Astronomy 110 Lecture 5 + 6 Fall, 2005 Astronomy 110 1
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
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
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
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
Parallax nearby objects appear to change position with respect to more distant objects when the observer changes position. Fall, 2005 Astronomy 110 6
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
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
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
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 110 10
The First Modern Cosmology Fall, 2005 Astronomy 110 11
Fall, 2005 Astronomy 110 12
Astronomy was forced on early societies for many reasons: 1. Calendars 2. Eclipses 3. Navigation 4. Farming 5. Weather Fall, 2005 Astronomy 110 13
http://www.geocities.com/astrologyages/babylonianprecession.htm Three Stars Each Tablets - the Ancient Baylonian 'Planisphere' or 'Astrolabe' [c 1000 BC] Fall, 2005 Astronomy 110 14
The Northern [Bottom] and Southern [Top] Panel 'Decan Chart from the Tomb of Senmut [c 1500 BC]. Fall, 2005 Astronomy 110 15
Rome Paris Time and the Calendar: Obilesk, an ancient clock? http://members.aol.com/sokamoto31/obelisk.htm#alllist Fall, 2005 Astronomy 110 16
Fall, 2005 Astronomy 110 17
It is generally felt that, among other things, Stonehenge functioned as a calendar. Fall, 2005 Astronomy 110 18
England: Stonehenge (1550 B.C.) Fall, 2005 Astronomy 110 19
There are similar circular calendars around the world: sacred stones in Scotland. Fall, 2005 Astronomy 110 20
Big Horn medicine Wheel (WY): one of several such medicine wheels. Fall, 2005 Astronomy 110 21
Ancient people of central Africa (6500 BC) could predict seasons from the orientation of the crescent moon Fall, 2005 Astronomy 110 22
The Sun Dagger: Chaco Canyon Marks the Summer Solstice Fall, 2005 Astronomy 110 23
Mexico: model of the Templo Mayor Fall, 2005 Astronomy 110 24
Yucatan, Mexico: Mayan Observatory at Chichen Itza Fall, 2005 Astronomy 110 25
"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 110 26
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 23.85 degrees. Fall, 2005 Astronomy 110 27
Measurements: Syene to Alexandria distance 5000 stadia angle = 7 Calculate circumference of Earth: 7/360 (circum. Earth) = 5000 stadia circum. Earth = 5000 360/7 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 110 28 S
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 110 29
Artist s reconstruction of Library of Alexandria Fall, 2005 Astronomy 110 30
Islamic Astrolabe, 14 th Century Fall, 2005 Astronomy 110 31