A Quick Tour of the Universe (and this course) Part 2

1. A Quick Tour of the Universe (and this course)Part 2

6. Charting the Heavens: Foundations of Astronomy • Learning Goals • Describe the Celestial Sphere and how astronomers use angular measurement to locate objects in the night sky. • Account for the apparent motions of the Sun and the stars in terms of the actual motion of the Earth. Explain why our planet has seasons. • Understand the changing appearance of the Moon and how the relative motions of the Earth, the Sun, and the Moon lead to eclipses.

7. If Earth rotated twice as fast as it currently does, but its motion around the sun stayed the same, then which of the following is true: A: the night would be twice as long B: the night would be half as long C: the year would be half as long D: the year would be twice as long E: the length of a day would be unchanged Clicker Question:

8. If Earth rotated twice as fast as it currently does, but its motion around the sun stayed the same, then which of the following is true: B: the night would be half as long Clicker Question:

9. Distances to nearby stars are measured by: A: bouncing radar signals off them B: using laser beams C: using geometry and parallax measurements D: measuring how long it took a spacecraft to get there and back and assuming a constant velocity. Clicker Question:

10. Distances to nearby stars are measured by: C: using geometry and parallax measurements Clicker Question:

11. How many stars are there in the observable universe? A: 1012 B: 1022 C: 1032 D: 1042 E: infinite Clicker Question:

12. How many stars are there in the observable universe? B: 1022 Clicker Question:

13. The Sky at Night What do we see? The Moon Planets Perhaps a meteor shower, comet, or other rare event Stars - about 3000 visible Patterns of stars - constellations 88 of them Useful for finding our way around the sky, navigating the oceans Satellites, airplanes, clouds, lightning, light pollution ...

14. The Celestial Sphere An ancient concept, as if all objects at same distance. But to find things on sky, don't need to know their distance, so still useful today. Features: - Does not rotate with Earth - Poles, Equator - Coordinate System

15. The "Solar Day" and the "Sidereal Day" Solar Day How long it takes for the Sun to return to the same position in the sky (24 hours). Sidereal Day How long it takes for the Earth to rotate 360o on its axis. These are not the same!

16. One solar day later, the Earth has rotated slightly more than 360o . A solar day is longer than a sidereal day by 3.9 minutes (24 hours vs. 23 hours 56 minutes 4.091 seconds).

17. Why is it warmer in Albuquerque in the summer than winter? A: The northern hemisphere is tilted towards the sun in summer. B: The Earth is closer to the sun in summer. C: The greenhouse effect increases in summer. D: The sun increases its intrinsic luminosity in the summer. E. All of the above. Clicker Question:

18. Why is it warmer in Albuquerque in the summer than winter? A: The northern hemisphere is tilted towards the sun in summer. Clicker Question:

19. The Earth's rotation axis is tilted with respect to its orbit around the Sun => seasons. Summer Winter Sun low in northern sky Sun high in northern sky Orion Day Night Scorpius Night Day Tilt is 23.5o

20. Summer Winter In winter, sunlight is spread out more thinly across the ground => each bit of ground receives less radiation => cooler

21. The Year Summer Winter Orion Day Night Night Day Scorpius The Earth revolves around the Sun in 365.256 days (“sidereal year”). But the year we use is 365.242 days (“tropical year”). Why?

22. Precession The Earth has a bulge. The Moon "pulls down" on the side of the bulge closest to it, causing the Earth to wobble on its axis (how do we know this?) Earth Moon Vega * * Polaris Spin axis Precession animation Precession Period 26,000 years!

23. The Motion of the Moon DEMO - Precession

24. Now Scorpius Night Day Orion Day Night Summer: July Winter: January 13,000 years from now Night Day Orion Night Scorpius Day Winter: July or January? Summer: January or July? We choose to keep July a summer month, but then in 13,000 years, summer occurs on other side of orbit!

25. The Motion of the Moon The Moon has a cycle of "phases", which lasts about 29 days. Half of the Moon's surface is lit by the Sun. During this cycle, we see different fractions of the sunlit side. Which way is the Sun here?

26. See Tutorial on book’s website for animation.

27. The Motion of the Moon DEMO - Phases of the Moon

28. Cycle of phases slightly longer than time it takes Moon to do a complete orbit around Earth. Orbit time or "sidereal month" Cycle of phases or "synodic month" 27.3 days 29.5 days

29. Eclipses Lunar Eclipse When the Earth passes directly between the Sun and the Moon. Sun Earth Moon Solar Eclipse When the Moon passes directly between the Sun and the Earth. Earth Moon Sun

30. Solar Eclipses Diamond ring effect - just before or after total Total Partial Annular - why do these occur?

31. Lunar Eclipse

32. Why don't we get eclipses every month?

33. Moon's orbit tilted compared to Earth-Sun orbital plane: Sun Moon Earth 5.2o Side view Moon's orbit slightly elliptical: Moon Distance varies by ~12% Earth Top view, exaggerated ellipse

34. Types of Solar Eclipses Explained

35. Certain seasons are more likely to have eclipses. Solar “eclipse season” lasts about 38 days. Likely to get at least a partial eclipse somewhere. Animation It's worse than this! The plane of the Moon's orbit precesses, so that the eclipse season occurs about 19 days earlier each year.

36. Recent and upcoming total and annular solar eclipses

37. Eratosthenes Determines the Size of the Earth in about 200 B.C. Sun's rays Syene Alexandria N 7.2o S Earth

38. He knows the distance between the two cities is 5000 "stadia". From geometry then, 7.2o 5000 stadia = 360o Earth's circumference => circumference is 250,000 stadia, or 40,000 km. So radius is: 40,000 km 6366 km = 2 pi (very close to modern value, 6378 km!)