Astronomy Guide September 2023

Dear Stargazer Friends,

I hope you have been well. September is here and with it the Autumnal Equinox on 23 September 2023. The heralds of autumn are already visible in early September. The summer constellations have notably shifted towards the West. In the East and Northeast, the Andromeda Group is now prominently visible with the primary constellations of Andromeda, Perseus, Cassiopeia, Cepheus and Cetus. All constellations with hundreds of notable dark sky objects and fascinating stars. Read more in the “Topic of the Month” chapter – this month it includes some Greek Mythology. 

On 11 August 2023, Japanese amateur astronomer Hideo Nishimura discovered a “new” Comet – C/2023 P1 (Nishimura). Closest approach to Earth is expected to take place on 13 September 2023 at a distance of 128 million kilometers. Maximum brightness is expected to occur on 17 September when it will appear in the constellation of Virgo. For the latest updates on the comet please go to The Sky Live.

All information on Planets and celestial phenomena not to miss in September see below. 

Enjoy the journey & clear skies!
Isa 

During the fall equinox on 23 September 2023, the Sun crosses the celestial equator, an imaginary line in the sky directly above the Earth’s equator. In the Northern Hemisphere this phenomena marks the beginning of autumn, while in the Southern Hemisphere it is the beginning of spring. The term “equinox” is derived from Latin meaning “equal night”. During the fall equinox, the duration of daylight and darkness is approximately the same all around the world. The Earth’s axis is tilted neither toward nor away from the Sun during this time, causing the Sun’s rays to strike the Earth’s surface almost directly over the equator. After the equinox, the Northern Hemisphere starts tilting away from the Sun, leading to shorter days. 

The heralds of autumn are visible in the night sky already early September. Although the summer constellations are still dominating the view especially around midnight, the “summer triangle” with Vega, Deneb and Altair has notably shifted towards the West. Deneb is now almost at the zenith while Vega and Altair have already crossed the meridian. In the South, just above the horizon, the constellation of Capricornus is crossing the meridian, followed by Aquarius in the southeast where Saturn can currently be observed. In the East and Northeast, the Andromeda Group is now prominently visible with the primary constellation of Andromeda, Perseus, Cassiopeia, Cepheus and Cetus. 

In Greek mythology, Andromeda was the daughter of King Cepheus and Queen Cassiopeia of the Kingdom of Aethiopia. Cassiopeia, who was known for her vanity, boasted that she and her daughter were more beautiful than the Nereids, sea nymphs who were the daughters of Nereus, “the old Man of the Sea”. Angered, the Nereids asked Poseidon, King of the sea, to punish Cassiopeia for her arrogance. Poseidon sent the sea monster Cetus to ravage the coastlines of Aethiopia as a punishment. To appease Poseidon, King Cepheus and Queen Cassiopeia sent a messenger to Delphi to consult the Oracle. The Oracle revealed that to stop the destruction, they must sacrifice their daughter Andromeda to Cetus.

Andromeda was then chained to a rock by the sea as a sacrificial offering to Cetus. At that moment, Perseus flies near the coast and upon seeing Andromeda bound to the rock, falls in love with her. He secures Cepheus’ promise of her hand in marriage if he can save her. Perseus then kills the sea monster Cetus with his sword and by using the head of Medusa, which could turn creatures to stone, rescuing Andromeda. They then married and lived happily together. According to the Catasterismi, Andromeda is placed in the sky by Athena as the constellation of Andromeda, in a pose with her limbs outstretched, similar to when she was chained to the rock, in commemoration of Perseus’ bravery in fighting the sea monster.

For us night sky observers, the constellations of Andromeda, Perseus, Cassiopeia and Cepheus offer many interesting objects. In Cepheus, look out for Alderamin (α Cephei), a white class A star near the northern pole, “only” 49 light years away from Earth. In the year 7’500 it will be the next North Star due to Earth’s precession. A well-known star in Cepheus is “The Garnet Star” (Mu Cephei or μCephei), a red supergiant at the edge of famous Elephant’s Trunk Nebula (IC 1396). The constellation of Cassiopeia contains many notable deep sky objects such as Messier 52, an open star cluster, as well as Messier 103, also an open star cluster. Another beautiful open cluster is NGC 7789 (Caroline’s Rose) discovered in 1789 by Caroline Herschel, sister of William Herschel. The Heart and Soul Nebulae are very notable, as well as the supernova remnant Cassiopeia A, and the “Pacman nebula” (NGC 281), a bright emission nebula and part of an H II region in Cassiopeia. 

Just below is the constellation of Perseus with many famous deep sky objects such as the Double Cluster, the California Nebula (NGC 1499), and the Little Dumbbell Nebula (Messier 76). The Double Cluster (Caldwell 14) consists of the open clusters NGC 869 and NGC 884 (h Persei and χ Persei, respectively). Both are well visible with the naked eye. 

The constellation of Andromeda not only consists of the Andromeda Galaxy (Messier 31), but according to The Sky Live at least 197 other deep sky objects, among them open star clusters such as NGC 752, and NGC 7662 (Copeland’s Blue Snowball), a Planetary Nebula. 

For more information I recommend visiting The Sky Live 

Sources and further reading: 

• «Kompendium der Astronomie. Einführung in die Wissenschaft vom Universum», Hans-Ulrich Keller, 6. Ausgabe, 2019, Kosmos Verlag, Stuttgart.
• ChatGPT
• Wikipedia
• The Sky Live