Sky Diary September 2026

Dark skies are back

For observers in Edinburgh, one of the best things about September isn’t a single astronomical event at all, it’s the return of proper darkness.

On 1 September, the Sun sets at 20:07 BST and astronomical twilight does not end until 22:32. By 22 September, sunset has moved forward to 19:11 and astronomical twilight ends at 21:19. By the end of the month astronomical darkness begins before 21:00.

This makes September an excellent month to return to deep-sky observing. The Summer Triangle of Vega, Deneb and Altair remains high during the evening, with the Milky Way running through Cygnus. Good targets include the Ring Nebula M57, Dumbbell Nebula M27, Albireo and the many star fields and clusters of Cygnus. (See our Cygnus Constellation project for some of these sights.)

Looking towards the east and north-east, the autumn sky is beginning to take over. Andromeda Galaxy M31 becomes increasingly well placed, while M33 in Triangulum makes a good challenge from a dark site. The Double Cluster in Perseus is another excellent binocular or telescope target.

Saturn returns to the evening

Saturn is probably the best planet to observe during September.

After being principally a morning object during the summer, it is now rising during the evening and is visible for most of the night by the end of the month. It is also approaching opposition, which occurs shortly after the end of September, on 4 October. That means Saturn is becoming brighter, larger and increasingly convenient to observe during September.

The rings are still presented at a relatively shallow angle following their 2025 edge-on appearance, but they are now beginning to open again. Even a modest telescope should show the rings clearly, while larger instruments may reveal the Cassini Division under good seeing.

The almost Full Moon passes close to Saturn on the night of 26–27 September.

Neptune and Triton by David Brett, 15 November 2024

Neptune at opposition

Neptune reaches opposition on 25 September, when Earth passes approximately between Neptune and the Sun. This is the best time of year to look for the most distant major planet. It lies in Pisces and will be visible for most of the night. At around magnitude +7.8, it is far too faint to see with the naked eye but a telescope makes identification easier. It has a tiny blue-grey disk, although at only a little over 2 arcseconds across it remains a very small target. You could try and capture its largest moon, Triton.

Mars and Jupiter before dawn

Mars rises during the early hours and is visible in the east before dawn. It remains quite small telescopically and is not yet at its best, but its distinctive orange colour makes it easy to identify. The waning crescent Moon passes Mars on 6 September, with the two separated by around 3°.

Jupiter is also emerging into the morning sky and becomes progressively easier to see during September. On the morning of 9 September, the waning crescent Moon passes close to Jupiter. A lunar occultation of Jupiter occurs from some parts of the world, but not from Edinburgh, so observers here will see a close pairing rather than the planet disappearing behind the Moon.

September equinox

The autumnal equinox occurs at 01:05 BST on Wednesday 23 September. At the equinox the centre of the Sun crosses the celestial equator heading southwards, marking the astronomical beginning of autumn in the northern hemisphere.

The name equinox comes from the idea of “equal night”, although day and night are not exactly twelve hours long on the date of the equinox because of atmospheric refraction and the way sunrise and sunset are defined. A term I learned of recently is “equilux“, the time when day and night are closest to 12 hours each This occurs a few days before the spring equinox and a few days after the autumn equinox. In 2026 this will be on September 25.

JUICE returns to Earth, 28 September

JUICE spacecraft: European Space Agency

ESA’s Jupiter Icy Moons Explorer (JUICE) makes another close flyby of Earth on 28 September. Current ESA trajectory data predicts closest approach at about 11:45 UTC, passing around 8,641 km above the Earth’s surface.

JUICE was launched in April 2023 and is taking a complex route through the inner Solar System to gain enough energy to reach Jupiter. It completed a unique Moon-Earth gravity assist in August 2024 and flew past Venus in August 2025. The September 2026 Earth encounter provides another gravitational boost before a final Earth flyby in January 2029. JUICE is due to reach Jupiter in July 2031.

The chart shows the sky at 22:00 BST on 15 Sep 2026.

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Much of the material is provided with thanks to Dominic Ford on his excellent site in-the-sky.org.