Planets, Stars, Galaxies, and the Universe



We're going to use the animation below to simulate taking observations of Jupiter and its Moons. For this exercise, we will be observing one of the outer moons, Ganymede. You will see Jupiter located in the center of the screen, and four dots which roughly line up horizontally across the screen (this is the plane in which the moons orbit). You cannot always see all four moons, sometimes one or two will be directly in front of or behind the planet.


Start the Revolutions of the Moons of Jupiter simulator (below) by entering the following information into the appropriate boxes:

  1. Enter your birthday month and day into the Month and Day boxes. (This just allows us to randomize the data a bit, so each student gets slightly different measurements.)
  2. Enter last year into the Year box.
  3. Enter 12:00 into the Time (UT) box.
  4. Increments should be set to 24 hours.
  5. Click on the Begin button and the view will change to show you Jupiter and the Galilean moons.
  6. Each time you click on the Next Interval button, it will advance by one day and show you the appearance of the moons 24 hours later.
  7. Click on all of the visible moons, and the lower portion of the screen will report information like the date and time of the observation, as well as the name of the moon. One piece of data it reports is the Julian Day, which is a method for recording the date. If day 1 is Julian day 2454431, then the next day it will be Julian Day 2454432. It will also report the moon’s position (X and Y), as well as its distance from Jupiter in units of Jupiter diameters. If the number is negative, it means it is to the left of Jupiter and if it is positive it is to the right. Distance is the value you are most interested in and will be recording.
  8. For week 1 of this lab, please click on "next interval" 20 consecutive times, and record the date and the separation between Jupiter and Ganymede (record—and save—the information in the format of your choice: old fashioned paper & pencil, a spreadsheet program, or any other easily accessible format). If you get a "Cloudy Night," you can count that as one of your 20 data points, but be careful to account for this in your data table.

Animation caption:  Simulator that displays Jupiter and its Galilean moons as observed from Earth on a given date and a give time.