Skywatch Line for Wednesday and Thursday, October 9 and 10, 2024, written by Alan French

This is Dudley Observatory’s Skywatch Line for Wednesday and Thursday, October 9 and 10, 2024, written by Alan French.

The Sun rises at 7:02 A.M. on Wednesday and sets at 6:22 P.M. On Thursday it rises at 7:03 and sets at 6:20. This Thursday has 20 minutes less daylight than last Thursday.

The Moon was new last Wednesday afternoon. On this Wednesday night a fat, crescent Moon will be low toward the south as the Sun sets. At 7:30 P.M. a 42% sunlit Moon will be just 16 degrees above the south southwestern horizon. It will set at 10:18. At 7:30 on Thursday evening the Moon will be toward the south, 19 degrees high, and 52% sunlit. It will set at 11:25.

The Moon reached first quarter, having completed one-quarter of its trip around Earth since new, at 2:55 Thursday afternoon and began moving toward full. It will reach full on Thursday, October 17.

Saturn now rises toward the east southeast at 5:12 P.M. By 8:00 P.M. the beautiful ringed planet is 27 degrees above the southeastern horizon and at 10:43 it is due south and highest, 39 degrees above the horizon. An astronomical or spotting telescope magnifying 60 times will show its rings,

Comet C/2023 A3 Tsuchinshan-ATLAS, which has been nicely visible from farther south in the morning sky, reached perihelion, its closest approach to the Sun, on September 27. It is now too close to the Sun to observe, but has been visible in SOHO’s camera. (SOHO is the Solar and Heliospheric Observatory, a European Space Agency spacecraft. It orbits the Sun-Earth L1 point, between the Earth and Sun, and has been studying the Sun for almost 29 years.)

By our next Wed-Thurs Skywatch line, Comet C/2023 A3 should be visible in our early evening sky. It will be 21 degrees above the west southwestern horizon on Wednesday, October 16, at 7 P.M. A bright, almost full Moon, will be low in the east. On Thursday night the comet will be 23 degrees high at 7 P.M. (The comet should first be visible in the early evening sky around October 12.)

Comet naming is interesting. Comets first get a provisional designation, starting with the year of discovery. This is followed by a letter indicating the half-month of discovery. The comet also gets a prefix to reflect its type. P/ indicates a periodic comet, a comet with an orbital period of less than 200 years or which has been seen at more than a single perihelion passage. A non-periodic comet gets a C/ prefix. There are a few other prefixes, such as I/ for an interstellar object.

Comets are also assigned a name by the International Astronomical Union, usually the names of their discovers, with a limit of three names. Comet C/2023 A3, Tsuchinshan-ATLAS, was named for the Purple Mountain Observatory in China and the Asteroid Terrestrial-Impact Last Alert System.