Lecture 9 2/4/2013 12:30:00 PM
Does the universe revolve around you?
France: Cave paintings from 15, 000 BC may suggest knowledge of
lunar phases (29 dots) and some star patterns.
Ancient people of central Africa apparently used the orientation of
the crescent moon to mark the seasonal. A carved bone shows
pictographs of the crescent moon.
This helped them to determine when to plant crops.
England: Stonehenge. Alignments of the stones mark the rising and
setting points of the sun at the solstices.
China: earliest known records of supernova explosion od a day
start. Also recorded solar, lunar eclipses continuously from 4 BC.
Eratosthenes measured the earths circumference by observing the
sun from two places on the same day of the year.
At noon on the first day of summer,
Syene: sun directly over head
Alexandria: sun at 7˚ from vertical (zenith)
Distance from Syene to Alexandria must be 7/360 of
Other Greek astronomer figured out distances and size of moon an
sun, using careful observations and geometrical reasoning.
Principles underlying ancient Greek astronomy:
o The earth is fixed at the centre of the universe.
o The heavens are perfect and unchanging.
o Circles are the most perfect shape—heavenly objects more in
A few Greek philosophers believed that the earth moved:
Earth, sun and all planets move around an invisible fire.
o Aristarchus of Samos:
Earth moves around the sun.
o But these ideas were not widely accepted.
Principles underlying ancient greek astronomy:
o Basic picture of the heavens: The earth is fixed at the centre of the universe.
Uniform circular motion:
Moon, sun, planets, and stars are all attached to
sphered of circles that rotate around the earth.
Problem: simple circles
Reason for believing the Earth is Stationary:
First of all, it feels solid and unmoving.
If the earth moves, we would expect the stars to show apparent
motion (parallax) due to different viewing angles. Constellations
ought to change their apparent shapes.
Some argued that if the earth moved, it would leave the moon
An apparent change of position caused by viewing from different
places (and different angles)
The farther away an object is, the smaller the parallax.
The Ptolemaic Model:
Explained retrograde motion with epicycles.
Each planet moves on a smaller circle, epicycle.
The centre of each epicycle moves on a larger circle, the deferent.
Centres of deferents are slightly displaces from earth.
Epicycles also explain changed in apparent brightness: planets
appear brighter when they are closer.
In order of distance:
o Fixed stars Ptolemy’s model was able to describe the motions of the planets
quite accurately. (although imperfections became more noticeable
In Europe and the Arab world, it was considered the correct
description for more than 1000 years.
Some historians consider it the first