JavaScript Utility: Jupiter's Moons

Sky & Telescope's JavaScript utility, which will open in a new browser window, shows the positions of the four Galilean satellites for any date and time from January 1900 through December 2100.

Here is what the routine looks like:

The display has several parts. At top is a diagram showing the positions of Io (I), Europa (E), Ganymede (G), and Callisto (C) with respect to Jupiter. Below the graphic are three buttons you can use to change the orientation of the diagram to match the view in your telescope. "Direct view" puts celestial north up and celestial east to the left; the routine opens in this orientation, which is the one used in most star atlases. "Inverted view" puts south up and west to the left, matching the view seen in a Newtonian reflector in the Northern Hemisphere. "Mirror reversed" puts north up and west to the left, matching the view in most catadioptric (mirror-lens) and refractor telescopes used with a star diagonal in the Northern Hemisphere.

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.

Our Jupiter's Moons JavaScript uses Universal Time (UT, the same as Greenwich Mean Time), and to the right of the date and time boxes it shows what we think is the offset between UT and your local time, based on your computer's current settings. When changing the time manually using the Time input box, enter the Universal Time that corresponds to the local time when you will be observing.

Basic Data. The next part of the JavaScript provides some basic data of interest to visual observers of Jupiter. First is the planet's apparent brightness, expressed in visual magnitudes. For reference, the brightest star in the night sky, Sirius, shines at magnitude –1.4. A difference of 1 magnitude corresponds to a brightness ratio of about 2½, so when Jupiter shines at its brightest, around magnitude –2.4, it's about 2½ times more brilliant than Sirius.

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 Earth–Sun 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."

Satellite Phenomena. The bottom section of the JavaScript displays a table of Jovian satellite phenomena, including transits of the moons (and/or their shadows) across the planet's disk, occultations of the moons behind the planet, and eclipses of the moons by the planet's shadow — all of which are also shown in the diagram at the top of the JavaScript. The calculated times of satellite events should be good to within a few minutes.

When you first launch the JavaScript, it displays a table of satellite phenomena for the current date. If you change the date, either by entering a new value in the Date box and clicking on the Recalculate button, or by clicking on the "+1 day" or "–1 day" buttons, the table of satellite phenomena remains unchanged. To update the table, click on the dark gray button labeled "Display satellite events on date above" to the left of the table. Depending on the speed of your personal computer, it may take a few seconds for the recalculated table to appear.

Whenever you change the date and recalculate the table of satellite phenomena, the diagram showing the positions of Jupiter's moons gets updated too.

• Launch S&T's Jupiter's Moons JavaScript

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.