Jupiter and its four brightest moons as they appear in a modest telescope. Jan Sandberg
Jupiter and its four brightest moons as they appear in a modest telescope. Jan Sandberg

Jupiter's easy to find any August evening. It's the brightest object in the southern sky at nightfall. Small telescope owners can easily see its four brightest moons called the Galilean satellites. Although discovered by Galileo on January 7, 1610, it was German astronomer Simon Marius who proposed their names (suggested by Johannes Kepler) in 1614. For more than four centuries we've known them as Io, Europa, Ganymede, and Callisto.

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Sometimes all four are visible at the same time, but often one or two go "missing." Because they revolve around Jupiter like the moon does around the Earth, one or more of the satellites can pass either behind or in front of the planet where it's temporarily blocked from view. Jupiter's shadow also occasionally eclipses its moons. All of these antics are fun to see in a small telescope especially shadow transits. These occur when a moon crosses directly in front of the planet and casts a perfectly black, pinpoint shadow on its cloud tops.

Shadow transits are relatively common. After you've observed a few you can even tell one moon from the other by the size of its shadow. Europa, the smallest of the quartet, looks like a pinprick compared to Ganymede's BB-pellet-sized dot. Observers with larger instruments can also try to spot the moons themselves as they cross Jupiter's disk. They're small and blend in well with the clouds, making for an excellent observing challenge.

Tonight (August 14-15) is very special because both Io and Ganymede will casts their shadows on Jupiter at the same time — a double shadow transit! Each shadow will first appear along the eastern edge of the planet's disk and slowly move westward. Double transits are relatively rare so it's worth the effort to see if skies allow.

Here are the particulars from start to finish:

  • Ganymede's shadow enters Jupiter's disk at 9:31 p.m. Central Daylight Time
  • Io enters Jupiter at 11:08 p.m. CDT
  • Both shadows are visible simultaneously between 11:10 p.m. and 12:50 a.m.CDT(12:10-1:50 a.m. Eastern; 10:10-11:50 p.m. Mountain and 9:10-10:50 p.m. Pacific)
  • Ganymede's shadow departs Jupiter at 12:54 a.m. (Aug. 15)
  • Io departs Jupiter at 1:25 a.m.

This simulation depicts Jupiter, the double transit and the Great Red Spot (GRS) tonight around 11:30 p.m. Stellarium
This simulation depicts Jupiter, the double transit and the Great Red Spot (GRS) tonight around 11:30 p.m. Stellarium

You'll need a 4-inch or larger telescope and a magnification of 75-100x to see both shadows distinctly. Ganymede's dot will be much more obvious than Io's. Note their positions when you first see them, then come back a half-hour later and you'll immediately see how much they've moved. Jupiter's moons are forever revolving about the planet, dragging their shadows along with them. As a bonus, the Great Red Spot — a hurricane-like storm 1.3 times the diameter of the Earth — will be squarely in view in the planet's southern hemisphere around 11 p.m. Central Time. A 6-inch telescope magnifying at 100-150x will show it.

As a fun exercise, imagine looking up from your orbiting space capsule as you passed beneath the shadows of Io and Ganymede. What would you see? A total eclipse of the sun, of course! They happen at Jupiter too and with great regularity because it has a lot more moons. Finding a place to stand and observe them isn't easy though because Jupiter has no surface — only clouds.

If you rise for tomorrow morning's conjunction you'll also see Orion climbing up the southeastern sky. Yes, the hunter has returned! Stellarium
If you rise for tomorrow morning's conjunction you'll also see Orion climbing up the southeastern sky. Yes, the hunter has returned! Stellarium

Speaking of moons, our own moon huddles near the planet Venus tomorrow morning. They'll be about six moon diameters (3) apart and well-placed in the eastern sky at dawn. The same telescope you used on Jupiter will show a splendid, cratered moonscape on the lunar crescent, while Venus will appear exactly like a half-moon. Astronomers have identified about 1,000 craters on Venus using radar to penetrate its dense cloud cover. Mead, the planet's largest crater, measures 168 miles (280 km) across and was named for American anthropologist Margaret Mead.

Clear skies!