Week 2 Lecture 1
Solutions at the bottom.

1)  In-video quiz at timepoint 02:29.

apparent retrograde motion - "from the Latin word retrogradus – "backward-step", the affix retro- meaning "backwards" and gradi to step or "to go". Retrograde is most commonly an adjective used to describe the path of a planet as it travels through the night sky, with respect to the zodiac, stars, and other bodies of the celestial canopy. In this context, the term refers to planets, as they appear from Earth, to stop briefly and reverse direction at certain times, though in reality they perpetually orbit in the same uniform direction."

Wikipedia, Apparent retrograde motion

2)  In-video quiz at timepoint 04:09.
Oops! Ronen credits Aristarchus with epicycles. As the subsequent slide states, he should have said Hipparchus. Aristarchus is later mentioned as being the first to propose a heliocentric model.

3)  In-video quiz at timepoint 04:49.

deferent - the base circular orbit of a planet about the Earth in the Ptolemaic model of the solar system. Planets were not thought to follow the deferent itself; instead the center of the planet's epicycle(s) traveled along the deferent.

epicycle - a circular planetary path in the Ptolemaic model of the solar system. The epicycle is smaller than the deferent and the center moves around the Earth along the deferent while the planet rotates about the center of the epicycle. Epicycles were proposed to explain the observed retrograde motion of the planets.

Wikipedia, deferent and epicycle

4)  In-video quiz at timepoint 08:43.
For more information on the heliocentric model, please refer to: 
http://astro.unl.edu/naap/ssm/heliocentric.html
http://www.youtube.com/watch?v=-hoI9510KPU

5)  In-video quiz at timepoint 14:23.

Imagine two planets that orbit the Sun, one with a period of 3 years for a full sidereal orbit, and the other with a sidereal period of 5 years. How long in years is the synodic period for the two planets (the time between alignments of the planets and the Sun)? Round your answer to two significant digits. 

[Open-ended]

Solutions:

5) (7.4999,7.5001)
By rearranging the formula, you can arrive at 1/S = 1/P1 - 1/P2, where S is the synodic period, P1 is the shorter sidereal period, and P2 is the longer sidereal period. Substituting the values we are given, 1/S = 1/(3 years) - 1/(5 years) = 2/(15 years). Taking the inverse of both sides, S = (15 years)/2 = 7.5 years. Because we are rounding to two digits, 7.5 years is the correct answer. Rounding to one significant digit would give us an answer of 8 years.