ASTR 390 – Motion of a Bright Planet
Goal: To chart the gradual motion of a bright planet through the stars of a constellation.
Materials: Planisphere, pencil, paper
Observations: every few evenings for several weeks
Background: Ancient sky watchers noticed 5 stars that looked exactly like stars but did not stay fixed against the backdrop of other stars. These they called “planets”, or “wanderers.” Now we know that these objects are earth-like masses shining in reflected light from the sun, and that they travel on elliptical orbits around the sun with periods ranging from 88 days for Mercury to 29 years for Saturn. This intrinsic orbital motion, plus the motion of the earth, causes the planets to drift among the stars.
For instance, Pluto’s position among the stars is plotted here for 2002-2004. Pluto’s actual velocity is slow (its period is 248.5 years) so the loops are caused by Earth’s 1-year motion, and the drift from loop to loop is caused by Pluto’s motion.
Procedure: Your instructor will assign you a planet. Sketch the star field around the planet, on a full sized sheet of paper. Every few nights, plot the planet’s position among the stars. Try hard for accuracy in placement. Label dates.
For accuracy: (1) To avoid having “tilt” problems, realize that the stars rise about 4 minutes earlier each night, so it is best to make your sketch [4 minutes times the number of days since your first observation] earlier on all sketches after the first. (2) To avoid biasing yourself by looking at previous measurements, make several photocopies of your original star map. Make a position measurement on a fresh page, then later transfer that point to your original master map.
Summer 2003 Instructions:
Your target this summer is Mars. Mars and Earth are both on the same side of the sun, and Earth “catches up” with Mars Aug 28. This is called opposition, and it is the closest Martian opposition in a long time.
Mars will be in Aquarius somewhere. It will be bright and will not appear on this chart, reproduced from your Field Guide book.
Make a larger, cleaner version of this chart using the stars that you actually see, then plot Mars’s position every few nights, like the example below.
In your report, (1) explain why Mars moves, (2) tell if Mars moves steadily, or if it appears to be accelerating in some way, (3) tell how Ptolemy would explain your observations, (4) tell how Copernicus would explain your observations.