Autumn and winter is a welcome time for astronomers: longer nights provide more observing opportunities, and colder winter air is less able to hold moisture so the haze of summer skies dissipates, leading to better transparency.
But this clarity can come at the cost of a scintillating atmosphere, which may rob Solar System observers of sharp views.
Make the best of each night’s conditions by selecting your targets carefully and there are some great ones coming up over the next few months.
As we approach autumn and winter, the summer constellations are on the wane and new ones loom into view.
Early September is a great time to take a last look at Cygnus in the heart of the Milky Way and, if you want a real challenge, the zodiacal constellation of Sagittarius, with its Teapot asterism, low to the south.
Then it’s time to move on to more seasonal targets.
Autumn is the time that the zodiacal constellations of Aquarius, Aries and Capricornus appear in the sky, along with some of the Perseus family: Andromeda, Cassiopeia, Cepheus, Cetus, Lacerta and Pegasus.
Cassiopeia and Cepheus are circumpolar, so can be seen all year round, but they’re particularly well placed at this time of the year.
The ‘W’ shape of Cassiopeia, rich in open clusters and emission nebulae, and the Great Square of Pegasus, with its beautiful globular cluster and galaxies, act as visual guides to their locations.
Andromeda is an enormous galaxy that lies between the two and also happens to be the most-distant object visible to the unaided eye.
As we move into winter, Perseus itself and family member Auriga become well placed.
Both have beautiful open star clusters, while nearby is another family member, Triangulum, with its famous spiral galaxy.
Cetus with its barred Seyfert galaxy and the zodiacal constellations of Pisces and Taurus are a clear reminder that winter is upon us.
Taurus, the constellation of the Bull, is packed with beautiful open clusters, a supernova remnant and unmistakable ‘V’ shape.
Orion and Gemini follow close behind. Orion is rich in nebulae and eagerly awaited as an imager’s paradise, while Gemini presents planetary nebulae, open clusters and emission nebulae.
In the New Year look out for the Winter Triangle formed by the bright stars Procyon in Canis Minor, Sirius (the brightest star in the Northern Hemisphere) in Canis Major and Betelgeuse in Orion.
Between Canis Major and Canis Minor you’ll find Monoceros with its beautiful open clusters and several nebulae.
The zodiacal constellation of Cancer slides into an excellent observing position as we approach spring, bringing with it beautiful star clusters.
Planets aren’t particularly well placed this season, especially for imaging. But that’s not going to stop us astronomers!
With Neptune at opposition in Aquarius on 10 September, this blue ice giant will be at its closest point to Earth.
Although even at full illumination it’ll only appear as a minuscule blue dot.
Uranus will be visible in Aries until January and will be at opposition on 27 October but, much like Neptune, this blue-green planet will appear only as a bright coloured dot.
Saturn will be in Sagittarius and best placed for observing in September, but as it heads for its rendezvous with the Sun on 13 January next year, you really only have until November to enjoy it.
Jupiter is an evening object in September and October, located in Ophiuchus. It’s then a morning object in Sagittarius in February and March, but it will be low in the sky.
Of the rocky planets, Mars will only be visible in December and January, as a morning object in Libra.
Venus will be visible from December through to March, when it’s best viewed in Aries. Bear in mind that its phase increases as the months progress.
Planetary transits are relatively rare events, only occurring with the inner planets, Venus and Mercury.
A transit of Mercury occurs on 11 November this year when the planet is near perihelion and its disc is near 10 arcseconds across.
The transit starts at 12:35 UT with greatest transit at 15:19 UT, and ends at 18:04 UT.
Remember, only view this event using a safe solar telescope.
Double stars and clusters
Nicknamed the ‘autumn Albireo’, Almach, a stunning gold and blue pair in Andromeda, is a lovely autumn target through a 4-inch or larger telescope and about 100x magnification.
Mesarthim, the faintest of the three stars at the western side of Aries, is a perfectly matched pair of white stars that can be split with a 4-inch telescope.
Eta (η) Cassiopeiae, 1.7° northeast of mag. +2.2 Shedir is a striking, colour-contrasting double.
Also in Cassiopeia, don’t miss the delightful open cluster M103, 1° northeast of Ruchbah (Delta (δ) Cassiopeiae) and best seen in an 8-inch telescope at around 100x magnification.
Moving to the point at which the two arms of V-shaped Pisces meet, double star Alrescha has two white-hot stars that can be separated in a medium-sized telescope and about 200x magnification.
Scan the sky between Perseus and Cassiopeia to find one of the finest binocular clusters, the Double Cluster in Perseus.
In Auriga, 10° southwest of Capella, you’ll find a fine open cluster, M38, with a 6-inch telescope.
Monoceros may be dim but the constellation is home to a lovely triple star system, Beta Monocerotis.
A 4-inch telescope will show this arc of stars. The stunning open cluster NGC 2244 is 12° north in the heart of the Rosette Nebula and visible through a 3- to 4-inch telescope.
As we head for spring, the double star Iota Cancri, the third brightest star in Cancer, is a fine sight in a small telescope.
Look 9° southwest and you’ll find the busy Beehive Cluster, M44. 10×50 binoculars give a wonderful view of the whole object.
Autumn is a great time to observe the wonderful Helix Nebula, NGC 7293, in Aquarius, 11° east of Deneb Algedi, even though it is always low in the sky.
Because of its large size this planetary nebula is best seen using binoculars or a rich-field telescope at low magnification.
The standout nebula for the season has to be the Orion Nebula, M42.
A wonderful sight in binoculars or telescopes of any size, it’s one of the few nebulae showing colour through the eyepiece.
Find it just over 4° due south of Alnilam, the centre star in Orion’s Belt.
Still in Orion, the Flame Nebula, NGC 2024, lies just to the northeast of Alnitak in the Hunter’s Belt.
Although Alnitak’s sheer brightness makes observing the emission nebula tricky, placing the star just outside the field of view of the eyepiece in a 6-inch scope will show its leaf shape.
Stepping into Monceros, 9.5° east-southeast of Betelgeuse you’ll alight on the beautiful Rosette Nebula (Caldwell 49).
Best seen in a rich-field telescope, an ultrahigh-contrast filter will make this nebulous region spring into view, although it is also worth observing some of the detail with an 8-inch or larger telescope.
Comets are thin on the ground this season but early in September, comet C/2017 T2 (PanSTARRS) is in Taurus.
It moves into Auriga early in October and passes close to M36 on the night of 27 October.
Although predictions can be a little tricky, early December should offer the best views as the comet can be found in Perseus and then Camelopardalis as it heads into Cassiopeia in February and March.
A favourite for autumn is the Andromeda Galaxy, M31, lying 15° to the west of Almach.
It’s the most-distant object visible with the naked eye – and a wonderful sight in binoculars or a rich-field telescope.
Moving 15° east into Triangulum, don’t miss the Triangulum Pinwheel Galaxy, M33 (not to be confused with the Pinwheel Galaxy, M101).
It appears quite faint despite its +5.7 magnitude as its light is spread over a wide area, so it’s best seen at low magnification.
Moving northwards and 3.5° northeast of Alkaid, the Whirlpool Galaxy, M51, in Canes Venatici attracts both observers and imagers thanks to its stunning shape.
Easily viewed in a 4-inch telescope, M51 is an interacting grand-design spiral galaxy comprising NGC 5194 and NGC 5195.
Remaining in the north, Bode’s Galaxy, M81, and the Cigar Galaxy, M82, in Ursa Major, 10° west-northwest of Dubhe, are a fantastic sight a couple of hours after midnight.
At a magnification of about 35x, a 6-inch telescope captures both galaxies in the same field, making for a most attractive pairing.
Part of our own Galaxy the Milky Way is still present in the skies over winter but we’re unable to observe the core.
Instead, we’re looking out at the outer reaches, to the Orion and Perseus Arms.
As we approach spring, the Leo Triplet comprising M65, M66 and NGC 3628 are a welcome sight in Leo, roughly 2.5° southeast of Chertan, early in the morning.
A 6- to 8-inch telescope will show oval M66 is the brightest, whereas M65 is more cigar shaped. Edge-on NGC 3628 is somewhat more elusive.
Although the Moon is tidally locked to Earth and we always see the same face, its surface always appears to be changing as shadows play an important part in what we observe.
Nowhere is this more evident than with the Lunar X, a clair-obscur effect that illuminates the ridges and walls of the La Caille, Purbach and Blanchinus craters, and occurs a few hours before first quarter Moon.
But libration (a small oscillation of the Moon) causes this effect to vary in its timing from month to month.
A small telescope will easily show the Lunar X and the best time to view it will be 4 November from about 17:00 UT onwards for around two hours.
The Moon’s surface has numerous rilles or faults, but the most notable one is probably Rupes Recta, also known as Huygens’ Sword or simply the Straight Wall.
With a length of approximately 110km and a height estimated to be between 250 and 350m, it is an imposing sight.
You can find it along the eastern shore of Mare Nubium and it’s best seen during the last quarter phase when the face of the wall turns white as it’s illuminated directly by the setting Sun.
The best times to observe it this season are the night of 22 September and morning of 16 March.
Craters of many different sizes cover the Moon’s surface and can be seen through binoculars.
There are some standouts, however: crater Petavius, found near to the Moon’s southeastern limb, southeast of Mare Fecunditatis (the Sea of Fertility).
This wonderful double-walled crater, 177km across, is best seen on a three-day-old Moon and will appear elongated as it’s close to the southeastern limb.
As well as examining the crater walls and central peak, enjoy tracing the path of Rimae Petavius, a rille that cuts across the crater’s plain from the peak to the inner southwestern wall.
A small telescope will show these features well and a great time to observe the crater would be on the evening of 1 October.
The first of the major meteor showers of the season is the Orionids, which peaks on the night of 21–22 October.
It’s produced by dust grains left behind by Halley’s comet, but the light from a last quarter Moon will obscure the fainter meteors.
Next up is the Leonids, peaking on the night of 17–18 November.
Unfortunately, the Moon will be 80 per cent illuminated, but you’ll still be able to see the brighter members. The Leonids are produced by dust grains left behind by comet 55P/Tempel-Tuttle.
The ‘must see’ shower of the year is the Geminids produced by debris from asteroid, 3200 Phaethon.
With its peak on the night of 14 December, this is normally a great show.
Frustratingly, the Moon will be nearly full at 91% illumination, but the shower is strong and bright so you could still be in for a treat.
The final major shower of the season, the Quadrantids is produced from debris from asteroid 2003 EH and peaks at the start of the New Year on the night and morning of 3-4 January.
This above-average shower could produce a good show this time as the first quarter Moon will set at around 01:00 UT on 4 January so be prepared for an early morning observing session.