The summer’s brilliant pageant of planets is now tarnishing, as Venus exits the stage and Mars continues to fade. This month the nights become longer and we move towards unsettled weather. The autumn constellations are starting to show. Ursa Major, or the Plough, can be found low in the North. The ‘W’ of Cassiopeia is not far from the overhead point
Also on view this month we have, Aquarius the water carrier, Cetus the sea monster, Capricorns the sea goat, Pices the fishes, Piscis Austrinus the southern fish and Delphinus the Dolphin.
The summer triangle of Altair, Deneb and Vega remains high up. The southern sky is dominated by the Square of Pegasus. The bright star Capella in the constellation of Auriga the Charioteer is becoming more noticeable in the east. It will be overhead in winter evenings.
Although the four stars that form the Square of Pegasus are not the brightest, once found they will be easily recognised again. It is always an interesting project to count how many stars you can see within the square; you might be surprised by the result.
If you use the two right hand stars of the square and draw a line to the south you will reach a bright star very low in the sky. This star is Fomalhaut, in the constellation of Piscis Austrinus.
This month on Sunday, 23rd September we have the autumn equinox. After this date the nights noticeably lengthen.
On the 10th of the month, Comet Giacobini-Zinner will be at its closest to the Sun. It should be a binocular object at magnitude +7. There will be some lovely views of it passing the stars clusters M37, M35 and NGC2264
Jupiter can be seen in the west soon after sunset at the start of the month. It shines at magnitude -1.9 (falling to -1.8 during the month) and has a disk some 35 (falling to 33) arc seconds across. Jupiter’s equatorial bands, sometimes the Great Red Spot and up to four of its Gallilean moons will be visible in a small telescope. Sadly, moving slowly eastwards in Libra during the month, Jupiter is heading towards the southern part of the ecliptic and will only have an elevation of ~10 degrees after sunset.
Saturn will be visible in the south at an elevation of ~14 degrees after sunset at the beginning of September. Its disk has an angular size of 17.5 arc seconds falling to 16.5 during the month. Its brightness reduces from +0.4 to +0.5 magnitudes as the month progresses. The rings were at their widest some months ago and are still, at 25 degrees to the line of sight, well open and spanning ~2.5 times the size of Saturn’s globe. Saturn, lying in Sagittarius, halts its retrograde motion on the 6th within a few degrees of M8, the Lagoon Nebula, and M20, the Trifid Nebula.
Mercury can be seen low in the east-northeast some 30 to 45 minutes before sunrise during the first week of September. On the 5th and 6th, Mercury, shining at magnitude -1, is just over one degree from Regulus in Leo (at magnitude +1). Around the 11th of the month Mercury disappears into the Sun’s glare as it moves towards superior conjunction (behind the Sun) on the 20th of the month.
Mars, which ceased it retrograde motion westwards in Capricornus (and just moving into Sagittarius) at the beginning of the month made its closest approach to Earth since 2003 on the night of July 30th/31st. After sunset, Mars can be seen just east of south shining at a magnitude of -2.1 but this falls to -1.3 by month’s end. Its angular size is 21 arc seconds at the start of the month falling to 16 arc seconds by the start of October. With a small telescope it should (but see below) be possible to spot details, such as Syrtis Major, on its salmon-pink surface.
Venus, was at greatest elongation east on the 17th August but is now only seen low in the west southwest after sunset setting at about 80 (reducing to 45) minutes after the Sun. The planet brightens from -4.6 to a dazzling -4.8 magnitudes making it easier to spot in the Sun’s glare. Binoculars might be needed to spot it but please do not use them until after the Sun has set. Its angular size increases from 29 to 46 arc seconds during the month as the percentage illuminated disk (its phase) narrows from 40% to just 17%.
The phases of the Moon are:-
Last Quarter September 3rd
New Moon on September 9th
First Quarter September 17th
Full Moon September 25th