Night Sky Watch for September

Written by David Pugh

There seemed to be few clear nights this August except when there was the bright Moon present and the Perseids proved to be mediocre with some high level hazy cloud spoiling things on the peak night of Aug 12/13. Let's hope the weather in September will be better and we can enjoy some glimpses of the northern Milky Way at least with averted vision if the light pollution is not too bad. Ask your neighbours if they would briefly turn off their security lights!

This September Mercury, Venus and Mars will not be favourably placed for observation. However, Jupiter and Saturn are fine evening objects in the southern sky, on view all night. You can not miss Jupiter beaming brilliantly at magnitude -2.8 in Capricornus and with a disk size of 47.6". (It was at opposition on August 20th). Saturn lies 15 degrees to Jupiter's right (west) and a bit nearer the horizon at mag +0.4 and disk size of 17.9". Although these gas giants are not particularly high in the sky, Saturn especially, this is the best time this year to observe or image them. To minimise the effects of our atmosphere in causing a soft focus watery view of these planets I recommend two things. Firstly, make sure that you let your telescope tube cool down for half hour outside by removing the dust cover. Secondly, try to observe them at their highest in the sky due south, known as "culminating". Jupiter will reach its highest in the south around 11.30pm mid month and Saturn around 10.30pm. Unlike many deep sky objects the presence of the Moon should not cause a problem.

A medium sized telescope should be able to show you some of the details in Jupiter's cloud bands whilst even binoculars should allow you to see Jupiter's 4 largest moons, appearing as bright dots on either side of the planet. For telescope owners the giant storm of the Great Red Spot is a favourite. Here are dates and times when the GRS will be crossing the centre of Jupiter during dark evenings this month - Sept 1st 10.03pm, 3rd 11.46pm, 6th 9.15pm, 8th 10.53pm, 13th 10.00pm, 15th 11.39pm, 18th 9.08pm, 20th 10.46pm, 23rd 8.16pm, 25th 9.54pm, 27th 11.32pm and 30th 9.02pm. Have a go if it is reasonably clear. For Saturn a medium sized or larger telescope will allow you to see Saturn's rings and a few of its brightest moons. Even a small 60mm scope can pick up Titan, Saturn's largest and brightest moon (8th mag) when at its furthest - 4 ring widths east or west of Saturn.

Uranus (mag +5.7, disk 3.7") in Aries rises about 10pm at the start of the month and 8pm at the end. It is observable with binoculars as a greenish star, but, through a telescope, requires high powers to see its tiny disk. The same applies to more distant Neptune (mag +7.8, disk 2.3" ) at opposition in Aquarius on Sept 14th. and visible all night.

Early in the month a lovely crescent Moon skims past the Beehive star cluster M44. However, this potentially stunning view with binoculars is on the morning of Sept 4th and so one for early risers. The Moon will be only 2 degrees from the cluster.

In the night sky generally, all of the objects that I covered last month are still very much on view - Vega, Deneb, Altair, the Cygnus Starcloud, Albireo, the North America Nebula NGC 7000, the Ring Nebua M57, the Dumbbell Nebula M27 and the Coathanger asterism Collinder 399. If you would like more why not start with the small constellation of Scutum the Shield below Aquila the Eagle. You will see a curved line of stars that ends at the rich star cluster M11 "The Wild Duck Cluster" (mag 5.8 dia 13') arguably the best open star cluster in the whole summer night sky. Lower down, if you have a good southern aspect towards the horizon, why not try for the famous Lagoon and Trifid Nebulae M8 and M20 in N. Sagittarius. However, you will have to catch these early, as soon as it gets dark. In contrast, almost overhead in N.Cygnus you will find the mag 4.6 open cluster M39 31' in diameter. With binoculars you can resolve about a dozen stars in the cluster in an area the size of the full Moon. If using a telescope overhead to view it I suggest that you use a right angled star diagonal prism to attach your eyepiece for more comfortable viewing.

Wishing you clear skies and hoping to see a number of you this thursday for CDAA's first meeting after lock down.

Best Regards