Observing the Moons of Jupiter
Project for Elementary Astronomy
In this project you'll follow in Galileo's footsteps, using a small telescope to observe
the motions of the moons of Jupiter over the course of a few nights.
For this project you need suitable equipment:
To conduct this observing project, follow this procedure:
- Telescope. Almost any telescope will do--it doesn't have to be fancy or expensive.
An excellent option is the new Galileoscope kit,
which is affordable ($50) and gives you a chance
to assemble the telescope yourself. If you already have a small telescope it will probably
work fine, but you should avoid high-magnification eyepieces. An ideal magnification is
about 15 to 50. Unless you are already an experienced observer, you should show your
telescope to your instructor to determine whether it is suitable.
- Tripod or other mount. This is absolutely essential. At the magnifications required
for this project, you can't see anything in a hand-held telescope because the shaking of your
hands will also be magnified. For the Galileoscope, you can get by with a discount store
tripod that costs about $25.
- Chair. Depending on the design of your telescope and mount, you may need to sit during your
- Clock or watch. You'll need to keep track of the time of each of your observations.
- Pencil, paper, clipboard, and flashlight for making sketches and notes.
For your report, analyze and discuss your observations as follows:
- Find a location where you have a clear view to the south. Your observing location
must be free of bright lights shining directly at you, but a moderate amount of urban
light pollution shouldn't be a problem.
- Prepare some sheets of paper with pre-drawn circles, about three inches in diameter.
Each of these circles will hold one sketch of what you see through the telescope, with
the edge of the circle representing the edge of the telescope's field of view.
- During the day, practice pointing and focusing your telescope by aiming it at a distant
mountaintop. Also make an approximate measurement of your telescope's field of view, by
comparing the width of the field to the width of the moon (half a degree), or to the
width of your little finger tip held at arm's length (about one degree).
- On a clear evening, set up your telescope and point it at Jupiter, which should be very bright
and somewhere in the southern sky. Be sure that the telescope is focused. Center Jupiter in the
telescope's field of view, and look around it. Unless your timing is very unlucky, you should
see three or four faint starlike objects strung along a line with Jupiter. These are Jupiter's
- Carefully sketch the appearance of Jupiter and its moons on one of your note pages.
Be sure to accurately represent the relative positions of Jupiter, the moons, and the edge
of your telescope's field of view. Also keep track of which way is "up" in the telescope's
field (which may be upside-down compared to the actual sky). If any stars are also visible, include those in your
sketch as well. Next to your sketch, write the date and time.
- Repeat your observation and sketch every half hour for a couple of hours. You should
find that at least one of Jupiter's moons moves noticeably, in relation to the Jupiter and
the others, during this time.
- After the first two hours you can gradually lengthen the interval between your
observations. But try to make enough observations that you can keep track of which moon
is which. You should observe for one full night if possible.
- Make some additional observations and sketches over the next few nights. You don't
need to stay up all night for more than one night; the idea here is to try to follow the motion of
Jupiter's slower-moving moons.
- Include all your sketches in your report. Also, if possible, include a photograph of
your telescope and other equipment. Describe your telescope and other equipment in any case.
- By comparing Jupiter's width to the field of view of your telescope, estimate Jupiter's
approximate angular size. (You should find that it is only a small fraction of a degree.)
- Describe the apparent motions of Jupiter's four moons, based on your obsevations.
Were you able to keep track of all four moons? Explain in as much detail as you can, with
reference to your sketches.
- For each of Jupiter's four moons, estimate both the time to orbit Jupiter once and the
maximum angular distance from Jupiter. These estimates should be based entirely on your
observations. Explain how you made each estimate.
- Look up the official values of the orbital periods of Jupiter's four moons, and compare
to your estimates.
- Optional: Look up the distance to Jupiter and orbital radius of each of Jupiter's four moons.
Use this information to check your estimates of the maximum angular separation between
Jupiter and each of its moons.
- Discuss any other interesting experiences and observations.