Sky notes for 2025 August & September

2025 August 1

Nick Hewitt

(Written for 23:00 BST in the UK on September 1)

The days shorten, and with the equinox on Sep 22, amateur astronomy can take off again. We also begin to see several planets back at a more reasonable hour. Saturn reaches opposition on Sep 21 and is a fine sight, even with the rings nearly edge-on with a mere two-degree tilt. The planet is fainter than when the rings are wider, but in a barren part of the sky it is easy to find. By mid-evening the Milky Way is at its best, arching from Sagittarius in the south through Scutum, Aquila, Cygnus and into Cepheus and Cassiopeia. Arcturus is now in the west, with Corona Borealis dragging behind – and with the tease that is T Corona Borealis yet to flare (at the time of writing). The huge figures of high summer, Hercules and Ophiuchus, remain well placed west of the meridian but the Summer Triangle of Vega (Lyra), Deneb (Cygnus) and Altair (Aquila) dominates, and will do so throughout late summer and well into autumn.

Low in the south lies the northern portion of Sagittarius, with a host of celestial wonders: globular and open clusters, as well as emission, reflection, and dark nebulae. However, they are low and are best viewed south of the UK. More accessible is the Scutum Star Cloud, a strikingly bright portion of the Milky Way.

The deep sky

Messier 11 (M11; Figure 1) is a bright, compact and rich star cluster, and technically visible to the naked eye at magnitude 5.8, but being in the middle of the Scutum Star Cloud it can be difficult to pick up. It lies south of the dark nebula Barnard 111. Any telescope shows it well, and hundreds of stars can be discerned, although it probably contains over 2,000. The semiregular variable R Scuti lies one degree northwest of the cluster. It varies between magnitude 4.8 and 6.0 over 143 days. The other Messier cluster in Scutum, M26 (Figure 2), is much less impressive, being smaller (8 arcmin), fainter (magnitude 8) and less rich with a mere 70 stars. It sits in a rich star field, so can be difficult to pick out. It does have a rare carbon star within it: FH Scuti, fading from 13.4 to 16.8 when it puffs out a sooty ejecta.

Moving northwards, but bypassing Aquila, we find the great Northern Cross asterism, the most prominent and central feature of Cygnus the Swan. First-magnitude Deneb is the faintest of the three Summer Triangle stars, but is prominent enough at magnitude 1.2 and is the 19th brightest star in the sky. Just to Deneb’s east is the huge and popular North America Nebula, NGC 7000, and the large, faint Pelican Nebula (IC 5070) to its west. Increasingly imaged, however, is the complex mess of nebulosity (Figure 3; p. 284) surrounding Sadr, the central star of the Cross. This is a fascinating area, now being imaged in staggering detail by astro-imagers using small telescopes and narrowband filters. The emission and reflection components contrast dramatically with the dark nebulae.

If imaging is not your interest, there are many fine visual targets to enjoy, such as M29, a prominent open cluster near Sadr, and M39, a large open cluster to the northeast of Deneb, which is best viewed in binoculars. For larger-aperture telescopes, the Crescent Nebula (NGC 6888) is derived from a Wolf–Rayet star. Small telescopes offer fine sights too, including some of the best-loved and most important double stars (Figure 4a) of summer, which deserve special mention. Albireo (beta Cygni) is one of the loveliest doubles in the heavens with its striking, contrasting components of gold and azure (Clerke, 1905).¹ Situated at the base of the Cross asterism, it has a comfortable separation of 34.3 arcsec, with magnitudes of 3.1 and 5.1. Another famous double is 61 Cygni, an easy pair for small telescopes; it is of spectral type K, so both stars are light orange in hue. Their large proper motion of 5.22 arcsec per year suggested their relative proximity to us, and they are indeed the sixth closest stars. They have widened from 11 arcsec when measured in 1650 to an anticipated 34 arcsec by 2100. The rapid motion gave F. W. Bessel the idea to attempt a trigonometric parallax in 1838, and he measured it as a very creditable 0.29 arcmin, giving a distance of 10.3 light-years. We now know it is slightly further at 11.4 light-years, but what a fine estimate for the time. The pair of Sun-like stars can be located east of the Swan’s southern wing, east of epsilon Cygni.

16 Cygni is another wide pair (39 arcsec), this time of G-type fifth-magnitude dwarfs, almost twins of our Sun. These were in the field of the Kepler space telescope (2009–2013), which searched for extrasolar planets, and indeed it detected a gas giant orbiting 16 Cygni B.

While in the vicinity, sweep to a personal favourite, omicron¹ or 31 Cygni (Figure 4b), a colourful and very wide triplet found between Deneb and delta (also a double) in the Swan’s northern wing. The colour of all three components is striking. The brightest of the trio (31 Cygni) is an orange giant and eclipsing binary (magnitude 3.9), but the period is 10 years and the amplitude tiny (3.73 to 3.89), so enjoy the hue instead! The magnitude 4.8 B-type star (30 Cygni) at 330 arcsec contrasts nicely with the golden primary, and there is a third in the group, nearer to the primary (109 arcsec), at magnitude 7 that is sometimes viewed as greenish, although it is also a B-type star.

To the northeast of Cygnus, we find the less distinct pattern of Cepheus the King, less adorned than Queen Cassiopeia but sitting on hidden treasure. Cepheus contains one of the most famous and important stars of all: delta Cephei. It lies in the southern aspect of the constellation, forming a convenient triangle of stars with zeta (magnitude 3.35) and epsilon (magnitude 4.15). Delta Cephei is a neat double star and the prototype of the hugely important Cepheid variables –  pulsating supergiants that have enabled the local Universe to be measured through the period-luminosity relationship discovered by Henrietta Swan Leavitt in 1908, and used by Harlow Shapley at Mt Wilson Observatory to map the globular star clusters associated with the Milky Way and relegate the Sun to the suburbs. Delta varies between 3.48 to 4.37 like clockwork, its period being 5.366249 d.

With the Milky Way passing through it, Cepheus holds exquisite emission and dark nebulae (see image, p. 282), which skilled imagers now tease out in extraordinary detail.

As summer slips into autumn, the last of the season’s best globular clusters come into view. The first of these late-summer treats is M15 (Figure 5), which lies to the west of the Square of Pegasus near epsilon Pegasi (Enif). Easily seen in a finder or binoculars, it can be fully resolved with a 6-in telescope. Large-aperture owners can look for the planetary nebula Pease 1 northeast of the core region. Further south is a very compact example of the type – M2 (Figure 6, previous page) in Aquarius. This bright cluster is magnitude 6.4 but is much more difficult to resolve, requiring at least a 12-in to break down. We are then into the season of mist and mellow fruitfulness, with more fun to be had as the outer planets become more prominent.

Solar system

Eclipses

The total lunar eclipse on Sep 7 will be best seen from east Africa, India and western Australia. From Britain, the Moon rises in eclipse with totality ending at 18:54 UT. The partial phases can be viewed as the Moon climbs higher. However, this is yet another compromised lunar eclipse!

There is a partial solar eclipse on Sep 21, but nothing is visible from Britain.

Sun & planets

The Sun has remained highly active over the summer with many sunspot groups on display. Even after the equinox, the Sun remains high and is perhaps less oppressive than in high summer.

Mercury reaches western elongation on Aug 19, becoming reasonably well placed in the predawn, brightening sky, around 10 degrees above the horizon. Its 7-arcsec disc is a waxing crescent, but it rapidly becomes gibbous before plunging out of sight as it nips behind the Sun on Sep 13. Appearing in the evening at the end of September, it will be too low to observe. Daytime observation should be possible at the end of October, as the fleeting inner planet reaches eastern elongation on Oct 29.

Venus is a beacon in the predawn sky, achieving its highest altitude as it shrinks from 13 to 10 arcsec. There is a close conjunction with Jupiter on Aug 12, when these luminaries are a mere 51 arcmin apart. In addition to this, we can look forward to a Sep 19 daytime occultation by the Moon, which will be a slender waning crescent, so difficult to see at 11:54 UT as Venus, much the brighter at magnitude −3.9, disappears behind the bright limb. It reappears at 13:13 UT from the dark limb. Venus shows an 11.5-arcsec gibbous disc.

Although Mars is technically visible in the early evening sky, it is low, faint and tiny.

Jupiter has left Taurus and lies in Gemini, beginning to reach a far better altitude throughout August, and by late September it dominates the predawn southeast. It does not reach opposition until 2026 Jan, but what a wonderful opportunity to enjoy the gas giant for many months at the peak of its ecliptic journey.

Saturn (Figure 7) rises mid-evening and lies in Pisces, with its best declination for many years, but the rings are still almost edge-on and so the planet’s apparent magnitude is only +0.7. The southern hemisphere is marginally more favoured and will remain so for several years to come. Opposition is reached on Sep 21. There are various moon events over the two months, but Titan’s occultation by the ringed planet at 01:40 UT on Sep 28 is the most accessible. As the moon’s disc is being occulted, timing the disappearance will be of interest.

Uranus remains in Taurus and becomes increasingly available as August progresses, lying a little south of the Pleiades in September.

Neptune, in Pisces, reaches opposition on Sep 23. Its tiny 7.8-magnitude disc is a subtle blue through larger telescopes. It is just 2.7 degrees northeast of Saturn at the end of August. Follow a gentle curve from 27 Psc through 29 Psc and Saturn, and the remote ice giant should be easy to locate.

Dwarf planets & asteroids

(2) Pallas is in Delphinus and can be picked up in large binoculars or small telescopes at magnitude 9.5. It lies between brilliant Altair and epsilon Del in August, moving westward into Aquila in September.

(3) Juno is low in Libra throughout August and, at magnitude 11, is too difficult to observe.

Dwarf planet (1) Ceres is a binocular target in Cetus, best viewed after midnight towards the end of September. At magnitude 7.6 and in a rather barren field, its passage west towards the tail of the Whale should be fun to watch.

Meteors

This year, the popular Perseid meteor shower is somewhat compromised by the waning gibbous Moon rising just before 22:00 BST on the peak night of Aug 12, but warm observing in groups remains fun, and the Moon and Saturn can be viewed around midnight if the sky is too bright to see the fainter meteors.

The Kappa Cygnids are much less rich than the Perseids but fare better this year, as they are not affected by the Moon until after midnight. They peak on Aug 17, and although the shower is often absent, there are occasional bright fireballs, so watch for anything brilliant arising from near Vega. The parent body remains unknown.

The Alpha Aurigids are at their maximum around Aug 31, and the Moon at first quarter sets mid-evening. It is rather an anticlimax after the Perseids, but occasional outbursts have been recorded. The meteors derive from comet C/1911 N1 (Kiess).

Comets

At the time of writing, there is a dearth of brighter comets from August to October. Perhaps the best on show is C/2025 K1 (ATLAS) which moves rapidly from below Cygnus into Serpens during August, but is only likely to be around magnitude 11. As it brightens, it moves southwest, so becomes more difficult to observe in the twilight.

C/2025 A6 (Lemmon) may brighten to 10th magnitude before dawn at the end of September, when it is on the Lynx / Leo Minor border in the wee hours.

1           Clerke A. M., Modern Cosmogonies, Adam & Charles Black, London, 1905