1. 2 The Moon and Planets A Backyard View

2. 2 Goals How does the Moon move through the sky? What causes its phases? What really is the “dark side” of the moon? What are eclipses? What are occultations? How do planets move through the sky? What is retrograde motion? How is the Universe ordered?

3. 2 The Moon and Phases The Moon takes 27.3 days to orbit the Earth. The Moon takes 27.3 days to turn on its axis. Result: We always see the same face. There is no “dark” side of the Moon. http://antwrp.gsfc.nasa.gov/apod/ap991108.html

4. 2 The Moon in the Sky FullNew S N

5. 2 Lunar Eclipse Sometimes the Moon passes through the Earth’s shadow as cast by the Sun.

6. 2 Composite photo of the January 2001 lunar eclipse.

7. 2 Solar Eclipse Sometimes the Moon passes between the Sun and the Earth (and casts a shadow on the Earth). August 1999 eclipse. Photo from crew of Mir.

8. 2 Composite photo of August 1999 solar eclipse.

9. 2 Types of Solar Eclipses Partial Total Annular

10. 2 Angular Size The Sun is a million times larger than the Earth. The Moon is a fourth the size of the Earth. The distance from the Earth determines their ANGULAR SIZE. Angular size: How big does something look as viewed from the Earth.

11. 2 Angular vs. Linear (True) Size How big an object LOOKS from the Earth depends upon its TRUE size (Linear Size) and distance from Earth

12. 2 D d  you In degrees!

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14. 2 Angles The sky is 360 arc degrees around. 60 arcminutes = 1 arc degree –The Full Moon is about half an arc degree = 30 arcminutes. 60 arcseconds = 1 arcminute –Jupiter is about 10 arcminutes now. 1000 milliarcsecond = 1 arcsecond –Polaris is 3 milliarcseconds in diameter –An astronaut on the Moon is 2 milliarcseconds tall!

15. 2 Occultations Eclipse: sometimes the moon passes in front of the Sun. Occultation: sometimes the moon passes in front of other things: –Stars –Asteroids –Planets

16. 2 The Plane of the Solar System The Earth, Sun, Moon and Planets all lie in the same plane. This is the plane of the solar system.

17. 2 Ecliptic As viewed from inside the solar system, the Sun, Moon and planets all move in a band on the sky around the Earth. This band is called the ecliptic. The constellations that the ecliptic passes through are the zodiac.

18. 2 Ecliptic and Occultations Result: Moon, Sun and planets aren’t just “anywhere” in the sky. Always found somewhere on the ecliptic. Distant planets pass behind nearby objects and are occulted.

19. 2 The Ecliptic in the Sky FullNew The Ecliptic

20. 2 Venus

21. 2 Retrograde Motion June, 2000 to May 2001 Credit & Copyright: Tunc TezelTunc Tezel

22. 2 The Wanderers The planets (or “wanderers”) move differently from stars –They move with respect to the stars –They exhibit strange retrograde motion What does all this mean? How can we explain these movements? What does the universe look like?

23. 2 Geocentric System The Earth is at the center of Universe. The Sun, stars, and planets orbit the Earth: explains daily movement. To account for unusual planetary motion epicycles were introduced. Fit the Greek model of heavenly perfection – spheres are the perfect shape, circular the perfect motion. The accepted model up until ~1630 AD.

24. 2 Heliocentric System What if? Sun at center (heliocentric). Moon orbits the Earth, the Earth and planets orbit the Sun. Stars don’t move. The daily motion of the stars results from the Earth’s spin. The annual motion of the stars results from the Earth’s orbit.

25. 2 In the heliocentric model, apparent retrograde motion of the planets is a direct consequence of the Earth’s motion

26. 2 Geocentric vs. Heliocentric How do we decide between two theories? Use the Scientific method: –These are both explanations based on the observation of retrograde motion –What predictions do the models make? –How can these predictions be tested?

27. 2 Phases of Venus Heliocentric predicts that Venus should show a full phase, geocentric does not Unfortunately, the phases of Venus cannot be observed with the naked eye

28. 2 Moons of Jupiter In a geocentric philosophy, there is only one center of motion: the Earth. If one thing can be found not to orbit the Earth, then maybe nothing needs to orbit the Earth. Using his new telescope, Galileo: –discovered four moons of Jupiter  The Galilean Moons. –Saw the phases of Venus.

29. 2 A Scientific Universe The heliocentric model fits the observations best. It’s the model of the solar system we have today. While our uncritical eyes tell us one thing, the scientific method, and critical thought, reveals another. Retrograde motion, phases of Venus, and moons of Jupiter: –We are the center of the Universe. –We as a single part, in no way special.