Who’s Who in Early Astronomy Part 1 ELIZABETH MEGONIGAL

Ptolemy (pronouced TAHL uh mee)- 140 AD (Greek – astronomer, mathematician, & geographer) He thought that Earth was the center of the universe and the sun and the other planets revolving around Earth. Claudius also discovered the irregularity of the moon's orbit  He claimed that everything in the universe moves either toward or around the Earth His theory was thought to be accurate for about 1500 years.

1543 -Nicolaus Copernicus (Polish descent ) (pronounced NIK uh lay koh PURH ni kuhs) His theory, Heliocentric model, stated that the sun was the center of the universe and the planets- including the Earth– orbit the sun. This model explained why the planets in our system naturally vary in brightness because they are not always the same distance from the Earth AND The retrograde motion (apparent backwards movement) could be explained in terms of geometry and a faster motion for planets with smaller orbits, as illustrated in the following animation. https://www.pas.rochester.edu/~blackman/ast104/copernican-move.gif

Tycho Brahe- (pronounced TIE koh BRAW uh) (work– 1550’s to 1600) The Earth is fixed in the center of the universe. The stars revolve around us everyday, the Moon every month and the Sun every year, while the other planets all orbit the Sun. In 1572 Tycho sees a supernova in the sky and records the finding of the 'new star' (before the telescope was invented). He developed many of the tools he used to study the universe.

Brahe’s Childhood (fun facts)-- Tycho Brahe was born December 14 1546 Tycho Brahe was one of two twin boys. His twin died shortly after Tycho was born. Before Tycho was born his dad told his uncle that if Tycho was a boy then the uncle could have Tycho. But later, Tycho's father changed his mind. After Tycho's parents had a second son Tycho's uncle kidnapped him. Some people say that the parents didn't try to get Tycho back, but some people think otherwise.

Johannes Kepler (around 1609)- He was the assistant of Brahe. Kepler didn’t agree with Brahe’s theory, but he recognized how precise and valuable Brahe’s data were. After reanalyzing the data, Kepler announced 3 new laws of planetary motion. And stated that the planets revolve around the sun in elliptical orbits and that the sun is not in the exact center of the orbits.

Kepler’s 1st law of Planetary Motion- “The orbit of a planet is an ellipse with the Sun at one foci”. Each ellipse has two foci (plural of focus) as shown. We can find the value of c by using the formula c2 = a2 - b2.

Kepler’s 2nd law of Planetary Motion- “A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.” I can calculate where an object will be in a certain period of time. A line that connects a planet to the sun sweeps out equal areas in equal times

Kepler’s 3rd law of Planetary Motion- “The square of the orbital period (the time it takes to make one complete revolution around the sun) of a planet is proportional to the cube of the semi-major axis (radius of the longest side of the ellipse) of its orbit.” If I know how long it takes for an object to complete a revolution (through observations), I can calculate how far away it is. Or vice versa. The semi-major (a) and semi-minor axis (b) of an ellipse