Motions of the Earth and Sky Part II

1. Motions of the Earth and Sky Part II

3. Solar Eclipses Sun planet moon Depending on the relative sizes and distances of the Sun and a moon, you might see an eclipse like this:

4. Solar Eclipses Sun planet moon Or you might see an eclipse like this:

5. Solar Eclipses Sun planet moon In our case, the Sun is 400 times larger than the Moon, and coincidentally is also 400 times further away, so they happen to have the same size on the sky: http://www.astro.psu.edu/users/kluhman/a5/Eclipses_Nav.swf

6. Solar Eclipses

7. Solar Eclipses

9. Total Solar Eclipse You only get to see this outer atmosphere (corona) of the Sun if the Sun’s entire body is blocked out.

11. Solar Eclipses

12. Partial Solar Eclipse If you’re on the edge of an eclipse path and only a slice of the Sun is blocked out, it’s called apartial eclipse. These are not very exciting, since the uneclipsed part of the Sun is still extremely bright.

13. The Moon’s orbit is not a perfect circle, so its distance from the Earth varies by a small amount, and so does its size on the sky

14. Annular Eclipses Because the Moon’s orbit about the earth is not perfectly circular, it is sometimes too far away to block out the whole Sun, even when perfectly aligned. When this happens, you get an annular eclipse. It is rarer than a total solar eclipse.

15. Shadows of the Earth and Moon The Moon’s orbital plane is tilted by 5.2° from the ecliptic plane. Hence, ½ the time, the Moon is slightly north of the ecliptic (and ½ the time, it is south of the ecliptic). The shadow of one body very rarely falls on the other.

16. Shadows of the Earth and Moon The Moon’s orbital plane is tilted by 5.2° from the ecliptic plane. Hence, ½ the time, the Moon is slightly north of the ecliptic (and ½ the time, it is south of the ecliptic). The shadow of one body very rarely falls on the other.

17. Lunar Eclipses If the Moon crosses the ecliptic plane while exactly opposite the Sun, it will fall in the earth’s shadow. This is alunar eclipse. It can be seen from anywhere on the half of the Earth facing the Moon.

18. Frequency of Eclipses • Under the most favorable conditions, the diameter of the shadow of a solar eclipse is 269 km at the Earth’s surface. • At the equator, the shadow moves at 1730 km/hr. • Totality can last as long as 7½ minutes. • A total solar eclipse occurs about once every 18 months somewhere in the world. • At any given location, a total solar eclipse occurs once every 360 years. • The next total solar eclipse in the U.S. is on Aug. 21 2017 • Lunar eclipses happen about twice each year. They are more common than solar eclipses because the earth is larger than the Moon and casts a larger shadow. http://eclipse.gsfc.nasa.gov/eclipse.html

19. Solar Eclipse Paths through 2020

20. Testing whether Earth orbits the Sun: Parallax Parallax is the apparent motion of a nearby object relative to a distant object due to the changing position of the observer.

21. If Earth orbits the Sun, then nearby stars should appear to move relative to distant stars over the course of a year. However, to the naked eye, the stars appear to remain fixed relative to each other. Because of the absence of noticeable parallax among the stars, ancient philosophers like Aristotle concluded that Earth must be stationary. If so, the motions of the Sun, Moon and planets across the sky must mean that they orbit Earth. This is the geocentric model of the solar system.

23. Observed Properties of the Original Planets • To ancient observers, the planets were distinctive from the stars because they moved relative to the stars. They were called “wanderers” in Greek, from which the word “planet” is derived. • The planets always stay close to the ecliptic, i.e., they move through the zodiac constellations. • Ancient observers noticed two distinct types of planets. Mercury and Venus were always fairly close to the Sun the the sky (i.e., always near conjunction). They were called theinferior planets. • The other planets, Mars, Jupiter, and Saturn, would appear near the Sun at one time and far from the Sun at another time (either conjunction or opposition). They were thesuperior planets. • Planets usually move west-to-east against the fixed stars. But sometimes the planets move backwards (east-to-west). This is called retrograde motion.

24. Retrograde Motion Path of a planet relative to the stars

25. Retrograde Motion

26. Retrograde Motion

27. Aristotle’s Geocentric Model (350 B.C.) • Because of the apparent absence of parallax, Earth must be stationary, and the center of the solar system • Sun and Moon orbit Earth • To explain retrograde motion, planets must move around small circles calledepicycles, which in turn orbit Earth This model could explain retrograde motion, but it didn’t do a very good job of predicting the positions of the planets the sky over time.

28. Ptolemy’s Geocentric Model (140 A.D.) Ptolemy revised Aristotle’s model and made it more complicated to try to improve the predictions of the positions of the planets, but it still didn’t do a great job.

29. Ptolemy’s Geocentric Model (140 A.D.)

30. Copernicus’ Heliocentric Model (1530 A.D.) Since the planets are in the heavens, Copernicus assumed that they must move in perfect circles at a constant speed. But otherwise, his model differed greatly from Aristotle’s: • The heavenly bodies do not all move around the same center. • The Earth is not at the center of the solar system. Only the Moon goes around the Earth. • The Sun is at the center of the solar system. This is the heliocentric model. • The daily motion of the Sun, Moon, and stars is due to the Earth’s rotation. • The Sun’s yearly motion is due to the Earth’s orbit round the Sun. • Retrograde motion is due to the Earth’s orbit round the Sun.

31. Copernicus’ Heliocentric Model (1530 A.D.)

32. Copernicus’ Heliocentric Model (1530 A.D.) According to Copernicus, retrograde motion is produced by parallax as Earth passes by a planet. This is the correct explanation for retrograde motion. http://www.astro.psu.edu/users/kluhman/a5/MarsRetrograde.swf http://astro.unl.edu/naap/ssm/animations/configurationsSimulator.html

34. Copernicus’ Heliocentric Model (1530 A.D.) The heliocentric modelnaturally explains why some planets (inferior) never stray far from the Sun in the sky. They are the planets with orbits smaller than Earth’s orbit. But the model was no better at predicting the positions of the planets than Aristotle’s model. Also, people still wondered why they couldn’t see parallax among the stars if the Earth is moving. (answer: stars are so far away that their parallax shifts are too small to detect with the naked eye.)