Observing Retrograde Motion One of the most difficult parts of constructing an a
ID: 108929 • Letter: O
Question
Observing Retrograde Motion
One of the most difficult parts of constructing an accurate model for planetary motions is that planets seem to wander among the stars. During their normal (or prograde) motion, planets appear to move from west to east over many consecutive nights as seen against the background stars. However, they occasionally (and predictably) appear to reverse direction and move east to west over consecutive nights is seen against the background stars. This backward motion is called retrograde motion.
1) Given the data in Table 1, plot the motion of the mystery planet on the graph provided in Figure 1 (record dates next to each position you plot). Then, draw a smooth line (or curve). Using your data points, illustrate the path of the planet through the sky.
Table 1 Mystery Planet Positions
Date of Observation
Azimuth (degrees)
(horizontal direction)
Altitude (degrees)
(vertical direction)
May 1
240
45
May 15
210
50
June 1
170
50
June 15
150
45
July 1
170
40
July 15
180
45
August 1
140
50
August 15
120
55
2) On what date was the mystery planet located farthest to the west? What was the azimuth value of the planet on this date?
3) On what date was the mystery planet located farthest to the east? What was the azimuth value of the planet on this date?
4) Describe how the mystery planet moved (east or west), as compared to the background stars, during the time between the dates identified in Questions 2 and 3
5) During which dates does the mystery planet appear to move with normal, prograde, motion, as compared to the background stars? In what direction (east-to-west or west-to-east) does the planet appear to be moving relative to the background stars during this time?
6) During which dates does this mystery planet appear to move with backward, retrograde, motion, as compared to the background stars? In what direction (east-to-west or westto-east) does the planet appear to be moving relative to the background stars during this time?
7) If a planet were moving with retrograde motion, how would the planet appear to move across the sky in a single night? Where would it rise? Where would it set?
8) Suppose your instructor says that Mars is moving with retrograde motion tonight and will rise at midnight. Consider the following student statement:
Student: Since Mars is moving with retrograde motion, during the night it will be moving west-to-east rather than east-to-west. So at midnight it will rise in the west and move across the sky and then later set in the east.
Do you agree or disagree with the student? Explain your reasoning.
Date of Observation
Azimuth (degrees)
(horizontal direction)
Altitude (degrees)
(vertical direction)
May 1
240
45
May 15
210
50
June 1
170
50
June 15
150
45
July 1
170
40
July 15
180
45
August 1
140
50
August 15
120
55
60 50 45 40 35 30 E 25 20 15 10 90 110 130 50 170 190 210 230 250 270 Azimuth East West Figure 1Explanation / Answer
Ans.
Q-8:
Ans. I disagree with the student. The reason is given above in the question- 7.
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