Major Astronomical Events of September 2023

September of the second year of full-scale invasion deserves the name “month of three comets”. We owe this to the Japanese astronomer Hideyo Nishimura, who, on August 12, 2023, first photographed a small nebula that turned out to be a new “tailed star”. It was soon calculated that on September 18, it would pass perigee (the point on its orbit closest to the Sun), and a few days before that, it would be visible in the evening navigation twilight, low above the northwestern horizon. Interestingly, the moderate latitudes of the Northern Hemisphere, including Ukraine, are possibly the best for observing Comet Nishimura. It might even be visible to the naked eye.

Comet Hartley 2 at its closest approach to Earth since its discovery in October 2010

Also this month, two short-period comets will be observed — Encke, whose periodicity was established back in the early 19th century, and Hartley 2, discovered in 1986. They will reach perigee in October, and in September, they will approach the minimum possible distance from Earth within a two-day interval. Their brightness might exceed the 9th or even the 8th magnitude, making them visible through small amateur instruments. Both will be observed in the pre-dawn sky.

September 1

The Moon is 4° south of Neptune (7.8 ᵐ)

September 4

The Moon is 2° north of Jupiter (-2.6 ᵐ)

September 5

The Moon is 3° north of Uranus (5.7ᵐ)

September 6

Mercury in lower conjunction with the Sun

September 7

The Moon is in last quarter phase

September 10

The Moon is 2° south of Pollux (Beta Geminorum, 1.2ᵐ)

September 11

The Moon is 10° north of Venus (-4.5ᵐ)

September 12

The Moon is at apogee, 406,288 km from Earth

September 13

The Moon is 3° north of Regulus (Alpha Leonis, 1.3ᵐ)

Comet Nishimura (C/2023 P1 Nishimura, 3.0ᵐ) is at 0.852 AU from Earth

September 15

New Moon

September 16

The Moon is 1° south of Mars (1.7ᵐ)

September 17

The Moon is 1° north of Spica (Alpha Virginis, 1.0ᵐ)

September 18

Comet Nishimura (2.0ᵐ) at perigee, 0.221 AU (33 million km) from the Sun

September 19

Neptune is in opposition

September 21

The Moon is 4° east of Antares (Alpha Scorpii, 1.0ᵐ)

September 22

The Moon is in first quarter phase

Mercury is in greatest western elongation (17°52’ from the Sun)

September 23

Autumnal Equinox

September 24

Comet Encke (2P/Encke, 9.0ᵐ) is at 0.901 AU (135 million km) from Earth

September 26

Comet Hartley 2 (103P/Hartley, 8.0ᵐ) is at 0.383 AU (57 million km) from Earth

September 27

The Moon is 4° south of Saturn (0.5ᵐ)

September 28

The Moon is at perigee, 359,910 km from Earth

The Moon is 2° south of Neptune

September 29

Full Moon