The Motion of the Sun, Earth, and Stars. Day 1: How to Describe the Sky The Motions of the Stars

Transcription

1 The Motion of the Sun, Earth, and Stars Day 1: How to Describe the Sky The Motions of the Stars

2 We know that the Earth rotates once every day (in what direction?)... that the Earth orbits around the sun once every year (in what direction?) What does that mean for our view of the sky? First, some terminology.

3 The Celestial Sphere Imaginary North Celestial Pole South Celestial Pole Celestial Equator Ecliptic: path of sun Is it aligned?

4 The Local Sky Horizon Zenith Meridian Identify an object by: Direction (Azimuth) Altitude

5 Angular Size The angle an object appears to span in your field of view. Depends on: Actual size of object Distance from us to the object

6 Angular Size The Sun and the Moon have about the same angular size. How can that be? Not to scale!

7 Card Question When an astronomer describes the altitude of something in the local sky, he or she means A: how high something is in the sky, in units of miles or kilometers. B: how high something is in the sky, in units of degrees. C: the direction toward something North, South, East, or West.

8 Do the stars that we can see change with the time of night?... your latitude on Earth?... your longitude on Earth?... the time of year (the season)?

9 Stars and the Time of Night Lecture Tutorial: Position Groups of 3 4 Try to take ~ 10 minutes

10 Stars and the Time of Night Lecture Tutorial: Position Groups of 3 4 Try to take ~ 10 minutes

11 Stars and the Time of Night Circumpolar stars: always up Others: rise East, set West Near South Celestial pole: never rise

12 Circumpolar Stars

13 Card Question How much time is there between when a star rises and when it sets? A: Less than 12 hours B: About 12 hours C: More than 12 hours D: It depends on the star.

14 Card Question How much time is there between when a star rises and when it sets? A: Less than 12 hours B: About 12 hours C: More than 12 hours D: It depends on the star.

15 Card Question Figure 2.13 animated The North Celestial Pole points towards Polaris, and stars appear to rotate counterclockwise. Which way do stars rotate around the South celestial pole? A: Clockwise B: Counterclockwise

16 Card Question Figure 2.13 animated The North Celestial Pole points towards Polaris, and stars appear to rotate counterclockwise. Which way do stars rotate around the South celestial pole? A: Clockwise B: Counterclockwise

17 Stars and Your Location on Earth Latitude (North/South) Affects location of horizon and zenith. Altitude of celestial pole = your latitude. Longitude: no change in local time (explain). Why?

18 Card Question Where on Earth would you see the most circumpolar stars? A: Oslo, Norway (latitude 60N) B: Boulder, Colorado (latitude 40N) C: Bangalore, India (latitude 12N)

19 Card Question Where on Earth would you see the most circumpolar stars? A: Oslo, Norway (latitude 60N) B: Boulder, Colorado (latitude 40N) C: Bangalore, India (latitude 12N)

20 Stars and the Time of Year What does your Zodiac sign mean? Figure 2.14

21 Lecture tutorial: Seasonal Stars ~ 15 minutes

22 Stars and the Time of Year Stars rise 4 minutes earlier each night due to the orbit of the Earth around the Sun.

23 Clicker Question You go out tonight and see the brightest star in the constellation Orion rise at about 10PM. One week from now this star will rise at about A: 9:30PM B: 10:00PM C: 10:30PM D: 10:00AM

24 Clicker Question You go out tonight and see the brightest star in the constellation Orion rise at about 10PM. One week from now this star will rise at about A: 9:30PM B: 10:00PM C: 10:30PM D: 10:00AM

25 Clicker Question One evening at midnight, you observe Leo high in the Southern sky. Virgo is to the East of Leo and Cancer is to the West. One month earlier, which of these constellations was high in the Southern sky at midnight? A: Leo B: Virgo C: Cancer

26 Clicker Question One evening at midnight, you observe Leo high in the Southern sky. Virgo is to the East of Leo and Cancer is to the West. One month earlier, which of these constellations was high in the Southern sky at midnight? A: Leo B: Virgo C: Cancer

27 Do the stars that we can see change with the time of night? Yes, due to the rotation of the Earth. Many stars rise and set at different times of night. Some are circumpolar, some we don t see at all.... your latitude on Earth? Yes. Your latitude affects your horizon and zenith.... your longitude on Earth? No.... the time of year (the season)? Yes, due to our orbit around the Sun.

28 Precession Gradual change in the orientation of Earth s axis.

29 Clicker Question Why are different stars seen in different seasons? A: Because of Earth s axis tilt. B: Because stars move during the year. C: Because as Earth orbits the Sun, we see the Sun in front of different constellations. D: Because of precession.

30 Clicker Question Why are different stars seen in different seasons? A: Because of Earth s axis tilt. B: Because stars move during the year. C: Because as Earth orbits the Sun, we see the Sun in front of different constellations. D: Because of precession.

31 Skills learned: Celestial Sphere: definitions Why is the ecliptic different from the celestial equator? Local Sky: definitions Angular vs. Physical Size Stars: rise and set; direction of travel; circumpolar stars Latitude and Longitude: altitude of polaris = latitude Motion through the Zodiac: why does it happen? Stars rise 4 min earlier each day; why? Precession: wobble of the Earth's axis over LONG periods of time