Before sunrise tomorrow – September 28, 2016 – rises in the east. You might catch it shortly before the sun’s first rays pierce the morning sky. Mercury, the sun’s innermost planet, swings to its greatest elongation on September 28 – its greatest apparent distance from the sun on our sky’s dome – when it will be 18o west of the sun. For reference, your fist at arm’s length spans about 10o of sky. As luck would have it, on this same date – September 28, before dawn – Mercury will appear beneath the waning crescent moon as darkness gives way to dawn.
A greatest western (morning) elongation of Mercury is most favorable in early autumn – and least so in early spring. That’s why the Northern Hemisphere enjoys the better view of Mercury in the morning sky – because it’s now early autumn in the Northern Hemisphere and early spring in the Southern Hemisphere.
It’ll be extremely difficult to spot Mercury from the temperate regions of the Southern Hemisphere. But try it, especially if you have binoculars that’ll let you sweep along the eastern, predawn horizon.
Assuming that you live at mid-northern latitudes, look for Mercury over the sunrise point on the horizon some 75 minutes (or sooner) before sunrise.
The moon will rise before Mercury does on September 28, so use the bow of the lunar crescent, which points to Mercury lurking close to the horizon. Remember that binoculars help our immensely with any Mercury search, especially if your sky is less than crystal-clear.
Mercury will remain visible in the morning sky for another week or two. If you’re really lucky, you might even spot the conjunction of Mercury with the king planet Jupiter, with the unaided eye or binoculars, before sunrise October 11, 2016.
Bottom line: Mercury beneath the waning crescent moon before sunrise on September 28, 2016. September 28 is also the day of Mercury’s greatest elongation, when the planet will be 18o west of the sun on our sky’s dome.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.