A monthly look at astronomical events in the sky and on Earth
After the short and light nights of summer, September is the first month in which the sky goes completely dark every night (i.e. astronomical twilight ends, and the sun dips to more than 18° below the horizon). At the start of the month, total darkness runs from 2211 to 0355 BST, but by month end, it runs from 2043 to 0504 BST. So in addition to other delights of the night sky, we can observe faint and diffuse deep sky objects without having to stay up really late.
Another feature of September is of course that it contains the autumnal equinox. This is the moment that the Sun crosses from the northern to the southern hemisphere, and everywhere on Earth, day and night are exactly 12 hours long. (‘equinox’ from medieval Latin equal night ). This is at 0850 on Monday the 23rd this year.
Finally, in September, the ecliptic (the path across the celestial sphere taken by the sun, the moon and most of the planets) is angled at its lowest at sunset. This means that the moon or any planet to the east (left) of the sun will be rather low in the sky in the hours following sunset. Conversely, for anyone able and wanting to observe solar system objects to the west (right) of the sun in the hours before sunrise, the ecliptic is aligned at its greatest angle, causing those objects to be at their highest.
Best seen as a thin waxing crescent low in the south-west after sunset on Mon 2nd to late gibbous low in the south on Tue 10th. It doesn’t climb much above 15° altitude throughout this period due to the angle of the ecliptic as explained above. Saturn will be seen 3½° to the right of the moon on the 8th.
Mercury, Venus and Mars
Are all on the far side of the sun and completely unobservable this month.
At the beginning of the month, Jupiter is low in the SSW at sunset and by the end of civil twilight (about 2030 hrs) has sunk to only 12° above the horizon. At over 38 arc seconds it’s still worth observing, but take the opportunity when you can because by mid-month it’s down to only 10° altitude and approaching the sunset glow.
Saturn is conveniently placed for observation for the whole of the month. It’s due south and highest in the sky at 2125 at the beginning of the month and 2030 on the 15th. From then on, it can still be seen in the SSW as it approaches the sunset glow. The moon is 4° to its left on the 9th.
The YAS has organised an event at Beetle Bank for the 7th Sept. when Jupiter, Saturn and the Moon will be observed, together with a number of deep sky objects. This is probably your last chance to see these two giant planets well this year. See the Events page for more details and to book a place.
Uranus reaches opposition in late October and will be conveniently placed for observation in November and December: leave it until then.
Neptune reaches opposition on the 10th of the month and is therefore due south and well placed at 0100 around that time. At magnitude 7.8 you’ll need a finder chart to locate its 2.32 arc seconds diameter blue disc, and to see it as a disc rather than a star, you’ll need a fair-to-decent telescope, say 6-inch or larger. Its apparent diameter is about the same as the diameter of the four main satellites of Jupiter. If you’re going to try to spot it, wait until November, when it can be well see without staying up late and because of its great distance from Earth, will only have reduced in magnitude to 7.9 and in size to 2.27 arc seconds. We’ll publish a finder chart in November’s What’s Up!
Non-Solar System Objects
September is a great month for deep sky observing because not only does the sky get really dark before bedtime, but the evenings are still quite often comfortably cool rather than cold. (September 1st is officially the start of meteorological autumn.)
I would recommend for September the following additional two objects. Both can be seen in the south at about 2300 on the 1st, 2200 on the 15th, and 2100 on the 30th. These are arrowed on the following sky map.
M2 is an exceptionally fine globular cluster. Its magnitude is 6.3 making it just discernable to the naked eye on a clear, dark night. It is 16 arc minutes in diameter, roughly half that of the Moon. It looks like a fuzzy patch in binoculars, partly resolvable into individual stars with a 6-inch telescope, and quite magical in a 12-inch.
Another globular, and even better than M2. Its magnitude is 6.2 and its diameter 18 arc minutes. It has a particularly condensed centre and is worth reading about in Wikipedia before you observe. There is an impressive HST image of it in that article.
It is not normally the practice of the YAS to run commercial advertising, so the following is simply a personal recommendation from the writer. I have been using various editions of the book Norton’s Star Atlas for over sixty years. There is no book that comes close to presenting such a wealth of interesting and useful knowledge and advice aimed right at people like you! It isn’t just a book of maps; it is a complete reference book that’s a joy to read. If you haven’t got a copy, do order one so you have it ready for the season. It was first published in 1910 and has now reached its 20th edition. If you order it (from Amazon or any decent bookshop), make sure you specify the latest edition, and make sure it’s the hardback binding.
ISBN-10: 0131451642 / ISBN-13: 978-0131451643
Passes of the International Space Station (ISS)
For up-to-date news on space missions, rocket launches etc. visit https://www.space.com/32286-space-calendar.html
Clear skies and good viewing and maybe we’ll see you on the 7th at Beetle Bank.
John Rowland 29/08/2019