This is Dudley Observatory’s Skywatch Line for Friday, February 26, through Sunday, February 28, 2016, written by Alan French.
Reaching full last Monday, a waning gibbous Moon rises late in the evening this weekend. The Moon rises at 9:36 PM on Friday, 10:33 PM on Saturday, and 11:29 PM on Sunday. It reaches last quarter this coming Tuesday.
Bright Jupiter now rises in the east close to 6:30 PM and is due south and highest just before 1:00 AM. Although detail on the planet is best observed around 1:00 AM, when the telescope is looking highest and through the thinner possible layer of our atmosphere, its four bright Galilean moons are easy to view during the more agreeable evening hours.
Even a modest telescope will show the four largest moons, Io, Europa, Ganymede, and Callisto. The moons look like stars, roughly in a line, and may lie on either side of the planet. From our viewpoint, as they orbit Jupiter, they appear to shuttle back and forth, passing in front of the planet as they move from east to west and behind it as they move from west to east. (The other 63 moons orbiting Jupiter are essentially beyond the reach of amateur telescopes, although Himalia is within the reach of experienced amateur astronomers with large telescopes.)
The moons themselves can be hard to see when they cross in front of Jupiter, but they cast shadows onto Jupiter’s cloud tops, and these inky black spots are fairly easy to spot through a telescope magnifying about 60 times, typical of many spotting scopes and small astronomical telescopes. Larger telescopes and higher magnifications are needed to see the moon itself in front of Jupiter – and even then the moon can prove hard to see.
At 9:30 on Friday evening you’ll find three moons stretched out to one side of Jupiter and one lone moon on the opposite side. The string of three moons, in order of their distances from Jupiter, consists of Io, Europa, and Ganymede. The lone moon is Callisto.
At 9:30 PM on Saturday night you’ll see only three moons, and one may be hard to spot. Io and Callisto will be on one side and the third, Ganymede, will be on the same side by very close to Jupiter, having just emerged from behind the planet, and perhaps hard to spot. Europa is passing behind the planet and out of sight. It will emerge, on the east side, at 9:08 PM.
If you stay up late, the shadow of Io will move onto Jupiter at 11:06 PM, appearing as an inky black dot. Io itself will move in front of Jupiter at 11:20 PM. Although we’ve said the moons can be hard to spot in front of the planet, it’s often easier to see one just as it moves in front of Jupiter.
If you look at 9:30 PM Sunday night you’ll see only three moons, forming a triangle, to one side of the planet. The closest to Jupiter is Europa, the brighter of the other two is Ganymede, and the third is Callisto. Io will be passing behind the planet and out of sight.
It’s fun and easy to watch the dance of the Jovian moons. I hope you’ll try it!