1. Chapter 1: The Scale of the Cosmos
Astronomy deals with objects on a vast range of size scales and time scales (时间空间尺度大).
Most of these size and time scales are way beyond our every-day experience （尺度非日常经历的）.
Humans, the Earth, and even the solar system are tiny and unimportant on cosmic scales （我们很渺小）.

2. A Campus Scene
16 x 16 m

3. A City View
1 mile x 1 mile

4. The Landscape of A State
100 miles x 100 miles

5. Diameter of the Earth: 12,756 km ~ 104 km ~ 107 m

6. Distance Earth – Moon: 384,000 km
Earth and Moon
Distance Earth – Moon: 384,000 km

7. Distance Sun – Earth = 150,000,000 km = 1.5x108 km
Earth orbiting around the Sun
Distance Sun – Earth = 150,000,000 km = 1.5x108 km

8. Earth orbiting around the Sun
为避免使用大的数字，引入新的单位:
1 Astronomical Unit (AU)
= 1 天文单位
= 日地距离
= 1.5x108 km (1.5亿公里)

9. Earth orbiting around the Sun
1 AU = 1.5x108 km
光速 C = 3 x 105 km/s
太阳光到地球要多长时间？
Live 8 more minutes
旅行时间 ＝ 1 AU / C = 500s

10. The Solar System（太阳系）
Approx. 100 AU

11. (Almost) Empty Space Around our Solar System
Approx. 10,000 AU

12. The Solar Neighborhood
Approx. 17 light years

13. The Solar Neighborhood
New distance scale:
1 light year (ly) =
Distance traveled by light in 1 year
= 63,000 AU = 1013 km
= 10,000,000,000,000 km
离太阳最近的恒星:
Proxima Centauri (比邻星), at a distance of 4.2 light years
大约 17 light years

14. The Extended Solar Neighborhood
Approx. 1,700 light years

15. Diameter of the Milky Way（银河系）: ~ 75,000 ly
The Milky Way Galaxy（银河系）
Diameter of the Milky Way（银河系）: ~ 75,000 ly

16. Distance to the nearest large galaxies: several million light years
The Local Group of Galaxies
Distance to the nearest large galaxies: several million light years

17. The Universe on Very Large Scales
Clusters of galaxies are grouped into superclusters.
Superclusters form filaments and walls around voids.

18. Chapter 2: The Sky

19. 三垣、四象（二十八宿）

20. 88 Constellations 星座
In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures.

21. Today, constellations are well-defined regions of the sky, irrespective of the presence or absence of bright stars in those regions.

22. Planetarium Software

23. Usually, this is only a projection effect.
The stars of a constellation only appear to be close to one another. （一个星座里的星星只是看上去互相很接近）
Usually, this is only a projection effect.
但这只是投影效应
The stars of a constellation may be located at very different distances from us.

24. Betelgeuse =  Orionis （猎户座 ）,
Betelgeuse
Rigel
Stars are named by a Greek letter (, , ,) according to their relative brightness within a given constellation + the possessive form of the name of the constellation:
Betelgeuse =  Orionis （猎户座 ）,
Rigel =  Orionis

25. The Magnitude Scale （星等）
First introduced by Hipparchus
（依巴古，希巴恰斯）
( B.C.):
Brightest stars: ~1st magnitude（一等星）
Faintest stars (unaided eye): 6th magnitude（6等星）
More quantitative:
1st mag. stars appear 100 times brighter than 6th mag. Stars
（亮度差100倍，星等差5等。）
1 mag. difference gives a factor of in apparent brightness (larger magnitude = fainter object!) 星等差1等，亮度差2.512倍。

26. 星光强度和星等的公式 或 如果星光亮度差100倍， log(102) = 2 mA - mB = -2.5 x 2 = -5 星等差
星等差
星光强度比 1
(2.512)1 2
(2.512)2=6.31 3
(2.512)3=15.8 4
(2.512)4=39.8 5
(2.512)5=100 6
251 7
631 8
1,580 9
3,980 10
10,000 … 15
1,000,000 25
1,000,000,000 或
如果星光亮度差100倍， log(102) = 2 mA - mB = -2.5 x 2 = -5

27. Example: Betelgeuse Rigel
Magn. Diff.
Intensity Ratio 1
2.512 2
2.512*2.512 = (2.512)2 = 6.31 … 5
(2.512)5 = 100
Betelgeuse
Magnitude = 0.41 mag
Rigel
For a magnitude difference of 0.41 – 0.14 = 0.27, we find an intensity ratio of (2.512)0.27 = 1.28
Magnitude = 0.14 mag

28. Sirius (天狼星 brightest star in the sky): mv = -1.42
The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects):
Sirius (天狼星 brightest star in the sky): mv = -1.42
Full moon满月: mv = -12.5
Sun太阳: mv = -26.5

29. The Celestial Sphere （天球）
Zenith（天顶） = Point on the celestial sphere directly overhead
Nadir = Point on the c.s. directly underneath (not visible!)
Celestial equator （天赤道）= projection of Earth’s equator onto the c.s.
North celestial pole （北天极）= projection of Earth’s north pole onto the c.s.

30. The Celestial Sphere (II)
From geographic latitude ℓ (northern hemisphere), you see the celestial north pole ℓ degrees above the horizon;（地理纬度=当地北天极到地平线的夹角） ℓ

31. The Celestial Sphere (III)

32. Apparent Motion（视运动） of the Celestial Sphere

33. Apparent Motion of the Celestial Sphere II

34. The sun’s gravity is doing the same to Earth.
Precession 进动 (I)
Gravity is pulling on a slanted top. => Wobbling around the vertical.
The sun’s gravity is doing the same to Earth.
The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the ecliptic takes about 26,000 years and is called precession.

35. It will be closest to Polaris ~ A.D. 2100.
Precession (II)
As a result of precession, the north celestial pole follows a circular pattern on the sky, once every 26,000 years.
It will be closest to Polaris ~ A.D
~ 12,000 years from now, it will be close to Vega in the constellation Lyra.
There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) 北极星并不特殊