Night sky notes for September 2017 Geoff Mitchell
The September night sky is perhaps offers some of the best observing conditions. With the autumn equinox for northern hemisphere observers on September 23rd the mix of summer and autumn contains many fine objects to view with both binocular and small telescopes. The constellations of Cygnus, Lyra and Aquila can be seen high overhead and into the south; these are noted for the bright stars of Deneb, Vega and Altair respectively forming the `Summer Triangle ` asterism.
The Milky Way stretches from Perseus rising in the north east, through the `W` shaped constellation of Cassiopeia in the north east , along the cross shaped constellation of Cygnus. This faint band of stars best seen on moonless evenings continues down through the obscure constellation of Scutum and on in the direction of the centre of the Milky Way (not visible from the UK) low on the horizon.
Look at the constellation of Sagittarius, close to the `spout ` of the Teapot Asterism, very low in the south / south west early evening. The faint band of light of the Milky Way is seen stretching high overhead and low into the south. Use binoculars to reveal the rich star clouds that show a myriad of stars and some of the dark rifts, regions of the Milky Way spiral arm obscured by dust clouds on clear moonless evenings.
Auriga has the bright star Capella and can be seen low in the north east early evening, Capella is circumpolar from UK latitudes and so is always visible. In the east the constellation of Pegasus, noted for its `Square ` shape and the adjacent constellation of Andromeda noted for The Andromeda Galaxy (M31) , visible to the unaided eye as a faint fuzzy patch on moonless evening can now be seen mid evening . The Square of Pegasus is a useful sign post constellation and also is a good test for sky conditions, (how many faint stars you see within the square is indicates just how good your seeing conditions are). Follow the two end stars (Scheat and Markab) down to find the star Fomalhaut in the constellation of Pisces Australis (The Southern Fishes), visible very low on the southern horizon late evening at the end of the month .
In the North West Ursa Major, The Plough or The Great Bear is seen low with its handle or tail parallel to the horizon late evening? Use the right hand pair of stars Dubhe and Merak (The pointers ) to find the faint pole star Polaris and hence the position of North . Follow the curve of Ursa Major’s tail to the orange star Arcturus in the constellation Bootes (The Herdsman) low in the west. To the east of Bootes find the `horse shoe ` shaped constellation of Corona Borealis (Northern Crown) and the `Keystone asterism ` in the constellation of Hercules also noted for the globular cluster M13 containing around 750,000 stars, a good view with modest telescopes.
Saturn is visible early evening , very low in the south west in twilight and is not well placed, however the rings are wide open, a classic view of this ringed gem. Uranus in the constellation of Pisces and Neptune in the constellation of Aquarius are now both evening objects that can be located using binoculars and a suitable finder chart. A good starting point is to locate the Square of Pegasus and the ring of stars below the Square known as the `Circlet` in the constellation of Pisces (The Fishes), then star hop using the finder chart. Uranus and Neptune are notable due to the blue and green colour, Telescopes show a tiny disc rather than the sharp pinpoints of stars in the field of view.
In the east mid evening the seven sisters’ (Pleiades, M45) star cluster and constellation Taurus heralds the forthcoming autumn skies of a new observing season.
Also in Taurus, newly discovered Comet 2017 01 ASASSN1 moves steadily northward, night by night and may brighten to binocular visibility during September The comet reaches perihelion , its closest distance from the Sun at 1.5AU in mid October when it is also closest to Earth at a distance of 0.72 AU.
Planets in September 2017
Mercury is at greatest elongation on September 12th, visible low dawn skies during September.
Venus shines brightly low in dawn twilight in the constellation of Leo, joined by Mercury and Mars.
Mars emerges into dawn skies rising some 2 hours before sunrise,
Jupiter is well into the evening twilight setting with the Sun by late September –poorly placed.
Saturn visible very low in the south west early evening, rings are wide open in the constellation of Scorpius.
Uranus rises late evening in the constellation of Pisces. (Binocular / Telescope required) (See notes)
Neptune in Aquarius rises early evening (Binocular / Telescope required). The constellation of Aquarius is positioned low in the south east by mid evening Neptune is at opposition on September 5th (See notes)
Moons phases in September 2017
New Moon Sept 20th Moonless, best time for deep sky observing and Comets .
First Quarter Sept 28th Best days to see shadow details in lunar craters (early evening)
Full Moon Sept 6th Best days to see bright ray craters like Copernicus / Tycho.
Last Quarter Sept 13th Moon visible in daytime skies. Do not look directly at the Sun.
Autumnal Equinox occurs on September 22nd 21:03 BST, Day and night being equal length
Piscids, range September to October, maxima September 9th and 21st – rather low rates.
The Highlights of the Month
- Comet 2017 01 ASASSN1 moves northward into our eastern evening skies ( Telescopic) – see notes
- September skies, Milky Way visible high over head on moonless evenings in darker skies.
- Look at the Moon illusion effect at moonrise around Full Moon the Harvest Moon rising
- Uranus and Neptune, binocular objects to find using suitable finder charts
- M31 the Andromeda Galaxy is visible on moonless evenings, best seen in binoculars.
- Double cluster, on the Perseus /Cassiopeia border, nice pair of star clusters
- Albireo, the star Beta Cygnii is a nice blue/ yellow double star seen telescopically
- A 5% waxing crescent moon is positioned close to Venus in twilight on September 3rd
- DSLR astrophotography – Sky photography on moonless evening – 28mm /50mm lens – manual focus to infinity. Set ISO / ASA 1600, exposure 30s, remote cable release, tripod mounted – Have a go at capturing some of the star fields through Cygnus around Deneb (NGC 7000 N.American Nebula) and Scutum
More detailed sky notes and LAS Newsletters, Finder charts are available to LAS members via the Members` page on the LAS Website www.lutonastrolink.org.uk Artificial satellite and International Space Station visible passes and bright Iridium flares – check the home page for posting s of details of favourable observing times.
Waxing crescent Moon visibility . Caution. Do NOT look at the Sun directly with or without optical aid.
New Moon occurs on September 20th, but the 0.4%, 15 hour old Moon is just too close to Sun to observe safely. However on September 21st, a thin (2.7%) waxing crescent Moon is visible for around 30 minutes after the sun has completely set. (Moonset 19:54 BST) Note the dimly lit part visible by Earthshine is readily seen with binoculars or small telescope. Jupiter is located to the left of the Crescent Moon
Only look for the crescent Moon after the sun has completely set.
More detailed sky notes and LAS Newsletters , Finder charts are available to LAS members via the Members` page on the LAS Website www.lutonastrolink.org.uk
Sky looking south in late September 2017 at 9pm BST
The number of stars visible inside the Square of Pegasus in dark moonless conditions is an indication of your sky conditions.
Stars shown to unaided eye limit 6th magnitude equivalent to a good dark moonless site
Sky looking east at 9.30pm (late September) 2017
Comet 2017 O1 ASASSN1 is brighter than predicted already within limits of a small telescope in a dark sky and my be a binocular object by early September – LAS newsletter No 189 ( available to LAS members from the members page ). Finder chart for Comet 2017 01 ASASSN1 – Refer to web site for updates
Sky looking north at 9.30pm mid September 2017
Sky looking west at 9.30pm late September 2017
The constellations of Aquarius and Pisces rising late evening
Look south east to find the constellation of Pegasus (Winged Horse), Pisces (The Fishes) and Aquarius (The Water Bearer)
LAS general finder chart for Uranus for September / October 2017, positions shown at 5 day intervals
Note – Circles show the field of view of typical 10×50 binoculars Uranus at opposition October 19th 2017
Uranus is located in the constellation of Pisces and rises late evening in September.
LAS general finder chart for Neptune for September / October 2017, positions shown at 5 day intervals
Neptune is visible using 10×50 binocular at magnitude 7.8m at opposition September 5th 2017
located in the constellation of Aquarius.
Note – Circles show the field of view of typical 10×50 binoculars