “Moon guy” rotate? Try it again and don’t rotate…

7. Earth’s Moon • OK, so the Moon revolves around the Earth and rotates on its own axis at the same rate. • What’s wrong with the diagram on the left?

8. To the Moon… • Answer: The scale! • The Moon is 30 Earth diameters away. • If the Earth was a basketball, the Moon would be a baseball, 30 basketballs away! • The average distance to the Moon is 384,000 km. (Seeds, page 33)

9. And finally, why does the Moon have Phases?

11. Those odd phase names: • New and Full I think you get. • 1st and 3rd Quarter are named because they occur one quarter and three quarters of the way through the cycle. (Also we only see a quarter of the Moon’s surface at those times.) • Wax—to gradually increase. • Wane—to gradually decrease. • Gibbous—Latin for humpbacked.

12. Lunar cycles… • When and where would you see each phase of the Moon? • How much time for 1 lunar orbit? • The sidereal period of the Moon is about 27 days (27.32 to be exact). • Sidereal period is measured relative to background stars and represents the actual time to orbit Earth once.

13. Lunar Cycles cont… • The synodic period of the Moon is the time to complete a cycle of phases. • This takes about 29.5 days—just over 4 weeks, a “moonth.” • The word synodic comes from the Greek words for together and path • Why does the phase cycle take longer than one lunar orbit?

14. Sidereal vs. Synodic • As the Moon orbits the Earth, the Earth orbits the Sun. • The Moon must then revolve more than 360° to reach the same alignment for New Moon. • Extra 29° takes an extra 2 days.

15. Eclipses • We will discuss various types of eclipses now. • What does the word eclipse mean? • The word eclipse means… to obscure or hide. • For example: “Politicians’ failures often eclipse their accomplishments.”

16. Lunar Eclipses • A lunar eclipse occurs at Full Moon when the Moon moves through the shadow of Earth. • Earth’s shadow consists of two parts:

17. The Umbra and Penumbra • The umbra is the region of total shadow. • The penumbra is the region of partial shadow. • Seeds, page 39

18. Lunar Eclipses • During a total lunar eclipse, the Moon passes completely into Earth’s umbral shadow. • It often appears to have a reddish cast because of sunlight refracting around the Earth.

19. Lunar Eclipses cont. • A partial lunar eclipse occurs when only part of the Moon enters the umbral shadow. • A penumbral lunar eclipse occurs when the Moon enters only the penumbral shadow of the Earth. They aren’t very impressive and are often not even noticed by the public.

20. Solar Eclipses

21. We are lucky… • Earthlings can see a phenomenon that is not visible on most planets. • Our Moon is large enough and close enough to completely eclipse the Sun. • The Sun may be 400 times larger than the Moon, but the Moon is about 400 times closer!

23. Total Solar Eclipses • During a total solar eclipse, the Moon completely covers the visible surface of the Sun called the photosphere. • Once the photosphere is covered, the outer atmosphere of the Sun, called the corona, is visible. • Sometimes the active lower atmosphere, the chromosphere, is visible as well.

24. Solar Eclipses cont… • A partial solar eclipse occurs when the Moon does not completely cover the Sun. • Both total and partial solar eclipses take place during the New Moon phase. • There is a third type of solar eclipse as well…

25. Annular Eclipse • An annular eclipse is a solar eclipse in which a ring (or annulus) of the photosphere is visible around the disk of the Moon. • This occurs when the Moon is slightly farther from the Earth than average because of its elliptical orbit.

26. Apogee and Perigee • The Moon’s closest approach to the Earth in its elliptical orbit is called perigee. • Apogee is the point in its orbit where it is farthest from the Earth. (annular eclipses) • Apogee and perigee are also used for other satellites orbiting Earth or other planets.

27. Eclipse Geometry • If lunar eclipses happen at Full Moon and solar eclipses happen at New Moon… • Why don’t we have eclipses every New and Full Moon?

28. The Moon’s orbit is tipped about 5° with the ecliptic.

29. Nodes • For an eclipse to occur, the Moon must reach new or full phase just as its orbital path intersects the ecliptic. • Such a point is called a node. • It is more likely the Moon will be just north or south of this node and an eclipse will not occur. • See Seeds, page 47