University of Michigan - Department of Astronomy




Motions of the Inner and Outer Planets

Part 1: The Inner Planets

The planetarium show will follow the motion of Venus, stepping forward 30 days at a time and pausing. Since there are 365 days in a year and 360º in a circle, you can treat each dot on the ecliptic as approximately equal to 1º. Pay attention to the motion and positions of all the planets relative to the Sun and stars. You will sketch Venus' path, but there also will be questions about the other planets.

  1. Mark the positions of Venus with respect to the Sun in the box below. Use the frame of reference in which the Sun is always at the origin in this box. The numbers on the grid represent angular distance from the Sun in altitude - azimuth coordinates, and are not the numbers on the meridian and ecliptic. Note and label the approximate dates for which Venus is farthest from the Sun.
  2. How is Mercury’s motion different from all the other planets? Give at least 2 differences.

  3. Refer to Figure 2 in the Introduction, and label the sketch you made with the following positions: (1) the start of the show, (2) greatest eastern elongation, (3) greatest westward elongation, (4) inferior conjunction, (5) superior conjunction.


Part 2: Retrograde Motion

For the first part of this activity, your GSI will run a show that demonstrates the motion of Mars in the sky. Note that the Earth’s rotation is turned off, and so the stars and sky are not moving; this is similar to time-lapse photography, returning to look at the sky at the same time every night. Your GSI will tell you the starting and ending dates of the show.

  1. Make a rough sketch of the path Mars followed, including a few stars to show its motion relative to them. Indicate the direction of motion.

  2. Label the following positions in your sketch: (A) start of show, (B) start of retrograde, (C) end of retrograde, (D) end of the show, (E) opposition and (F) conjunction. If you cannot include any of the positions on the sketch, explain why not. Figure 5 of the Introduction may be helpful.

  3. What direction (eastward or westward) does Mars normally move relative to the stars? When in retrograde?

  4. Label your sketch in Question 4 above, with the positions shown in this diagram. Figure 5 may be helpful.

Concluding Questions

In the following, you may use sketches in your answers, or refer to figures from the Intro and Worksheet in this activity.

  1. How do the motions of the planets reveal whether they are inner vs. outer planets? Give at least two ways to distinguish them.

  2. Venus is often seen as the Morning Star or Evening Star. Can we ever see it at midnight? Explain.

  3. Jupiter is an outer planet. Can we see Jupiter at a position in the sky similar to the Morning Star or Evening Star? Can we ever see it at midnight? Explain.

  4. Can we see an occultation of Mars by the Sun? Can we see a transit of Mars in front of the Sun? Explain.

  5. Galileo first observed the phases of Venus in 1610. He observed that Venus goes through all the phases, including new, crescent, and full, yet never goes through opposition. Explain why this shows that the heliocentric solar system is correct, rather than the geocentric system.

Updated: 8/28/14 by SAM & MSO

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