SKY TALK – SEPTEMBER, 2017
Celestial Traffic Jam!
Most of our columns (in addition to telescope-related topics) are devoted to celestial objects and events visible in the evening sky. But occasionally, something spectacular happens in the pre-dawn morning sky worth rising early for—as is the case of this month’s lunar-planetary-stellar conclave.
On the early morning of September 18th about 45 minutes before sunrise, facing due east, a wondrous spectacle will greet the eye. There, highest in the sky and brightest, will be the radiant planet Venus. Below it will be the blue-white first-magnitude star Regulus. Beneath it will be a razor-thin young crescent Moon. Lower still will be the ruddy planet Mars (currently rather faint), and just above the horizon the elusive planet Mercury. A cosmic lineup involving a star, the Moon and three planets!
This near–perfect alignment will be visible for several mornings both before and after the 18th. However, the Moon won’t be part of it. It moves its own diameter eastward every hour in its never-ending monthly orbit. Before the morning of the lineup, it will likely be barely visible to the lower right and afterward to the upper left as it climbs higher in the sky. While this is a naked-eye event, binoculars will provide a fascinating scene, with several objects in the same field of view at once.
The Moon itself will offer an added bonus to the lineup since it will be in its crescent phase. Even with the unaided eye, you may notice that the “dark” portion is not really completely dark at all but faintly lit up. This is the famed “Earthshine” resulting from the light of the morning Sun shining over the Atlantic Ocean, reflecting up onto the Moon, and then back down to the Earth where it is still in darkness. The view of the Moon with the ghostly Earthshine within its illuminated slender crescent is especially spectacular in binoculars and small telescopes.
Mention should be made of another naked-eye sight prior to the actual lineup itself. On the morning of September 10th about 30 minutes before sunrise, look for the planet Mercury nearly touching the star Regulus to its left! As the innermost fastest-moving world closest to the Sun, it’s usually seen only briefly in the evening or morning twilight, making it the most elusive of the five naked-eye planets. Indeed, many skywatchers have never even seen it. Here’s a great opportunity to finally spot this “shy” inner world. Again, binoculars and small telescopes should provide a fascinating sight of star and planet in the same field of view.
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.