Comet C/2023 A3 to light up sky, visible to naked eye starting Oct 9

Just as astronomers and skywatchers are getting ready to witness and take pictures of Comet C/2023 A3 (Tsuchinshan-ATLAS), which should be visible to the naked eye after sunset for a few weeks starting around Oct 9, the ATLAS survey telescope has discovered a comet that is getting closer to the sun.

Comet C/2023 A3, also known as Tsuchinshan-ATLAS, will be visible in the morning sky at magnitude 2.2. Starting on October 14, it will also be observable in the evening sky.

The comet will be in the far eastern region of the constellation Virgo on Oct 14. It will have moved across into Serpens on Oct 15, and for Eastern time zone observers, it will be located somewhat to the south of globular cluster M5.

Both objects could be seen simultaneously with binoculars if the comet is bright enough. Tsuchinshan-ATLAS's motion will locate it somewhat southeast of M5 in the western United States.

Although comet brightnesses are notoriously unpredictable, if the comet's main body sticks together, we may be in for a spectacular celestial show.

There are two reasons every spectator would be prudent to think carefully about when to view the comet.

The moon is the first. On Oct 17, it will be full, falling in the center of Tsuchinshan-ATLAS's optimal viewing window. After that date, each evening the satellite will rise later, which will greatly facilitate comet observation.

Secondly, the comet never reaches a high altitude in the sky. On Oct 15, Tsuchinshan-ATLAS will be 16° high in the west one hour after sunset. On October 18, it will rise to a more visible 25 above the horizon.

On Oct 19, Tsuchinshan-ATLAS will enter the constellation Ophiuchus, when the Moon will no longer obstruct vision.

The comet will be 28° high in the west and very near to the magnitude 3.8 star Marfik an hour after sunset. It is anticipated by observers to be far brighter than the star.

The comet will go across Ophiuchus until it lies between two of the brightest stars in that constellation on Oct 28: magnitude 3.7 Muliphen and magnitude 2.8 Cebelrai.

Both stars are separated by two degrees. An hour after sunset that night, the comet will be 40° high in the southwest.

The long-period comet C/2024 S1 (Atlas) was discovered on Sep 27, 2024, by astronomers at the ATLAS survey in Hawaii. It is a member of the Oort Cloud, a sphere that surrounds our solar system and is home to millions of comets.