Name: ___________________________ ID: __________________
Astronomy 104
Observing Laboratory
Spring 2009
Observing Retrograde Motion This is a required exercise worth 40 points toward your final grade. Carefully read the lab and complete the exercise as instructed. If something is not clear, do not hesitate to ask the instructor. This lab is due Mar 19. 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 seem against the background stars. However, they occasionally (and predictably) appear to reverse direction and move east to west over consecutive nights as seen against the background stars. This apparent backward motion is called retrograde motion. For this lab, we will use data already collected by an observer. Table 1 gives the data of Mars positions observed at the 1st and 15th of each month from May to August. The positions are recorded in azimuth and altitude. The azimuth is measured along the horizon in degrees, starting at north, going through east, south, and west in a circle. The altitude is measured upward from the horizon in degrees. We will use the data to see how Mars has moved over the course of 4 months and examine how we can use a heliocentric model to explain its retrograde motion.
Table 1: Positions of Mars Azimuth (horizontal direction)
Altitude (vertical direction)
May 1
240
45
May 15 June 1 June 15 July 1 July 15 August 1 August 15
210 170 150 170 180 140 120
50 50 45 40 45 50 55
Date of observation
1
Name: ___________________________ ID: __________________ 1) Use the data in Table 1 to plot the motion of Mars on the graph provided in Figure 1. Write down the dates next to each position you plot. Then, draw a smooth line (or curve) connecting your data points to illustrate the path of the planet through the sky.
Figure 1
2
Name: ___________________________ ID: __________________ 2) Figure 2 is a heliocentric model showing Earth’s and Mars’s orbits. Since Mars is located farther from the Sun than Earth is, Mars moves more slowly than Earth in its orbit. For instance, in the time it takes Earth to move from its position 1 to 2, Mars will also have moved from its position 1 to 2, which is a shorter distance. The curves on the right are the background lines. This is where you will see background stars. With straight lines, connect all the same-numbered points along the two orbits, extending each line to intercept the same-numbered background line located on the right, and mark and label each point where your line crosses the background line. For example, draw a straight line from Earth 4 to Mars 4, extending it to the right until the line meets the curve labeled 4, and mark the position where they meet with a dot and a “4”. Repeat for all numbers. Draw a smooth curve through the dots you marked to show the path of Mars against the background sky.
Figure 2 3) According to Figure 1, on what date was Mars located farthest to the west? What was the azimuth value of Mars on this date?
4) According to Figure 1, on what date was Mars located farthest to the east? What was the azimuth value of Mars on this date?
3
Name: ___________________________ ID: __________________ 5) After completing Figure 1, you will find that Mars was moving in different directions at different times. List the dates Mars moved from east to west, and the dates Mars moved from west to east, respectively.
6) During which dates does Mars appear to move with normal, prograde, motion, as compared to the background stars? In what direction (east-to-west or west-to-east) does Mars appear to be moving relative to the background stars during this time?
7) At what positions in Figure 2 do the dates in Question 6 correspond?
8) During which dates does Mars appear to move with backward, retrograde, motion, as compared to the background stars? In what direction (east-to-west or west-to-east) does Mars appear to be moving relative to the background stars during this time?
9) At what positions in Figure 2 do the dates in Question 8 correspond?
10) 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?
11) 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, that means that 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.
4
