Astronomy 2 Overview of the Universe Winter Lectures on Greek Astronomy Joe Miller

The earliest days of astronomy: 1. It was dark at night! No artificial lighting. 2. The sky is very impressive. 3. Certain behaviors and patterns became obvious: The concept of the celestial sphere. The sky appeared to be a huge sphere turning on an axis. There was a north and south pole and an equator. Stars rose in the east and set in the west as if they were attached to this vast sphere.

The annual changing aspects of the sky Note that as the stars rise and set, they don’t move with respect to one another, but remain fixed as if attached to the celestial sphere. Hence their name: the FIXED STARS. Various patterns or groups of stars were identified and given names by all cultures. Today we call these constellations, but they have no scientific meaning. In some cases elaborate stories were developed about large groups of constellations. The same group of stars was observed to rise about 4 minutes earlier each night. Thus different constellations were observable in different seasons.

Remember: same stars rise 4 minutes earlier each day. Why? Because the earth goes around the sun. 4 minutes/day x 365 days = 1460 minutes. But the day is 1440 minutes long. Actually stars rise a few seconds less than 4 minutes earlier each day and it takes the earth to go around the sun once with respect the stars. This is called the siderial year. If we kept time by the stars (siderial time), the day would be about 4 minutes shorter. We use solar time.

How did the Greeks make sense of all this? They assumed that the earth is at the center of the universe- the geocentric theory. They believed that the stars were attached to an extremely large sphere- the celestial sphere. The celestial sphere must rotate once a day around the earth, carrying around it with it the sun and moon. The sun and moon must slowly drift with respect to the celestial sphere over the course of a month (moon) and year (sun).

But there were problems!!! Not all stars were fixed! A few moved around and were called “wanderers” or “planets” in Greek. From night to night they gradually drifted in the sky relative to the stars, generally toward the east. But occasionally they stop their eastward motion and reverse direction, moving toward the west. This is called retrograde motion. For the Greeks, who believed in uniform circular motions, retrograde motions were a horrible problem.

The solution: epicycles!! If the planets travel on small circles (epicycles)attached to the big circles (deferents), and the speeds are adjusted properly, retrograde motion can be established.

Aristotle (? BCE) Argued that the moon must be spherical -he understood the phases of the moon. Argued the world was spherical for several reasons. –Traveling south brought new constellations into view. –He understood eclipses, and the shadow of the earth on the moon always had a curved edge. –Elephants!

Aristotle believed in the geocentric picture picture. Why? The earth was too big to rotate once a day. It would fly apart. Stars did not show parallaxes during the year, which they must if the earth goes around the sun. This argument was conclusive for most people. The only way out: the stars must be very far away!

Aristarchus (? BCE) Proposed that the earth went around the sun. The stars were very far away, and thus no parallaxes could be observed. Parallax is an apparent shift in a star’s position caused by the motion of the earth around the sun. Attempted to measure relative distance of sun and moon. Clever, but wrong. He found the sun’s distance was 20 times the moon’s distance.

Eratosthenes (? BCE ) Measured the size of the earth and got it right to about 1%!!! (We think.) The basic assumption was that the sun was very far away compared to the size of the earth, so that the sun’s rays hitting the earth could all be considered parallel to one another.

Hipparchus- the greatest Greek astronomer Builder of excellent instruments to measure positions of stars. Led to excellent star catalog. Produced extensive tables of the sun, moon, and planet positions. Hypothesized that the “fixed” stars might actually move. Devised a method to check this over centuries. Predicted eclipses, including solar ones: their time and place. Very hard to do! Invented trigonometry because he needed it. Measured length of year with error estimate of 15 minutes. He was accurate to 6 minutes! Greatest discovery: precession.

Claudius Ptolemy (c. 150 AD) The Almagest “Greatest Work”- a great synthesis of knowledge of his time. Became the standard text on astronomy for 15 centuries. Elaborate presentation of the geocentric system, also called the Ptolemaic System.