This Interactive illustrates how the different planetary orbital velocities lead to “looping” or retrograde motion in the night sky. The Interactive will show the view at successive times from the Earth as well as from a “birds-eye” perspective in space. Users will be able to manipulate the size of planetary orbits, plus be able to view the retrograde motion from different perspectives, like what the retrograde motion of Earth looks like from Mars.

For the Greeks, the motions of the planets were most confusing; while the Sun and Moon always moved eastward night by night, on occasion the other five moving bodies would slow down, stop, and for weeks move westward against the background of stars. But over time, they recognized set patterns that did repeat.

 1 Venus always appears to retrograde when she: Need a Hint? A) goes behind the Sun at superior conjunction. B) comes out from behind the Sun into the evening sky. C) reaches greatest eastern elongation, high in the western evening sky, thens heads back toward the Sun. D) reaches greatest western elongation in the morning sky, and starts to head back toward the Sun.

For the Greeks, the motions of the fast moving inferior planets had a very different appearance than those of the slower superior ones. Contrast the retrograde motion of Venus in the previous question with the retrograde motion of Jupiter.

 2 Jupiter will appear to retrograde near: Need a Hint? A) greatest eastern elongation. B) oppostion. C) quadrature. D) inferior conjunction.

For the Greeks, the motions of the planets were most confusing; while the Sun and Moon always moved eastward night by night, on occasion the other five moving bodies would slow down, stop, and for weeks move westward against the background of stars. How Ptolemy attempted to solve this riddle would be one of the great scientific detective stories of all time.

 3 According to Ptolemy, what geometric device was needed in his model to produce retrograde motion? Need a Hint? A) Ellipse B) Eccentric C) Epicycle D) Deferent

As Copernicus studied the sizes of the retrograde loops for the superior planets, he came to a profound conclusion relating the loops to the ordering and distances of these worlds from the Sun.

 4 Copernicus found the retrograde loop the smallest, and thus the distance to the planet the greatest for: Need a Hint? A) Mercury B) Mars C) Saturn D) Neptune

From the Copernican, heliocentric perspective, retrograde motion can produce a rare but striking alignment for the inferior planets with the Earth. Millions of people observed this for Venus on June 8, 2004, and will be looking for it to happen again on June 6, 2012, and not again for over another century.

 5 When Mercury or Venus retrograde directly between us and the Sun, this is called a ____ and occurs at _____ . Need a Hint? A) solar eclipses, nodes. B) planetary eclipses, superior conjunction. C) transits, inferior conjunction. D) oppositions, quadratures.