Observing Projects
It is quite probable that within a few weeks of your reading this chapter one of the planets will be near opposition or greatest eastern elongation, making it readily visible in the evening sky. Select a planet that is at or near such a configuration by searching the World Wide Web or by consulting a reference book, such as the current issue of the Astronomical Almanac or the pamphlet entitled Astronomical Phenomena (both published by the U.S. government). At that configuration, would you expect the planet to be moving rapidly or slowly from night to night against the background stars? Verify your expectations by observing the planet once a week for a month, recording your observations on a star chart.
If Jupiter happens to be visible in the evening sky, observe the planet with a small telescope on five consecutive clear nights. Record the positions of the four Galilean satellites by making nightly drawings, just as the Jesuit priests did in 1620 (see Figure 4-17). From your drawings, can you tell which moon orbits closest to Jupiter and which orbits farthest? Was there a night when you could see only three of the moons? What do you suppose happened to the fourth moon on that night?
If Venus happens to be visible in the evening sky, observe the planet with a small telescope once a week for a month. On each night, make a drawing of the crescent that you see. From your drawings, can you determine if the planet is nearer or farther from Earth than the Sun is? Do your drawings show any changes in the shape of the crescent from one week to the next? If so, can you deduce if Venus is coming toward us or moving away from us?
Use the Starry Night™ program to observe retrograde motion. Select Favourites > Explorations > Retrograde from the menu. The view from Earth is centered on Mars against the background of stars and the framework of star patterns within the constellations. The Time Flow Rate is set to 1 day. Click Play and observe Mars as it moves against the background constellations. An orange line traces Mars’s path in the sky from night to night. Watch the motion of Mars for at least 2 years of simulated time. Since the view is centered on and tracks Mars in the view, the sky appears to move but the relative motion of Mars against this sky is obvious. (a) For most of the time, does Mars move generally to the left (eastward) or to the right (westward) on the celestial sphere? Select File > Revert from the menu to return to the original view. Use the time controls in the toolbar (Play, Step time forward, and Step time backward) along with the Zoom controls (+ and − buttons at the right of the toolbar or the mouse wheel) to determine when Mars’s usual direct motion ends, when it appears that Mars comes to a momentary halt in the west-east direction, and retrograde motion begins. On what date does retrograde motion end and direct motion resume? (b) You have been observing the motion of Mars as seen from Earth. To observe the motion of Earth as seen from Mars, locate yourself on the north pole of Mars by selecting Favourites > Explorations > Retrograde Earth from the menu. The view is centered on and will track Earth as seen from the north pole of Mars, beginning on June 23, 2010. Click the Play button. As before, watch the motion for 2 years of simulated time. In which direction does Earth appear to move for most of the time? On what date does its motion change from direct to retrograde? On what date does its motion change from retrograde back to direct? Are these roughly the same dates you found in part (a)? (c) To understand the motions of Mars as seen from Earth and vice versa, observe the motion of the planets from a point above the solar system. Select Favourites > Explorations > Retrograde Overview from the menu. This view, from a position 5 AU above the plane of the solar system, is centered on the Sun, and the orbits and positions of Mars and Earth on June 23, 2010, are shown. Click Play and watch the motions of the planets for 2 years of simulated time. Note that Earth catches up with and overtakes Mars as time proceeds. This relative motion of the two planets leads to our observation of retrograde motion. On what date during this 2-year period is Earth directly between Mars and the Sun? How does this date compare to the two dates you recorded in part (a) and the two dates you recorded in part (b)? Explain the significance of this.
Use Starry Night™ to observe the phases of Venus and of Mars as seen from Earth. Select Favourites > Explorations > Phases of Venus and click the Now button in the toolbar to see an image of Venus if you were to observe it through a telescope from Earth right at this moment. (a) Draw the current shape (phase) of Venus. With the Time Flow Rate set to 30 days, step time forward, drawing Venus to scale at each step. Make a total of 20 time steps and drawings. (b) From your drawings, determine when the planet is nearer or farther from Earth than is the Sun. (c) Deduce from your drawings when Venus is coming toward us or is moving away from us. (d) Explain why Venus goes through this particular cycle of phases. Select Favourites > Explorations > Phases of Mars and click the Now button in the toolbar. With the Time Flow Rate set to 30 days, step time forward, and observe the changing phase of Mars as seen from Earth. (e) Compare this with the phases that you observed for Venus. Why are the cycles of phases as seen from Earth different for the two planets?
Use Starry Night™ to observe the orbits of the planets of the inner solar system. Open Favourites > Explorations > Kepler. The view is centered on the Sun from a position in space 2.486 AU above the plane of the solar system and shows the Sun and the inner planets and their orbits, as well as many asteroids in the asteroid belt beyond the orbit of Mars. Click the Play button and observe the motions of the planets from this unique location. (a) Make a list of the planets visible in the view in the order of increasing distance from the Sun. (b) Make a list of the planets visible in the view in the order of increasing orbital period. (c) How do the lists compare? (d) What might you conclude from this observation? (e) Which of Kepler’s laws accounts for this observation?