Mercury reaches greatest elongation east on September 4, 2015

On September 4th, Mercury the innermost and fastest moving planet reaches greatest eastern elongation at 27 degrees. From our perspective this is about as distant as Mercury ever draws itself from the Sun in the sky. For observers at tropical and Southern Hemisphere latitudes the planet is now visible as an "evening star" low down above the western horizon just after sunset. Unfortunately from northern temperate latitudes, Mercury is inconveniently placed and not readily observable this month.

To spot the smallest planet of all you'll require a relatively unobstructed view of the western horizon and of course some clear skies. For example from latitude 35S (approx. equal to Sydney - Australia, Cape Town - South Africa and Santiago - Chile) Mercury appears 17 degrees above the horizon 45 minutes after sunset on September 4th. With a magnitude +0.1, it should be easy to spot the planet as the sky darkens. Also visible are first magnitude stars Spica (α Vir - mag. +1.0) and Arcturus (α Boo - mag. -0.04). Of the three, Mercury appears slightly fainter than Arcturus with both Mercury and Arcturus about 2 1/2 times brighter than Spica.

After greatest eastern elongation, Mercury will still be visible for a few days. Each subsequent evening the planet dims slightly and draws closer to the Sun. By the third week of the month it will be very low above the horizon, down to magnitude +1.7 and effectively lost to the bright evening twilight.

The diagram below shows the August / September evening apparition of Mercury from a latitude of 35S. Positions of the planet are displayed 45 minutes after sunset.