Applet controls:

Buttons:

Step: takes a new observation
Reset: clear the data and image windows

Pull down menu:

sets the time elapsed between observations. You can change this in the middle of your observations, if you'd like.

Slider bars:

These control the parameters of your fitted sinusoids. There are four sets from top to bottom, one for each moon, and each set consists of:

a:     the distance of the moon from Jupiter (this controls the height, or amplitude, of the sine wave)

P:     the period of the moon's orbit (this controls the period of the sine wave)

Shift:    this moves the whole sine wave to the left or right, so you can align your sine wave with the datasets (in the lingo, it's called a "phase" term)