Sky Talk May 2008: Mars Flies By the Beehive

For several evenings this month, skywatchers will have a chance to watch the planet Mars racing across the Beehive Star Cluster in the dim constellation Cancer, currently visible a third of the way up the western sky after evening twilight ends. And this is an event that can be enjoyed with the unaided eye, binoculars and low-power telescopes alike. If you’ve ever wanted to see a demonstration of planetary motion, this is your opportunity!

Circle the evenings of May 21, 22 and 23 on your calendar of upcoming celestial happenings, for on those dates Mars will pass across the face of the famed Beehive Star Cluster in Cancer providing a striking demonstration of the planet’s orbital motion. Facing west about 10 p.m. local time, Mars itself will be immediately obvious. But it will take careful attention to spot the big dim cluster with the eye alone, despite the fact that it’s about twice the apparent size of the full Moon. (A useful technique here is that known as “averted vision” — looking slightly to one side of the cluster, which brings the light-sensitive rods at the periphery of the eye into play and more than doubles the visibility of faint objects.)

Begin your vigil on the evening of the 21st, which will find Mars sitting just to the west (right) of the Beehive’s outer fringes. By the following evening, the planet will have moved into the center of the cluster, being positioned north of its sparkling core. On the 23rd, Mars will lie near the eastern (left) edge of this stellar clan, and by the following evening it will have completely cleared the Beehive. And while this night-to-night eastward movement of Mars will be obvious to the unaided eye even if you have difficulty spotting the Beehive itself, your Edmund binoculars will provide a thrilling view of both planet and cluster with plenty of sky around them — giving the overall effect of their floating in space (which, in fact, they are!).

But without question the very best way to fully enjoy this spectacle is through the eyepiece of Edmund Scientifics’ Astroscan telescope. This superb instrument was expressly designed for viewing events like the Beehive fly-by requiring a wide expanse of sky combined with lots of light-gathering power and more magnification than typical binoculars. This combination makes it possible to actually see Mars moving against the cluster’s starry background from hour-to-hour!

As an interesting aside here, visibility of the Beehive Cluster has been used since antiquity as a kind of “celestial barometer.” Its non-visibility on an otherwise apparently clear night indicates the presence of high altitude moisture and an advancing weather front, often bringing with it rain or snow depending on the temperature and season.

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of five books on stargazing.