Retrograde Motion of Mars

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Science, Earth and Space Science

Grade 5- 8

Objective

Students will plot the path of Mars through the constellations.

Directions

Background
Ancient astronomers (astrologers) mapped the positions of planets moving gradually through zodiac constellations. They saw that planets moved across the sky from east to west each day. However, they also slipped eastward throughout the year. Since astronomers believed the planets were traveling around Earth, they assumed the paths would always move eastward, like the sun. What they observed was very different. The planets did move eastward, but from time to time they would slow down, stop, and then begin moving westward. Gradually, they would slow down again until they stopped and resumed their eastward motion again. Thus, they appeared to be making loops in their paths. The loops varied in size with each planet, and not every planet was looping at the same time. This was finally explained by the theory that the planets were traveling around Earth within their own circular orbits, called epicycles. Later, it was proved that the planets revolve around the sun, not Earth. Now, a different explanation for this backwards, or retrograde, motion was needed.
Mars will be used as the example in this activity to show students what the retrograde loop looks like when plotted against the constellations and why we see this happening.
  • Make transparencies of Mars Motion in 1971 and Explaining Retrograde Motion. Cut and assemble the latter according to the instructions.
  • Show the transparency of Mars Motion in 1971. Read the information about the diagram.
  • Have students refer to drawing and chart to answer the following questions:

  1. (Trace the path of Mars from #1 to #13.) Look at the path. What is the direction of Mars from #1 position to #4? (eastward)
  2. Describe what happened after that. (The planet began to move westward after #5 until #9 when it began to move eastward again.)
  3. Was there any change in Mars' appearance at this time? (Yes, it increased in size from #1 to its largest size on August 11 at position #7. It then began to become smaller)
  4. What happened to its brightness? (Mars became brighter as it grew in size and then dimmer as it reduced in size.)
  5. Why would Mars change in size and brightness? (As it gets closer to Earth, it would appear to increase in size and brightness.)
  • Show the students the transparency of Explaining Retrograde Motion of Mars. Point out the orbits of Mars and Earth are not centered on the sun. Show that Mars takes longer than 12 months to make one complete trip around the sun. Place a finger from each hand on the position 1 for both planets. Rotate Earth's orbit, keeping your fingers moving to 2. As you do this for all positions, pause to analyze the distances and direction of Mars. Notice that Mars appears to lag behind Earth (retrograde) from 4 until 10, after which Mars begins to catch up with Earth again.
  • Have the students find where Mars appears brightest (4 or 5). Dimmest (12).

Resources

overheads of Mars Motion in 1971 and Explaining Retrograde Motion of Mars (see the link below for these two information pages)

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