Astronomy

Homework

# Scholars Online Astronomy - Chapter 4: 1-4: Planetary Orbits

## Homework

Reading: Astronomy, Chapter 4: Gravitation and the Motions of the Planets

• Section 1: Geocentric models dominated the Greek and Roman accounts of stellar motion, especially the theories of Eudoxus, Aristotle, and Ptolemy, who had to account for prograde and retrograde motion, differences in the apparent speed of the planets even in prograde motion, and changes in the brightness of the planets.
• Section 2: Copernicus' heliocentric model addressed the above issues by changing the relative positions of the sun and earth.
• Section 3: Brahe's observations of the supernova of 1572 and the comet of 1577 proved that stars changed and there were no physical spheres separating the planetary orbits.
• Section 4: Kepler effectively destroyed the last premise of classical and medieval astronomy by showing that planets move in ellipses with the sun at one of the foci, that thier velocity varies so that a line from the sun to the planet sweeps out equal areas in equal times, and that the period of the planet's orbit around the sun and radial distance of a planet from the sun are related as p2 = a3.

### Key Formulae to Know

Synodic (phase to phase as seen from Earth S) and Sidereal periods (complete revolution on orbit P)

• Inferior planets: add synodic and Earth inverses to get period inverse
• Superior planets: subtract synodic inverse from Earth's inverse to get period inverse

### Web Lecture

Read the following weblecture before chat: Aristotle, Ptolemy, Copernicus, Brahe: First steps to planetary theory, sections 1-4

### Study Activity

Planetarium program: Set your planetarium location to your home location, and center it on a point between Libra and Scorpius. Set the time to 1 June 2018, and advance the planetarium one day at a time until 15 August. When does Mars stop moving eastward along the ecliptic? How long is it in retrograde? When does it stop moveing westward? How long is it "stationary"? When does it start moving eastward again?

Optional websites:

Play with the animations at the astronomy website at the University of Nebraska-Lincoln.

• Select Venus on the Planetary Parameter drop down and start the animation. What pattern does the planet describe over time? Where is the sun as Venus moves around earth?
• Select one of the outer planets (Mars, Jupiter, Saturn), and rerun the animation. What happens to the pattern? Where is the sun as this outer planet moves around earth?
• Can this model account for change in brightness (planet moves nearer/farther from earth)?
• Can this model account for stationary and retrograde motion?
• Can this model account for variation in the rate of progress of the planet relative to the background sky as seen from earth?

### Chat Preparation Activities

• Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
• Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
• Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
• Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

### Chapter Quiz

• There is no chapter quiz yet -- we haven't finished the chapter.

### Lab Work

Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.