Astronomy 110 Lecture Fall, 2005 Astronomy 110 1

Transcription

1 Astronomy 110 Lecture Fall, 2005 Astronomy 110 1

2 Planets Known in Ancient Times Mercury difficult to see; always close to Sun in sky Venus very bright when visible morning or evening star Mars noticeably red Jupiter very bright Saturn moderately bright Fall, 2005 Astronomy 110 2

3 Planets could be told from stars because planets moved on the celestial sphere. Over many nights, planet motions showed retrograde loops. Fall, 2005 Astronomy 110 3

4 For some planets the motion can be very slow taking months or years to observe, but the nature of the motion is the same. Fall, 2005 Astronomy 110 4

5 Retrograde motion comes from a combination of relative motion - We describe planetary motion while riding on one of them - and Fall, 2005 Astronomy 110 5

6 Parallax nearby objects appear to change position with respect to more distant objects when the observer changes position. Fall, 2005 Astronomy 110 6

7 When we move to the right the planet appears to move to the left. When we are moving right at it, it appears to be standing still. Fall, 2005 Astronomy 110 7

8 Explaining Apparent Retrograde Motion Easy for us (we have the book!): Retrograde loops occur when we overtake another planet or when Mercury or Venus overtakes us. But very difficult to explain if you don t think the Earth is moving. Early theorists felt that since the stars did not show parallax effects, the Earth was not moving so a complex mechanical model was developed in order to explain it with the Earth fixed. Actually stars do show parallax but it is very small (<1 arc second) and can not be seen without a telescope and a a way to record images. Fall, 2005 Astronomy 110 8

9 Without observable parallax the Earth was assumed to be unmoving so all retrograde motion had to come from the planet s motion. But in 260BC Aristarchus argued that the Earth went around the sun and that the stars did not show parallax because they were very far away Fall, 2005 Astronomy 110 9

10 Conventional scientists went with the Ptolemaic, Earth-centered universe and retrograde motion was explained by the combination of on circle (epicycle) rolling on another (deferent) Notice that this idea also uses the combination of two circular motions to explain retrograde loops but both circles are part of the planet s orbit. Fall, 2005 Astronomy

11 The First Modern Cosmology Fall, 2005 Astronomy

12 Fall, 2005 Astronomy

13 Astronomy was forced on early societies for many reasons: 1. Calendars 2. Eclipses 3. Navigation 4. Farming 5. Weather Fall, 2005 Astronomy

14 Three Stars Each Tablets - the Ancient Baylonian 'Planisphere' or 'Astrolabe' [c 1000 BC] Fall, 2005 Astronomy

15 The Northern [Bottom] and Southern [Top] Panel 'Decan Chart from the Tomb of Senmut [c 1500 BC]. Fall, 2005 Astronomy

16 Rome Paris Time and the Calendar: Obilesk, an ancient clock? Fall, 2005 Astronomy

17 Fall, 2005 Astronomy

18 It is generally felt that, among other things, Stonehenge functioned as a calendar. Fall, 2005 Astronomy

19 England: Stonehenge (1550 B.C.) Fall, 2005 Astronomy

20 There are similar circular calendars around the world: sacred stones in Scotland. Fall, 2005 Astronomy

21 Big Horn medicine Wheel (WY): one of several such medicine wheels. Fall, 2005 Astronomy

22 Ancient people of central Africa (6500 BC) could predict seasons from the orientation of the crescent moon Fall, 2005 Astronomy

23 The Sun Dagger: Chaco Canyon Marks the Summer Solstice Fall, 2005 Astronomy

24 Mexico: model of the Templo Mayor Fall, 2005 Astronomy

25 Yucatan, Mexico: Mayan Observatory at Chichen Itza Fall, 2005 Astronomy

26 "On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star." "On the Xinwei day the new star dwindled." Bone or tortoise shell inscription from the 14th century BC. China: Earliest known records of supernova explosions (1400 B.C.) Fall, 2005 Astronomy

27 Eratosthenes measured the radius of the Earth by measuring the angular difference in the Sun s rays between two points on the surface of the Earth and knowing the physical distance between them. He also measured the tilt of the Earths axis to be degrees. Fall, 2005 Astronomy

28 Measurements: Syene to Alexandria distance 5000 stadia angle = 7 Calculate circumference of Earth: 7/360 (circum. Earth) = 5000 stadia circum. Earth = /7 stadia 250,000 stadia Compare to modern value ( 40,100 km): Greek stadium 1/6 km 250,000 stadia 42,000 km Fall, 2005 Astronomy S

29 Zephyros is the wind that blows from the point where the sun sets at the equinox, Aristotle, Meteorologica Tower of the Winds, Roman Agora, Athens (50BC) Fall, 2005 Astronomy

30 Artist s reconstruction of Library of Alexandria Fall, 2005 Astronomy

31 Islamic Astrolabe, 14 th Century Fall, 2005 Astronomy