Here is what the routine looks like:
Next comes the date and time; when the routine opens, it is initialized to the present (as determined from your computer's clock). Change the date and time by entering new values in the corresponding boxes and clicking the dark gray Recalculate button on the next row. Or click on the adjacent buttons to step backward or forward in increments of 1 day, 1 hour, or 10 minutes.
Next comes the angular diameter of Jupiter's disk in arcseconds, measured along the equator (the planet is noticeably "squashed" at the poles). There are 3,600 arcsec in 1°. The Moon spans about ½°, or 1,800 arcsec. Jupiter's angular size is typically between 30 and 45 arcseconds that's why you need a telescope to see any detail in the planet's cloudtops.
The next entry is Jupiter's distance from Earth, in astronomical units (a.u.). One a.u., based on the average EarthSun distance, is 149,597,870 kilometers or 92,955,807 international miles.
The last item in the list of basic data is the System II longitude of Jupiter's central meridian, the imaginary line down the middle of the planet's disk from pole to pole. This is useful as an indicator of whether you can see the Great Red Spot, which lies at a System II longitude of around 100°. For more information about observing this famous Jovian feature, see our companion article, "Transit Times of Jupiter's Great Red Spot."
Whenever you change the date and recalculate the table of satellite phenomena, the diagram showing the positions of Jupiter's moons gets updated too.
Once you have Jupiter and its moons in view, try to make a quick sketch of their relative positions. Do this on several consecutive evenings. By placing each drawing beneath the previous night’s, your series of sketches will resemble a page out of Galileo’s notebook. With these data in hand, see if you can estimate the orbital period of each satellite. These are the same data that Galileo had to work with in 1610 data that provided the first observational evidence supporting Copernicus’s assertion that the Earth is not the only center of motion in the solar system.