University of Michigan - Department of Astronomy


Retrograde motion of Venus

Venus favors the bold




The ecliptic crosses the sky from the eastern to the western horizon (though not from due east to due west). Your latitude determines how far north or south the ecliptic is. It is defined as the path of the Sun through the stars, or the projection of the Earth's orbit on the stars. The Sun, Moon, planets, and many asteroids move to the east or toward the west along it. Most objects travel eastward most of the time, so this is considered the "normal" direction and is called prograde. Occasionally, they will appear to stop, turn around and go backwards for a sort time. This is called retrograde motion.

Retrograde motion was incredibly difficult but not impossible to explain in the geocentric universe. Explaining the behaviors of the planets required more and more complicated sets of eccentrics, deferents and epicycles. However, when Kepler finally devised his laws of planetary motion, retrograde motion became a simple phenomena explained simply as the appearance of backward motion due to the Earth passing or being passed by the other planet.

In this lab, you will observe retrograde motion of Venus in the planetarium.

Part 1:Overview

The first thing the planetarium does is move to our latitude (42.3º north) and a date and time just before Venus next goes into retrograde motion. 

Observe Venus go through retrograde.

  1. What horizon is Venus over at the beginning? (e.g. N, E, NE, ENE,…)

  2. Sketch the shape of the path Venus followed

  3. What horizon is it on at the end?

  4. In general, what direction did Venus move, toward the east or toward the west?

  5. Did the stars move?

Part 2: Details

Now you'll watch it again, but the planetarium will stop at selected points for you to observe the position of the Sun on the ecliptic.

  1. What does the position of the Sun on the ecliptic tell you?

  2. What was the date at the beginning?

  3. What was the date when Venus began retrograde (approximately)?

  4. What was the date when Venus stopped retrograde (approximately)?

  5. What was the date at the end of the demonstration?

  6. How long did it take Venus to go through retrograde?

Concluding questions

  1. Can retrograde motion be observed in a single evening? Explain.

  2. Does the Sun ever go through retrograde?  Why or why not?

  3. Why did we observe Venus instead of one of the outer planets? 

  4. Label the sketch you did in part 1 with 4 positions: (1) the beginning, (2) the start of retrograde, (3) end of retrograde, and (4) end of the demo. In the figure below, sketch and label the relative positions of the Earth and Venus for the same positions.

Last modified: 8/17/05

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