This is Dudley Observatory’s Skywatch Line for Monday and Tuesday, June 24 and 25, written by Alan French.
The Moon reaches last quarter on Tuesday morning and now rises after midnight, leaving the evening skies nice and dark.
Mercury and Mars are in the evening sky and are now fairly close together, but you’ll need a good view to the west northwest and clear, haze free skies to see the pair. If you look at 9:20 pm the pair will be about 8 degrees above the horizon, with Mercury brighter and a little higher and to the left of Mars. Don’t confuse them with another pair of stars, Castro and Pollux, to their north (right) and higher in the sky. By 9:30 pm Mercury and Mars will be only about 6 degrees above the horizon, but the western skies will be a bit darker. (A fist held at arm’s length spans 10 degrees across the knuckles.)
While Mercury and, especially, Mars, may be challenging, there is no missing Jupiter, shining like a beacon low in the south southeast at 10:00 pm. The solar system’s largest gas giant is highest and due south at 11:51 pm, when it will lie 25 degrees above the horizon.
Jupiter has a retinue of 79 known satellites or moons. Four are easily visible in virtually any telescope and are fascinating to watch as they appear to shuttle back and forth, alternately passing in front of and behind the planet. Any modest telescope can reveal the four Galilean moons, Io, Europa, Ganymede, and Callisto.
Through an astronomical telescope or spotting scope magnifying 30 to 60 power the four moons look like faint stars on either side of the planet. If you have a correct image spotting scope, such as used for birding or target shooting, and look at Jupiter around 10 pm Monday night you’ll find two moons to Jupiter’s left (east) and two to its right (west). The two to the left, in order of distance, are Europa and Ganymede, and to two to the right are Io and Callisto, with Callisto a considerable distance from the planet. An astronomical telescope may reverse the side of the planet they appear on. (Some astronomical telescopes give a mirror reversed view.)
At 10 pm Tuesday night Io and Ganymede will be to Jupiter’s east, fairly close together, and Europa and Ganymede will be to its west. Again, Callisto will be farther from the planet than the others.
If you don’t own a telescope but have binoculars, steadily held binoculars can reveal the four moons when they are well away from Jupiter.
None of the remaining 79 moons are within range of a typical amateur telescope. Himalia has been spotted by a handful of experienced amateurs with telescopes of 15-inch aperture or so.