The fall Equinox occurs on September 22nd at exactly 20:44 UT (4:44pm edt). On this date and time the Sun crosses the equator making the day and night of equal length. After that, the daylight hours will get shorter and the nights longer – which is of course good if you’re an astronomer. At the start of the month, nautical twilight begins at 8:47pm – the time when the brighter stars can be seen (those used for navigation). By the end of the month, nautical twilight will be at 7:48pm – almost an hour earlier. Astronomical twilight – the start of the darkest part of the night – begins at about 9:26pm on Sept 1st. By Sept 30th, Astronomical twilight begins at 8:23pm.
By mid September at 9pm, the constellations Lyra (the Harp) and Cygnus (the Swan) are directly over head. Hercules is a little to the west but still in a good position to see. The large constellation Pegasus (the Winged Horse) is rising in the east. The tail of Pegasus is actually the constellation Andromeda (daughter of Cassiopeia) and home to the Andromeda Galaxy which will be featured next month. The Milky Way is still prominent in the south with Sagittarius (the galactic centre) a little west of the meridian.
The Summer Triangle – an asterism formed by the three bright stars Vega (Lyra), Deneb (Cygnus) and Altair (Aquila) is due south and stretches from the celestial equator to almost the zenith. These stars are in the top 20 list of the brightest stars and are the brightest stars in that area of the sky. So the “summer triangle” is easy to spot.
A fun binocular object is Brocchi’s Cluster (Collinder 399) but better known as the Coathanger. In 7×50 binoculars it looks like a perfectly formed coat hanger in the centre third of the FoV – albeit upside down. The Coathanger is in the lower third of the Summer Triangle along the Altair-Vega line. To find it, locate Altair with binoculars and then scan a third of the way towards Vega. The Coathanger should be easy to spot. (While Cr399 fits nicely between Sagitta and Vulpecula, these constellations don’t stand out well.)
Another great binocular target is Ptolomy’s Cluster (M7). It’s a large open cluster that will occupy about a quarter of the FoV of 7×50 binoculars. It’s only 6° above the horizon though so requires an obstructed view.
M13 – the Hercules cluster and M92 are still the best bets for binocular globular clusters of the season. M27 and M57 are nice Planetary nebulae for moderate scopes either as visual or photography objects. The diffuse nebula of M8 and M20 are best seen in photographs due the red colour. Although the brighter reflection component of M8 is visible in small scopes and the open cluster M21 near M8 is also visible.
The photography project for this month is the Iris Nebula (NGC 7023, Caldwell 4) – a bright reflection nebula in the constellation Cepheus to the north. NGC 7023 refers to the open cluster while the nebula is LBN 487. The grouping is listed as mag 6.6 and 18′ in diameter. Although the region of interest is more like a degree wide. Images of it in real colour show it to be rather blue-purple with significant brownish dust lanes. The scope of choice is the AT106 with the Canon T2i which yields of FoV of 1.8×1.2°. 6-8hrs of imaging should be enough for a decent result which usually takes two clear nights to acquire.