BAS Observer September 2017

20 BAS OBSERVER The sky in September by Peter Allison B y September the nights are getting a little bit shorter and the centre of our Milky Way Galaxy is heading for the western horizon. So make the best use of the central region of our galaxy while you can – but also search new constellations such as Sculptor for some fantastic deep-space galaxy hunting. Dark-sky nights So . . . when will be the best time for an all-nighter observing session this month? From about Monday, 11th September, the Moon starts setting late enough to give some reasonably dark observing hours before it rises late in the evening. After New Moon you will have to wait until the Moon sets to have a properly dark sky, so 11th–21st September looks best this month. BEST DARK-SKY OBSERVING DATES FOR SEPTEMBER Day Date Moonrise Dark sky from sundown until moonrise Monday 11 th Moon rises at 10.35 p.m. Tuesday 1 2 th Moon rises at 11.3 8 p.m. Wednesday 13th Moon rises after midnight. Thursday 1 4 th Moon rises at 12.40 a.m. Friday 1 5 th Moon rises at 1.40 a.m. Saturday 16th Moon rises at 2.36 a.m. Sunday 1 7 th Moon rises at 3.28 a.m. Monday 18th Moon rises at 4.15 a.m. Tuesday 1 9 th Moon rises at 4.59 a.m. Moon interference until evening moonset Wednesday 20th New Moon Thursday 21st Moon sets 1 hr 28 min. after sunset. Alignment stars Two prominent stars in the southern sky that could be used for telescope alignment are Alnair in the constellation Grus (the Crane) and Achernar in Eridanus (the River). Eridanus is a bit hard to recognise, as it is just a string of unremarkable stars that have been mapped together as a wandering line; however, Achernar is reasonably bright in a fairly sparse region of sky. Alnair is a little easier to find, as the constellation Grus has a fairly distinct angular alignment that can catch the eye. Alnair forms the apex of one of the angular alignments and marks the wing of the Crane. Grus Eridanus Alignment stars: • Alnair • Achernar