Our monthly guide for stargazers

◼ Jupiter and Saturn are visible as soon as darkness falls. Venus is shining brilliantly before dawn. Mars is prominent in the morning sky. Look for brightening variable star Mira. Here’s more about the planets, stars and other objects you can see in the night sky in September 2020.

The stars in September

For countries in the northern hemisphere, the nights are getting longer now with more hours of darkness in which to enjoy the stars. One of the two equinoxes, when the Sun crosses the celestial equator, heading south, occurs on September 23. On that date, neither of the Earth’s poles is tipped towards the Sun. All parts of the world enjoy day and night of equal length.

The Sun moves from the constellation of Leo into Virgo on the 17th, so this area of the sky is in daylight and unobservable.

As it gets dark in early September, northern skywatchers can spot the Summer Triangle roughly due south in the heavens. This is not a constellation in itself, but a pattern made up of the brightest stars in three others – Vega in Lyra, Deneb in Cygnus, and Altair in Aquila. The Milky Way runs overhead, though Cygnus and down to the southern horizon. From northerly latitudes, the centre of the galaxy, with the richest part of the Milky Way, lies low over the southern horizon at this time, in Sagittarius.

Over to the east, the Great Square of Pegasus hangs above the horizon, though the star marking the upper left (north-eastern) corner of this square actually belongs to the neighbouring constellation of Andromeda. Look along the line of stars of Andromeda to see the extended glow of Messier 31, the Andromeda Galaxy, which is unmissable with binoculars.

Not far below Andromeda, as seen from northern latitudes, you can find a smaller galaxy, Triangulum, that is home to another nearby galaxy, Messier 33.

North of Andromeda lies Cassiopeia which looks like the letter “w” in the evening at this time of year. Between Andromeda and the bright star Capella in the north-east, you can find Perseus, a constellation with some fine sights including the Double Cluster and a famous eclipsing variable star, Algol.

Over in the western sky, constellations sinking towards the horizon during September evenings include Hercules, Bootes and Corona Borealis.

This photo of Mars shining brightly in wispy cloud was taken on the morning of 21 August. The cloud exaggerated the apparent size of the Red Planet. Image credit: Paul Sutherland

The planets in September 2020

Mercury

Mercury is in the evening sky during September, but clings too closely to the western horizon after sunset to be seen easily from northern latitudes. From southern hemisphere locations, however, it will become prominent after dusk during the mid to latter part of the month. A fine crescent Moon will lie close to Mercury on the 19th. Look out for an interesting conjunction on 22 September, when Mercury will pass very close to the 1st-magnitude star Spica. At zero magnitude, Mercury will be about twice as bright as Spica (also known as alpha Virginis), and you will probably need binoculars or a telescope to see this conjunction well. At their closest, the star and planet will be just a third of a degree apart. Read more about Mercury.

Venus

Venus is still brilliant in the morning sky now, shining at -4.2 magnitude as September opens. It rises in the east about four hours before the Sun at the start of September. The planet is shrinking in diameter as it recedes from the Earth towards the far side of the Sun, and its phase is becoming gibbous. During the second week of the month, Venus passes south of the open star cluster Praesepe, M44, in Cancer. The Moon will lie close to Venus in the sky on the 14th. Read more about planet Venus, and also see our guide to observing Venus.

Mars

Mars is steadily increasing in brightness as its orbit carries it closer to the Earth, and its magnitude increases from -1.8 to -2.4 over the course of the month. It will be at its brightest when. at opposition in October. It rises during the early evening. in Pisces. Mars no longer looks like a star, but more like a very bright planet, and you will be able to detect its orange hue. Mars stops its eastward motion against the stars on the 10th and begins to move backwards in the sky – something called retrograde motion. This effect is purely due to Earth catching up with the planet and overtaking it on the inside. Over the course of September, Mars’ apparent diameter will also grow from 18 to 22 arcseconds, making features more visible in small telescopes. The waning gibbous Moon will lie close to Mars on the night of 5th-6th September. Read our guide to Mars.

Jupiter

Giant planet Jupiter is visible from nightfall until the early hours. It is another planet that is currently on a southerly part of the ecliptic, in the constellation of Sagittarius, the view is much better from southern latitudes, such as Australia, New Zealand, South Africa, or South America, where Jupiter will be high in the sky.

It is now shining brightly, though fading slightly from magnitude -2.4 to -2.2 over the course of the month, so is easy to spot, despite being low down for observers in the northern hemisphere.

Even a small telescope will reveal the cloud belts on Jupiter and its four main moons, the Galilean satellites Io, Europa, Ganymede and Callisto. The planet Saturn lies a little to the east of Jupiter. The Full Moon will be close by on the night of the 29th. Read more about planet Jupiter.

Saturn

The ringed planet, another gas giant, is also visible from dusk until the early hours. Saturn shines at around magnitude 0.7, like one of the brightest stars, and with a yellowish light. Sadly again for northern hemisphere observers, Saturn is low in the sky, lying not far from Jupiter, in the constellation of Sagittarius. The pair are gradually closing together and will be in very close conjunction in December. From the southern hemisphere, Saturn rises higher in the sky, and so this will be an ideal time to observe it and its beautiful rings. Read more about Saturn.

Uranus

Uranus lies in the constellation of Aries, rising in the mid-evening at the start of September. In a dark sky, this planet is theoretically visible with the unaided eye, shining at a dim magnitude 5.7. However, at northern latitudes where summer twilight still hangs around for much of the night, you will definitely need binoculars to see it this month. Read more about Uranus.

Neptune

The outer ice giant reaches opposition on 11 September and so will be visible all night long, in the constellation of Aquarius. It is a couple of magnitudes fainter than its twin, Uranus, and shines at magnitude 7.8 this month. You will be able to glimpse it with binoculars, or a small telescope. Read more about Neptune.

Variable star Mira is brightening

Mira is indicated in this photo taken from Preston Montford, Shropshire, England, on 30 November, 2018. Image credit: Paul Sutherland

One of the most interesting stars in the night sky for casual sky-watchers is Mira, or ο Ceti, because it has a huge range in brightness. Mira can brighten to second magnitude, making it easy to see with the unaided eye, but regularly fades to as faint as tenth magnitude when a small telescope is needed to see it! At the end of August, Mira had brightened to magnitude 4, making it a naked-eye sight in a dark sky. It is expected to be brightest in October. Why not check it out for yourself? Here is our special guide to observing Mira.

The Moon

Full Moon: Sep 2

Last Quarter: Sep 10

New Moon: Sep 17

First Quarter: Sep 24



The Moon always makes a great target for a small telescope, which come into view as its phases change. We’ve a guide to where to find 50 of the best lunar features, plus a checklist to download so you can tick them off as you spot them! Don’t have a telescope? Here’s our guide to choosing one.

Fifty fantastic features – our amazing Moon

Here’s our guide to observing some of the finest sights on the Moon with small telescopes. Our pages of charts will help you find interesting features to seek out yourself, including craters, lava flows, mountain ranges and deep chasms!

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