Ptolemaic
What are the names of the two circles in Ptolemaic astronomy?
The two circles are called deferent and epicycle.
(The deferent is centered on the earth. The epicycle is centered on a guide point which moves around the deferent.)
What two celestial bodies orbit the earth in Tychonian astronomy?
According to Tycho, the sun and the moon orbit the earth.
(Tychonican astronomy is geocentric; the earth stays still at the center.)
Is Copernican astronomy heliocentric or geocentric?
Copernican astronomy is heliocentric.
(It was the first heliocentric astronomy to become widely held.)
What was Johannes Kepler's profession and nationality?
Johannes Kepler was a German astronomer.
(He was also a math teacher and a court scientist.)
What does it mean that the deferent is eccentric?
The deferent is eccentric because it is not centered on Earth.
(Earth is not the center of the deferent, but it is still supposed to be the center of the universe. Therefore the deferents are off center from the universe.)
Which celestial bodies orbit the sun in Tychonian astronomy?
Mercury, Venus, Mars, Jupiter, and Saturn orbit the sun.
(The sun carries the planets around the earth.)
What geometry did Copernican astronomy use in common with Ptolemaic?
Copernican astronomy used deferents and epicycles.
(They were centered on the sun instead of on the earth.)
What is Kepler's first law?
A planet moves in an ellipse with the sun at one focus.
(Kepler departed from circular orbits.)
How did Ptolemy know that the Earth was not flat?
If the earth were flat, people in different parts of the world would see the stars rise at the same time, but in fact different places see them at different times.
(Ptolemy also rules out other geometries, such as cylindrical, concave, and polyhedral.)
The Tychonian system was able to explain his observations without a moving earth.
(Galileo disproved Ptolemy but could not prove Copernicus over Tycho.)
What does the epicycle explain about planetary motion?
The epicycle explains variations from circular motion.
(For instance, it explains why planets change speed and brightness. The deferent only describes the general pattern of motion.)
What is Kepler's second law?
The line between the sun and a planet sweeps out equal areas in equal times.
(This law determines the rate at which a planet moves along its orbit.)
What is the equant?
The equant is the location at which the guide point of a planet's epicycle appears to be moving at a uniform rate.
(In fact, the guide point speeds up near the earth and slows far from it. The equant and the earth are on opposite sides of the center of the deferent.)
What did mediaeval astronomers observe about the stars which led them to believe in geocentrism?
The mediaeval astronomers did not observe parallax, that is, the stars did not appear to become nearer to or farther from each other.
(This was evidence for geocentrism because, unless the stars were ridiculously far away, a moving earth would see the stars differently from different places.)
What three discoveries of Galileo were evidence against Ptolemaic astronomy?
Galileo discovered the phases of Venus, the moons of Jupiter, and the earthlike surface of the moon.
(Even before this, there were problems with inaccurate calendar predictions and with the unchanging size of the moon.)
What is Kepler's third law?
The square of a planet's period is proportional to the cube of its semi-major axis.
(Mathematically, T2 ∝ a3, or T2 / a3 = k (k is a constant.))