On September 4, 2015, the sun’s innermost planet Mercury swings to its greatest evening elongation (27o east of the setting sun). So is this a good time to see Mercury? Yes, if you’re in the Southern Hemisphere. This greatest elongation comes in the same month as the September equinox. That means the ecliptic – or path of the sun, moon and planets – makes a narrow angle with respect to the evening horizon for us in the Northern Hemisphere, but a steep angle for our fellow skywatchers on the southern part of Earth’s globe. That means a grand evening apparition of Mercury for the Southern Hemisphere – and poor one for the Northern Hemisphere.
Mercury is slightly brighter than Spica and takes stage in the same binocular field of view for several days, centered around September 20.
The most favorable time to see Mercury in the evening sky is when this planet’s greatest eastern (evening) elongation closely coincides with a spring equinox. Because the September equinox is the Southern Hemisphere’s spring equinox, southerly latitudes have a wonderful evening apparition of Mercury throughout this September.
On the other hand, an evening apparition of Mercury is most subdued near an autumn equinox. So we lose out this month, but, never fear, Mercury will put on a good show for us before dawn, beginning in October.
For us in the Northern Hemisphere, there is a consolation prize! Even at mid-northern latitudes, we should be able to catch the planet Saturn in the southwest sky at nightfall. See the chart below.
Bottom line: Mercury, the innermost planet of the solar system, reaches its greatest evening elongation of 27o east of the setting sun on September 4, 2015. It’s an awesome apparition for the Southern Hemisphere, and a poor one for the Northern Hemisphere. If you get pics, post them at EarthSky Facebook, or submit photos here.