In Spring 2003 we will be able to observe several bright planets. Jupiter and Saturn are well-placed for viewing in the evening sky throughout the semester, while Mercury will make a brief appearance in mid-April. We will track Jupiter and Saturn to chart their motion over time.
Background Reading: Stars & Planets, p. 298 to 301 (The Solar System)
As seen from the Earth, the Sun, Moon, and planets all appear to move along the ecliptic. More precisely, the ecliptic is the Sun's apparent path among the stars over the course of a year. (Of course, it's actually the Earth that moves about the Sun, and not the other way around, but the result of our orbital motion is that the Sun seems to move against the stellar backdrop.) The planets don't remain exactly on the ecliptic, but they stay pretty close to it at all times.
Unlike the Sun, however, the planets don't always move in the same direction along the ecliptic. They usually move in the same direction as the Sun, but from time to time they seem to slow down, stop, and reverse direction! This retrograde motion was a great puzzle to ancient astronomers. Copernicus gave the correct explanation: all planets move around the Sun in the same direction, and retrograde motion is an illusion created when we observe the other planets from our moving point of view, the planet Earth.
It's easiest to understand the retrograde motion of Mercury and Venus. These inner planets are closer to the Sun than we are, and they orbit the Sun faster than we do. From our point of view, the Sun moves slowly along the ecliptic (due, of course, to our orbital motion), while Mercury and Venus run rings around the Sun. So at some times we see them moving in the same direction as the Sun, while at other times we see them moving in the opposite direction. For Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto the explanation is a bit more subtle. These outer planets are further from the Sun than we are, and they orbit the Sun more slowly than we do. When the Earth passes between one of these planets and the Sun, we see it going backwards because we're moving faster than it is.
When the Earth passes between one of the outer planets and the Sun, we see the Sun and the planet in opposite parts of the sky; the planet will rise about the time the Sun sets, remain visible all night, and set about the time the Sun rises. At this time, the planet is said to be in opposition to the Sun. Opposition is a good time to observe an outer planet; it's visible all night, and relatively close to the Earth.
An outer planet's apparent motion is always retrograde for a month or two before and after opposition. The length of time when a planet appears retrograde depends on the planet; it's shortest for Mars, and generally longest for Pluto. The moment when a planet's apparent motion changes direction is called a stationary point, because at that exact instant the planet appears to be stationary with respect to the stars. An outer planet always has one stationary point before opposition, and another stationary point after opposition.
As it turns out, both Jupiter and Saturn will be in retrograde motion at the start of the semester, and both will have turned around and gone back to normal motion by the end of the semester. Some positions of Jupiter and Saturn are shown in the star charts on the next page. In both charts, the ecliptic is shown as a slanting line; the normal direction of planetary motion is West to East, parallel to the ecliptic.
|Positions of Jupiter. The first stationary point is on 12/04/02, opposition is on 02/02/03, and the second stationary point is on 04/04/03; during this entire period, Jupiter's motion with respect to the stars is East to West (retrograde). The stars labeled with Greek letters are Gamma and Delta Cancri, and Lambda Leonis.|
|Positions of Saturn. The first stationary point is on 10/11/02, opposition is on 12/17/02, and the second stationary point is on 02/22/03; during this entire period, Saturn's motion with respect to the stars is East to West (retrograde). The stars labeled with Greek letters are Beta and Zeta Tauri.|
The two charts handed out in class on Feb. 4th should be used to plot the positions of Jupiter and Saturn. (You can get fresh copies by following the links below.) These charts show more stars than you can see with your naked eyes, but under typical urban conditions most of these stars will be visible with binoculars. Each chart has a scale of 2 cm per degree; thus, two stars separated by 0.5° in the sky appear 1 cm apart on these charts. At the top of each chart is an arrow pointing toward the North celestial pole. Finally, the three small crosses on each chart show the positions of Jupiter and Saturn during the first three classes of this semester.
Your assignment is to plot Jupiter and Saturn on these charts every time we observe. This should be pretty routine after a few weeks, and a reasonably complete set of plotted positions will nicely show the tracks of both planets as they end their retrograde phases and resume normal (West to East) motion along the ecliptic. Here's how to plot planetary positions:
The point of this exercise is to track the planets over the entire semester as they switch back from retrograde to normal motion. Don't worry if we miss a few observations due to bad weather; we can just pick up again when the weather improves. If you want, you can make additional observations whenever you have the chance. For example, if we miss an observation due to bad weather on Tuesday, you can go out the next clear night and fill in the gap (of course, always write the current date next to your mark).
Make the observations described in the section on TRACKING PLANETARY POSITIONS, and write a report on your results. This report is due on May 6th (the last class of the semester); if the weather is good that night, we won't collect the reports until you've had a chance to make one final observation. Your report should include, in order,
In more detail, here are several things you should be sure to do in your lab report:
These charts show 12.7° by 9.5° regions of the sky. If printed at a resolution of 100 dpi, the GIF images have an scale of 2 cm per degree; the Postscript plots should automatically print at this scale. The faintest stars shown are magnitude 8.5, which is about the limit for our binoculars from Kapiolani park.
Last modified: February 16, 2003