Skywatch Line for Monday and Tuesday, December 15 and 16, 2025, written by Alan French
This is Dudley Observatory’s Skywatch Line for Monday and Tuesday, December 15 and 16, 2025, written by Alan French.
The Sun rises at 7:19 A.M. on Monday and sets at 4:22 P.M. On Tuesday it rises at 7:20 A.M. and sets at 4:22 P.M.
The Moon was at last quarter this past Thursday afternoon and now approaching new. It rises at 4:19 A.M. on Tuesday and an hour before sunrise, at 6:19 A.M., it will be 15 ½ degrees above the southeastern horizon. Its visible face will be 11% in sunlight. On Wednesday it will rise at 5:23 and an hour before sunrise will find it 7 degrees above the southeastern horizon and 6% in sunlight. By 6:35 it will be almost 9 degrees high and by 6:50, a half hour before sunrise, it will be approaching 11 degrees. The Moon reaches new Friday evening.
The ISS (International Space Station) crosses the sky twice on Tuesday morning. On the first pass it emerges into view from the Earth’s shadow at its highest point and less than half of its passage is visible. We see satellites by reflected sunlight. If they are in the Earth’s shadow, they are invisible. On the second pass the space station emerges from the shadow when low in the sky and we see it cross most of the sky.
Before 5:05 A.M. on Tuesday look for the constellation Boötes in the eastern sky. Look for orangish Arcturus, the brightest star in Boötes, 41 ½ degrees above the eastern horizon. Arcturus, as magnitude -0.05, is the fourth brightest star in the night sky and the brightest star in the sky’s northern hemisphere, and has no rivals nearby, so it is easy to spot. The stars of Boötes form a kite-shaped pattern stretching north (left) from Arcturus.
The ISS will emerge from the Earth’s shadow just over 12 degrees south (right) of Arcturus just before 5:06. (A fist held at arm’s length spans 10 degrees.) It will quickly brighten to magnitude -2.6 and will be moving toward a passage just below Arcturus so it should catch your eye. By 5:06 the ISS will be passing below Arcturus. It will be headed downward toward the north northeastern horizon and at 5:07 will pass below Corona Borealis, the Northen Crown. By 5:08 the ISS will be passing through southern Hercules and below the well-known “Keystone” formed by four of the constellation’s stars. It will be too low to see around 5:10.
The second pass emerges from the Earth’s shadow very low in the west just before 6:39 A.M. Tuesday and the actual emergence will be harder to catch. It will be headed roughly north and will pass well below bright Jupiter. Its path with then take it through Gemini above the twin’s feet. By 6:40 it will be in Gemini.
By 6:41 the ISS will be passing just above Auriga, the Charioteer, toward the northwest. Capella, the brightest star in Auriga, is 19 ½ degrees above the northwestern horizon and the space station will pass above Auriga. By 6:42:40 (HH/MM/SS) it will be passing above the familiar “W” star pattern of Cassiopeia, low in the north. Its journey will then take the ISS through Cepheus and then through Cygnus as it moves down to the northeastern horizon. By 6:47 the ISS will be gone from sight. At its brightest it will shine at magnitude -2.9.