Fall has officially arrived in September 2023 bringing a new set of celestial events to observe in our night sky. At the top of the month, the Moon makes a close approach to open star cluster Pleiades, the Seven Sisters. Then, in its morning apparition, Venus shines brightest in the sky reaching magnitude -4.5. Shortly after mid-month, Neptune reaches opposition and Mercury reaches its greatest western elongation. On the 23rd, the September Equinox marks the official first day of fall as the Sun crosses the equator and the month closes out with the Harvest Moon on the 29th.

(Looking to review last month's astronomy events? View August 2023)

Our highlighted list of astronomy events for September serves as your reference for key celestial sights to expect and to plan for.

6 Astronomical Events in September 2023:

Close approach of the Moon and Pleiades - 9/5
Venus at greatest brightness - 9/18
Neptune at opposition - 9/19
Mercury at greatest western elongation - 9/22
September equinox - 9/23
Full Harvest Moon - 9/29

For easy navigation, click on a specific event listed above to go directly to that section of the article. Below, we will explore each of these events in greater detail.

First, a few technical terms used in this article:

Magnitude - Magnitude is simply the measure of an object's brightness. The lower the number the brighter the object. Conversely, the higher the number the fainter the object. For example, a magnitude -7.2 object is brighter than a magnitude +3.6 object. Without optical assistance, the naked eye can see a celestial object as dim as roughly magnitude +6.0.

Right Ascension - Right ascension is the celestial equivalent of geographic longitude. It is measured from the Sun's position during March Equinox at 00h00m00s (h=hours, m=minutes, s=seconds). This measurement increases towards the east until it completes a full circle at 24h00m00s.

Declination - Declination is the celestial equivalent of geographic latitude. It is measured northward or southward of the celestial equator as degrees (°), minutes(') and seconds ("). For example, the celestial equator has a declination of 0°0'0", the north celestial pole has a declination of +90°0'0" and the south celestial pole has a declination of -90°0'0".

AU (astronomical unit) - AU is a unit of length that measures the distance from the Earth to the Sun.

Angular Size - Angular size refers to a celestial object's apparent size as seen from an observer on Earth. It is measured in arcminutes(') and arcseconds(").

Astronomy events to mark on your September 2023 calendar:

September 5th - Close approach of the Moon and Pleiades

Messier 45

On Tuesday, September 5th, the Moon and open star cluster Pleiades (also known as Messier 45 or M45) will make a close approach, passing within 1°06' of each other in the Taurus constellation. The pair will be at an angular separation of 102° from the Sun in the Leo constellation.

These celestial objects will be too widely separated to fit within the field of view of a telescope, but will be visible to the naked eye or through a pair of binoculars.

At a distance of about 444 light years, the Pleiades (M45) is the nearest Messier object to Earth, and is the most recognizable open star cluster visible to the naked eye in the night sky.

Moon & Pleiades close approach

Moon's magnitude: -12.1
Moon's right ascension: 03h48m50s
Moon's declination: +23°02'
Moon's angular size: 31'03"2
Pleiades magnitude: +1.3
Pleiades right ascension: 03h47m30s
Pleiades declination: +24°06'

September 18th - Venus at greatest brightness

Venus

Venus will reach its greatest brightness shining at magnitude -4.5 on Monday, September 18th. The morning star will be positioned in the Cancer constellation.

As the third brightest object in the sky after the Sun and Moon, you can easily spot this planet with the naked eye. However, this is also a great time to observe the planets details with even your small aperture telescope.

Venus at greatest brightness

Magnitude: -4.5
Right ascension: 09h06m10s
Declination: 11°31'N
Angular size: 38.3"

September 19th - Neptune at opposition

Neptune

Neptune will be at opposition and reach its highest point in the sky at about midnight local time on Tuesday, September 19th.

When a celestial object is "at opposition", this refers to its 180° position opposite to the Sun. This means that Neptune will rise at around the time the Sun sets, and set at around the time the Sun rises.

At the moment of opposition, Neptune will lie at its closest distance of 28.90 AU. This close approach to Earth is referred to as its perigee and makes the planet appear brightest to us.

Shining at magnitude +7.8, a moderate sized aperture telescope will be necessary in order for you to adequately observe.

Neptune at opposition

Magnitude: +7.8
Right ascension: 23h47m00s
Declination: 2°47'S
Angular size: 2.4"

September 22nd - Mercury at greatest western elongation

Mercury

Mercury will reach its widest separation from the Sun on Friday, September 22nd. The planet will be positioned 17° west of the Sun shining at magnitude +0.4 in the Leo constellation.

This is referred to as its greatest western elongation. These wide separation moments occur roughly once every 3-4 months.

Due to Mercury's close proximity to the Sun, the planet is often difficult to safely observe. Therefore, for stargazers, the exact dates of Mercury's greatest eastern and western elongation are significant moments that provide an opportunity for optimal observations.

At its greatest western elongation, Mercury will be visible before sunrise.

Mercury at greatest western elongation

Magnitude: -0.5
Right ascension: 10h51m10s
Declination: 8°10'N
Angular size: 7.1"

September 23 - September Equinox

September Equinox

The September Equinox marks the first day of autumn here in the Northern Hemisphere. In 2023, this day occurs on Saturday, September 23rd, when the length of day and night are equivalent at 12 hours each.

Subsequently, the days will continue to get shorter and the nights will become longer, until we reach the Winter solstice on December 21, 2023.

The exact moment of the September equinox occurs when the Sun appears to cross the celestial equator at 2:46am EST.

Sun at the moment it passes over the equator

Constellation: Virgo
Right ascension: 11h58m
Declination: +00°07'
Angular size: 31'52"

September 29th - Full Moon | Harvest Moon

Full Moon

On Friday, September 29th, the Moon will reach the full phase of its 29.5 day lunar cycle rising at around dusk and setting at around dawn. This astronomical event occurs each time the Moon's ecliptic longitude appears 180° away from the Sun's ecliptic longitude.

According to the Farmers' Almanac, traditionally the Full Moon that occurs closest to the fall equinox is referred to as the Harvest Moon. Unlike other full Moons, this full Moon rises at nearly the same time—around sunset—for several evenings in a row, giving farmers several extra evenings of moonlight and allowing them to finish their harvests before the frosts of fall arrive.

Full Harvest Moon

Constellation: Pisces
Right ascension: 00h23m30s
Declination: 0°41'N
Angular size: 33'02"
Distance from Earth: 361,000 km (224,315 miles)

Plan your observations

Now that you know which celestial events are occurring in the night sky this month, make sure to plan for your observation sessions. Some of the events discussed above can be seen without any special optical equipment. However, a quality pair of binoculars or a premium telescope will significantly enhance your viewing experience.

If you're in the market for purchasing or upgrading your astronomy gear, we have a curated selection of binoculars, telescopes and accessories from which to choose. Our online store offers high quality optics from industry-leading brands at value prices to help you explore the world above.

Feel free to browse our telescope collection or browse our binoculars.

If you are unsure where to begin, you may read our blog post How To Choose The Right Telescope or How To Choose The Right Binoculars for additional guidance.

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September 2023 Astronomy Events