September observing guide

The following describes objects in our night sky for September and is especially written for beginning observers.

Astronomy from the city is not easy. Still, this month Saturn is in great position for observing, and even under city lights a small telescope will show the rings. A free app such as Stellarium will show you the position of Saturn rising after sunset.

Also see if you can pick out the brighter stars and constellations. The area around Scorpius, shown in the picture below, is overhead after Sunset. As the night progresses it approaches the western horizon. To the south – left as you look west, the Southern Cross and its pointers, α- and β-Centauri, are also low at this time of year.

Further and further from the city, in the suburbs and outer suburbs, as the sky gets darker you can see more. A decent pair of binoculars will give first views of most of the objects described here. Binoculars are a great way to home in on bright objects before aiming a telescope at them.

And of course, there is always the Moon, which this month has a treat in store.

Eclipse of The Moon

A Total Lunar Eclipse will take place on the morning of September 8. See the post by Toner Stevenson: Total lunar eclipse : Monday 8 September 2025 .

Saturn will also be at prime position for observing during the eclipse, above the Moon in the west. And while you’re out on this morning, turn to the north-east to see the evening summer constellation of Orion, along with the naked eye star clusters, the Hyades and Pleiades.

Moon Phases

The Deep Sky Objects described later should be visible with the Moon present from the outer suburbs and darker locations, although they will be better seen at times when the Moon is not visible. Still, don’t be put off by the Moon if the sky is clear. See what you can and try again on a moonless night.

Sep 8: Full Moon, including the Lunar Eclipse. Afterwards, it wanes through gibbous phases toward…

Sep 14: Last Quarter – rising around midnight. Afterwards, it wanes through crescent phases toward…

Sep 22: New Moon – rising and setting with the Sun. No moon visible all night. Afterwards, it waxes through crescent phases toward…

Sep 30: First Quarter - the Moon rises about midday and sets about midnight.

Planets

Saturn is the only bright planet easily visible to observe this month. It comes to Opposition on Sep 21. This means it lies on the opposite side of the Earth from The Sun, is at its brightest and largest, and is visible all night. A telescope will show the planet’s rings, currently nearly edge-on from our perspective. Titan, the largest of Saturn’s moons, will be visible in a small telescope. Surface features will require a larger telescope, which may also show several fainter moons.

Deep Sky Objects

[For a primer that gives basic information about Deep Sky Objects refer to this external link: DEEP SKY 101 | Shoalhaven Astronomers]

The picture below shows the area of Scorpius and Sagittarius at 9PM during September. The beautiful constellation Scorpius, one of the few to easily resemble its moniker of Scorpion, is “claws down” toward the western horizon, its tail winding majestically upwards. Above it lies Sagittarius, the Archer; more commonly known today for its teapot asterism. [The picture was created in a free program, Stellarium, which can be downloaded from the internet].

This area of sky abounds in deep sky objects, mainly clusters and nebulae. Those described here are bright examples which can be located with binoculars in relatively dark skies and are good objects for a small aperture telescope.

Antares, or α-Scorpii, the named star in the picture, is the brightest star in the constellation Scorpius. It shines red and bright to the naked eye, being a red supergiant star 550 light years away. It is a binary star with a faint green companion nearby. Due to the brightness of Antares, however, the fainter star is difficult to see. A much easier binary is object 1 in the picture.

The following are descriptions of the numbered objects [see the end of this article for a primer on terms such as magnitude, M and NGC]

1.     β-Scorpii, the second brightest star in Scorpius, is a Binary Star. The magnitudes of the components are 2.6 and 4.9, separated by 13.6”. They are 404 light years away.

2.     M4, also called NGC 6121, is a Globular Cluster. It has a magnitude of 5.9 and is 10,000 light years away.

3.     M7, also called NGC 6475, is an Open Star Cluster. It has a magnitude of 3.3 and is 800 Light Years away. It is also known as “Ptolemy’s Cluster”. Visible with the naked eye in dark skies, it is visually spectacular even with a small aperture telescope.

4.     M6, also called NGC 6405, is an Open Star Cluster in the same binocular field as M7. It has a magnitude of 4.2 and is 1,300 Light Years away. It is also known as “The Butterfly Cluster”.

5.     M22, also called NGC 6656, is a Globular Cluster. It has a magnitude of 5.1 and is 7,800 light years away. This is one of the very bright examples of this type of object.

6.     M8, also called “The Lagoon Nebula” and NGC 6514, is an Emission Nebula. It has a magnitude of 5.8 and is 4,900 Light Years away.

7.     M20, also called “The Trifid Nebula” and NGC 3372, is an Emission Nebula in the same binocular field as M8. It has a magnitude of 6.3 and is 2,300 Light Years away.

Some Useful Terms

Aperture is the diameter of the lens or mirror that the telescope uses to collect light.

Magnitude is how bright an object is. Smaller Numbers have Brighter Magnitudes – e.g. Magnitude 1 is brighter than magnitude 2. A very dark sky will allow stars as faint as magnitude 6 to be visible. Very bright objects – bright stars and planets, the Moon and Sun, have negative magnitudes.

M stands for objects in the Messier Catalogue of 110 objects. This was one of the earliest lists of deep sky objects compiled in the 1700s. Most of the objects in this catalogue will also have an NGC number.

NGC stands for New General Catalogue. All Deep Sky Objects will have a catalogue number, of which the NGC is the most well-known, with more than 7,000 objects. Many brighter, more well-known objects have names such as Lagoon Nebula, Butterfly Cluster etc. All of these will also have a number in one of the many astronomical catalogues.

FOR THOSE NEW TO AMATEUR ASTRONOMY

I strongly advise that you purchase or borrow books, or read online sites, devoted to teaching basic Astronomy. Teach yourself about:

·      Different types of telescopes

·      Aperture and Focal Length

·      Magnification

·      Eyepieces, Barlow lenses and filters

·      Navigating the night sky

o   Planispheres

o   Star Atlases

o   Right Ascension and Declination

·      Deep Sky Objects – clusters, nebulae and galaxies

·      Red Light - to preserve what we call Dark Adaptation. Astronomers use red light, which in the visible light spectrum has the lowest energy, to preserve what we call Dark Adaptation. Torches with red LEDs can be purchased or get a small torch and tape several layers of red cellophane over the globe.

o   Astronomy Apps, either on a phone, computer or tablet, usually have a setting to that turns the screen red in order to preserve dark adaptation.

The Australasian Sky Guide is an easy to read guide to the stars and planets each month written by Sydney City Skywatchers member Nick Lomb. Nick is an honorary professor with the University of Southern Queensland and previous curator of Sydney Observatory. The 2025 edition is out of print but you can now pre-purchase the 2026 edition.

Andrew Wood

amwood1961@outlook.com