Skywatch Line for Wednesday and Thursday, April 9 and 10, 2025, written by Alan French

This is Dudley Observatory’s Skywatch Line for Wednesday and Thursday, April 9 and 10, 2025, written by Alan French.

The Sun rises at 6:23 A.M. on Wednesday and sets at 7:31 P.M. On Thursday it rises at 6:22 A.M. and sets at 7:32 P.M. Thursday gained just under 20 minutes of daylight compared to last Thursday.

The Moon reached first quarter last Friday and is now moving toward full, so a bright, waxing gibbous Moon will dominate most of the night. At 8:30 P.M. Wednesday a 92% sunlit Moon will be toward the southeast. It will be due south and highest at 11:02 P.M. and will not set until 5:25 A.M. Thursday.

At 8:30 P.M. Thursday a 97% full Moon will be 29 degrees above the horizon toward the southeast. It will be 46 degrees above the southern horizon at 11:41 P.M. and will not set until 5:42 A.M. Friday. The Moon will be full on Saturday.

Jupiter, moving farther west each night, is lower in the west and sets earlier each night. Our fine window for observing Jupiter with a telescope is closing. The planet started April 60 degrees east of the Sun and will end it 45 degrees east. Jupiter will be easily visible in evening twilight as May begins, and seen with increasing difficulty as the month progresses.

At 9:30 P.M. Wednesday Jupiter will be 29 degrees above the western horizon, getting a bit low and visible through a thicker layer of our atmosphere, with the chances of catching fine detail in the clouds of Jupiter through a telescope diminishing. The four brightest moons, appearing to the east and west of the planet, remain nicely visible in even a modest telescope.

On Wednesday at 9:30 P.M. two moons will be to Jupiter’s east and two will be to the west. In order of their distance from the planet, closest to farthest, the moons to the east are Io and Callisto, the moons to the west are Ganymede and Europa. Since some telescopes reverse the view, Ganymede and Europa are closer together than Io and Callisto. There is also a fainter star, HD 32408, forming a triangle with Ganymede and Europa, that might be visible.

By Thursday at 9:30 P.M., three moons, Io, Europa, and Ganymede, will be to the west, with Io closest to the planet, and Callisto will still be to the east. Callisto is the outmost of the four Galilean moons, taking just under 17 days to make one orbit. Innermost Io takes 1.769 days, and changes the most during the night. Europa takes 3.551 days, and Ganymede takes a little over 7 days to make one trip around Jupiter.

Jupiter is now recognized to have 95 moons. Only the four largest and brightest are accessible through amateur telescopes. If you see another “star” near the planet, such as HD 32408 Wednesday night, it is a star that Jupiter happens to be passing nearly in front of.