Astronomy 101 Chapter 4 Review

Describe in detail the motions of the planets according to Ptolemy.

All the planets orbit around Earth in their own circles with uniform motion, but have epicycles. Therefore, the planets do not travel at a constant speed

Assume the night is clear and the Moon's phase is full so you can see it all night long. How would you measure the Moon's parallax during the night?

As a difference in the angular distance between the Moon and stars at the beginning and end of the night.

Do the centers of all planetary epicycles move at the same speed and in the same direction according to Ptolemy's model?

The speed of the epicycles' centers is different but the direction is the same.

Why did Copernicus have to keep small epicycles in his model?

He needed epicycles because he used uniform circular motion instead of elliptical motion.

How do you know that the planet traveled faster from point A to B rather than from point A' to B' in the figure below?

It moved farther along its orbit from A to B in the same amount of time.

How did the discovery of the Galilean moons disprove Plato's and Aristotle's perfect heavens first principle(s)?

It proved that there could be other centers of motion besides Earth.

How does Kepler's second law of planetary motion overthrow one of the basic beliefs of classical astronomy?

It states that the planets do not move at a constant speed in their orbits, hence, it contradicts the idea of uniform circular motion.

How does Kepler's first law of planetary motion overthrow one of the basic beliefs of classical astronomy?

It states that the planets do not orbit Earth and also do not follow circular orbits; hence, it contradicts the idea of uniform circular motion.

When Tycho observed the new star of 1572, he could detect no parallax. Why did that undermine the belief in the Ptolemaic system?

Not observing a parallax of a new star indicates that this star is not moving around Earth but has its own direction of motion.

Name one example of a famous observer and of a famous theoretician.

Observer Tycho; theoretician Kepler.

When Tycho observed the new star of 1572, he could detect no parallax. Why did that undermine the belief in the perfect heavens idea of Aristotle?

Observing a new star undermined the perfect heavens idea because the starry sphere was no longer unchanging.

Why did Plato propose that all heavenly motion was uniform and circular?

Plato proposed uniform circular motion because he viewed the world as perfect, and he considered the sphere and uniform circular motion to be perfect.

How did the Alfonsine Tables, the Prutenic Tables, and the Rudolphine Tables differ?

The Alfonsine Tables are based on the Ptolemaic model. The Prutenic Tables are based on the Copernican model. The Rudolphine Tables are based on the heliocentric model with Kepler's three laws.

Which planet has the longest duration of retrograde motion as viewed from Earth? The shortest?

The duration of the retrograde motion is shortest for our neighbor Mars and generally longest for Neptune.

In Ptolemy's model, how do the epicycles of Mercury and Venus differ from those of Mars, Jupiter, and Saturn?

The epicycles of Mercury and Venus were attached to a line joining Earth and the Sun. The epicycles of Mars, Jupiter, and Saturn revolved around Earth independently of the Sun.

Does Tycho's model of the Universe explain the phases of Venus that Galileo observed? Why or why not?

Yes. In Tycho's model, all planets except Earth orbit the Sun, and the Sun orbits Earth once each day. Venus shows a full set of phases.

When Mercury is at aphelion

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