Galilean Project Two: Jupiter

Possibility #1
moons' orbits are tilted
from Jupiter's orbital plane
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Galilean Project 2:
Jupiter
Possibility #2
moons' orbits are seen edge-on
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INTRODUCTION. In this Galilean exercise, you yourself will see Galileo's first TWO rebuttals to the popular knowledge which said that all the planets and stars revolved around the earth.

Jupiter has moons of its own!

Even with a very low power telescope or binoculars (that's all Galileo had!) you can see up to four moons in the vicinity of Jupiter. While Jupiter has a couple of dozen moons, only these four are large enough to see with low power from earth. They are sometimes called the Galilean moons in honor of their discoverer. Usually, however, you will see only three of them as one (or more) might be hiding out of sight behind or in front of the planet. The main thing that Galileo did that got him into trouble was that he showed that these moons revolved around Jupiter and not the earth - meaning that the earth is NOT the center of the universe. For you to show that these moons go around Jupiter will take a number of weeks as you keep sketches of where the moons appear EVERY NIGHT. And that leads to a near impossibility for most students because clouds often like to sneak in a block the view. So you will have to live atop a very tall mountain in a desert. Thus you will probably prefer to do the next Galilean "rebuttal".

But just in case you wish to tackle this project, here is a guide for things to look for. Initially you will want to number the moons from 1 to 4, with the innermost #1. Unfortunately, you will not know immediately what you are looking at because a moon that is actually far from Jupiter might be in a position in its orbit such that it will look almost in line with the planet from earth's point of view. But a few days later, it will have moved, and eventually it will reveal itself as being further away from the planet than any other moon ever gets. Thus it will be #4. Another thing you might look for is how fast a moon moves relative to the other moons. Knowing that close moons move faster than further moons will perhaps help you in your number system. (Of course, if you are lucky enough to have earth's view looking straight down from the top onto the Jupiter system, you will have no problem at all! But the chances are that you will be looking at the system at some odd angle!)

Side View →
Top View

Jupiter revolves around the sun and not the earth!

From the introduction page, you will recall that you will see phases if and only if the planet revolves around the sun and inside the orbit of the earth, and no phases only if the planet has an orbit exterior to that of the earth. Night after night, month after month, whenever you look at Jupiter, you will only see a circle and never a crescent. What is more is that you will never see a transit of Jupiter across the face of the sun, and transits are only seen of bodies that are inside the orbit of the earth. (When earth's moon transits the sun, we call it a solar eclipse. And relative to the moon, when the earth transits the sun, it is a lunar eclipse as the earth blocks the sunshine onto the lunar surface.)

Being Patient

For many of the sciences there could be an adage: "There ain't no instant gratification." You have seen how you must collect data for months to ascertain that what you are seeing of Jupiter's moons it there revolving around the mother planet. For many months all your sketches will show are four dots that move back and forth on a line. The only sense that you will be able to make out of that is that you have a side-view (or edge view) of their orbits - AND that does tell you something else! It tells you that the plane of the orbits of the Jovian moons is parallel with our own earth orbit.

Meanwhile you have been keeping track of whether Jupiter's disk ever shows hints of being a crescent. Just remember that Jupiter's year is much, much longer than an earth year. But why doesn't that matter? Why would looking at Jupiter for only 6 months be sufficient? (Humpf! You didn't expect to get only answers on this page, did you?-!)

"Ah, ha! Something for Instant Gratification!

If your optical device is 12-power or more you will be able to glimpse the cloud structures on Jupiter. They are lined up in the way the winds are blowing there. Are they or are they not lined up parallel to the plane of the moons. And what does your observation say about the position of the polar axis of Jupiter? What is the "tilt" of earth's axis? The tilt relative to what?

Outside Reading

You should read about the space exploration projects dealing with these Galilean moons. Some of these moons are so large that they have atmospheres. Indeed, they are so large that one might say that the Jupiter system is a multiple-planet system of its own! And even more interesting, there are ongoing expeditions to see if there is life on one of those Galilean moons!

A reference