Russell Eberst's Sky View

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September 2018

The first month of Autumn in the northern hemisphere sees the evenings drawing in rapidly. For astronomers this has the advantage of longer nights when the presence of misty conditions can indicate that the atmosphere is very stable and that the lack of turbulence means that higher telescope magnifications could be rewarding. Autumn (or Fall as it is called in U.S.A.) is the time for harvesting crops. Farmers can take advantage of an useful phenomenon when, for a few nights around the Full Moon in September, the bright Moon will rise at almost the same time every evening, allowing the harvest period to extend into the hours of darkness. This is referred to as the "Harvest Moon" and this year occurs around September 25 with the Moon in the constellation of Pisces. In Edinburgh, moonrise on September 21 is at 18.25 B.S.T., and by September 29, the Moon rises at 21.06 B.S.T., so getting later by only just over 17 minutes per night. It is a coincidence that this year the Harvest Full Moon falls on nearly the same day as the Autumn equinox. September 23 sees the Sun crossing the celestial equator going southwards. Around this date the length of day and night are roughly equal giving a period of 12 hours with the Sun above the horizon, and 12 hours with the Sun below the horizon. Dark skies will last about nine or ten hours.

During these early Autumn nights in the northern hemisphere, stars can be seen that appear regularly each and every year at this time. Constellations visible include Pegasus, Andromeda, Perseus, Cetus, as well as Cassiopeia and Cepheus which can be seen on any night of the year from high northern latitudes. This year, however, all these star groupings are outshone by the planet Mars. This is found low in the south during the evening hours, but despite receding from us and fading (halving in brightness over the course of the month) it is still prominent enough to attract many observers. Mars has now resumed its more normal prograde (eastward) motion. At the beginning of September it is on the border between the two constellations Sagittarius and Capricornus. It slowly but steadily builds up speed traversing much of Capricornus by the end of the month. The next few months will see it gaining in elevation at the times of its meridian crossing. This increase in elevation will compensate somewhat for its increase in distance, and good views will be available throughout the month. The Moon swings past Mars on September 20, being about 5° away from the red planet. The other bright planet visible in the evening is Saturn. It can be seen due south as the sky darkens, but at an elevation of only 11°. Within a couple of hours it is too low for observing, with atmospheric effects causing unacceptable shimmering. With Venus and Jupiter already lost in the dusk twilight, observers might like to turn their attention to Uranus. It now reaches in excess of 45° when due south. It's located in the south-west corner of the constellation of Aries, the Ram. At magnitude +5.7, it is possible to spot it with the unaided eye, but only from places with no light pollution and when moonlight is absent. Binoculars or a telescope will show a pale blue-green disc, allowing it to be picked out from any background stars. It retrogrades westwards slowly from night to night, again easing identification in comparison with nearby stars. After Uranus, we have to travel a long distance along the zodiac until we reach Mercury.

During September, Mercury is found in Leo, and has its best morning apparition of the year for those in the northern hemisphere. At around magnitude -1, it should be easily picked out in the dawn twilight, passing close to the star Regulus on September 6. Two mornings later the thin decrescent Moon joins the group, and provides an opportunity for astrophotographers. Although there are no major meteor showers expected in September, there may well be an abundance of these 'shooting stars', since the (northern) Autumn, or Fall, displays more than its fair share of these transient "sparks". Many will radiate from the region of Aries and Taurus, including some extra bright fireballs for which this month has a good reputation. Most meteors originate from comets, and it is many months since we had a really bright comet grace our skies. This month does however bring a comet that should be fairly easy to detect in binoculars or telescope. Named Giacobini-Zinner after its two discoverers, its could reach magnitude +7 around the time of its closest approach on September 11 some 36 million miles (58 million km.) from Earth. Back in 1946, on a similar approach, this comet experienced a number of outbursts in brightness, so it's worthwhile monitoring its brightness in case something similar happens. The comet begins the month in Auriga; on September 2 it is only about 2° south-west of Capella. By September 10 it will pass close to the open star cluster M37. A great opportunity for a comparison in brightness of these two different types of object. Star charts for the comet are found at the heavens-above.com website, which also has maps for the transits of I.S.S., visible September 3-16 in the morning sky, and during the last week of the month in evenings.

The weekend of September 29-30 sees the annual Open days at the Royal Observatory Edinburgh on Blackford Hill. From 10 a.m. until 5 p.m., the Observatory will be open (free of charge) for the public to learn about matters astronomical, and about the work and research being performed at one of Britain's leading scientific sites.

Moon phases:

  • Last Quarter September 3
  • New Moon: September 9
  • First Quarter: September 17
  • Full Moon: September 25

Sky View Image

The maps show the sky at 23:00 BST on the 1st, 22:00 on the 16th and 21:00 on the 30th. An arrow depicts the motion of Mars.