Sky Talk March 2011: Mercury’s Spring Apparition

Many skywatchers have never seen the elusive innermost planet Mercury due to its rapid orbital motion and the fact that it never strays far from the Sun. (This includes lots of professional astronomers as well!) March offers a great opportunity to spot it at one of its periodic elongations. During the second half of this month, Mercury will be readily visible to the unaided eye in the darkening western sky after sunset.

As Mercury orbits the Sun every 88 days, it periodically appears briefly either east (left) of the Sun in the evening sky or west (right) of it in the morning one. These excursions are known as elongations, and when one occurs in the evening sky of spring it presents the best opportunity to see the planet due to the high inclination of its ecliptic pathway at that time of year with respect to the horizon. On the evening of March 22, Mercury will be 19 degrees east of the setting sun and stand 12 degrees above the western horizon 30 minutes after sunset.

Adding to the fun — and making it even easier than usual to locate the planet this apparition — on the evening of March 16, Mercury passes just 2 degrees north of much brighter Jupiter. A nice sight even to the unaided eye, this pairing will be spectacular as seen in binoculars and wide-field telescopes like the Edmund Astroscan Plus. Viewed in the great little Scientifics 60mm refractor with its 35x eyepiece, Jupiter will show a slightly gibbous disc while the 87x eyepiece will show Mercury like a tiny half-full Moon. While larger scopes will provide a bigger image, Mercury may well not appear as distinct due to the atmospheric turbulence typically found this close to the horizon. After the 22nd, this rapidly-moving “Winged Messenger of the Gods” quickly slides back towards the Sun and drops ever-lower into the sky until disappearing into the sunset by month’s end. Telescopically, its phase changes from half to a narrow crescent.

As a budding stargazer myself over half a century ago, I had heard how difficult Mercury was to see and so made no conscious effort to find it. Legend even had it that among others the famed astronomer Copernicus never saw Mercury! (Recent historical research now casts doubt on this.). But then once evening I saw this obvious “star” shining in the western twilight sky and checking found that it was indeed Mercury! I was quite thrilled — but also embarrassed that I had waited so long to locate it. Suffice it say that this shy and elusive planet requires careful attention to its favorable but brief apparitions to be seen. If you’re among those who have yet to spot it, I can promise you a sense of excitement and accomplishment once you do. And you’ll never have a better opportunity to look for it than this month. Don’t miss it!

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