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）.

A Campus Scene 16 x 16 m

A City View 1 mile x 1 mile

The Landscape of A State 100 miles x 100 miles

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

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

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

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)

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

The Solar System（太阳系） Approx. 100 AU

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

The Solar Neighborhood Approx. 17 light years

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

The Extended Solar Neighborhood Approx. 1,700 light years

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

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

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

Chapter 2: The Sky

三垣、四象（二十八宿）

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

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

Planetarium Software

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.

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

The Magnitude Scale （星等） First introduced by Hipparchus （依巴古，希巴恰斯） (160 - 127 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 2.512 in apparent brightness (larger magnitude = fainter object!) 星等差1等，亮度差2.512倍。

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

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

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

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.

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

The Celestial Sphere (III)

Apparent Motion（视运动） of the Celestial Sphere

Apparent Motion of the Celestial Sphere II

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.

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. 2100. ~ 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