CELESTIAL FIELD TRIPS: May is the month for Mars, Mercury, and the Moon
By Jim Walker
Mars has moved far from the Earth since its close approach last year, but it still has delights for the naked-eye or binocular observer. On the evenings of May 21 to 23 the red planet will pass through the heart of the Beehive cluster (M44), a spectacular open cluster of stars that is easily seen with the unaided eye. Recognized as a “nebulosity” (a fuzzy patch in the sky) by the ancient Greeks and Romans, the Beehive can be found in the constellation Cancer. The cluster is only 577 light years from Earth, but its exact nature was unknown until 1609 when Galileo observed it in his newly constructed telescope. Delighted at the swarm of sparkling stars revealed by his primitive instrument, Galileo reported that the nebula “is not one star only, but a mass of more than 40 small stars.” And so it is, as you can readily verify by pointing your binoculars or spotting scope at the center of the Cancer constellation. The Beehive is always worth a look, but the sight of a ruddy Mars in the midst of the “bees” should be a sight to savor, especially with Mars shining brighter than the combined output of all of the Beehive’s stars.
Beehive Cluster
May is also the month for the best views of Mercury. Look to the west-northwest about 45 minutes after sunset during the first couple weeks of May. If you are blessed with clear skies, and an unobstructed view of the horizon, the brightest object in that portion of the sky will be the elusive innermost planet. It begins the month just below and to the left of the Pleiades, and over the next few days will move upward until it is above that cluster of stars. Mercury reaches its greatest elongation—greatest height in the sky—on the evening of May 13/14. At that time, it will set a full 2 hours after the Sun. If you take a look at the planet through a spotting scope, you should be able to observe its changing phases—in fact, the illuminated portion of Mercury’s disk will shrink from half on May 8 to only 37 percent on May 13.During all this activity with Mercury, the Moon will also be vying for attention. A 2-day-old crescent Moon enters the scene on May 6, just 3? above and to the right of Mercury. The tight grouping of Mercury, the Moon, and the Pleiades should make for a most memorable sight. As you observe the Moon, it’s interesting to reflect on the case of Mars and its inner moon Phobos. Whereas our moon orbits beyond the geosynchronous limit, and moves from east to west across the sky, Phobos orbits well inside that limit and therefore moves across the sky in the opposite direction, rising and setting approximately twice a day. Because of its low orbit, Phobos moves closer to Mars by about 6 ft per century, meaning it will ultimately crash into Mars, or break up into a ring of particles. More good reasons—especially on Mars—to keep looking up!
