The sky in May

Natural History, May, 2007 by Joe Rao

Mercury reaches superior conjunction, behind and roughly in line with the Sun, on May 3rd. It passes the point of its orbit closest to the Sun (its perihelion) on the 8th, and quickly enters the evening sky. By the night of the 10th it sets fifty minutes after the Sun and shines at magnitude -1.5, just a trifle brighter than Sirius, the brightest star in the sky. The planet, in hues of topaz, can be picked up in binoculars. Look for it far to the lower right of Venus, near the west-northwest horizon; Mercury is the brightest starlike object in its part of the sky.

Mercury becomes easy to see with the naked eye after midmonth. It sets near the close of evening twilight. On the evening of the 17th Mercury is close to the horizon and about two and a half degrees to the lower left of a young crescent Moon. Although slowly fading, the little planet gains altitude rapidly day by day. By the 27th it climbs to within twenty-two degrees of Venus (your clenched fist held at arm's length is roughly ten degrees wide). Then, for about the next ten days, the two planets seem to stay almost fixed in their respective positions above the dusk horizon.

Venus is the brightest evening "star" in the west; at a dazzling magnitude of-4.2, it is bright enough to show through the blue sky soon after sunset. Appearing at its greatest height in the evening twilight for 2007, the planet stands nearly forty degrees above the western horizon at sunset. By the third week of the month it's setting in the northwest shortly before midnight local daylight time (LDT). On the evening of the 19th, Venus and the crescent Moon make a stunning celestial tableau as they descend the western sky side by side, less than a degree apart. Planetariums--and police precincts--will likely get a few calls inquiring about the "UFO" hovering next to the Moon! By month's end Venus is within several degrees of Pollux and Castor, the bright stars in the constellation Gemini, the twins. Telescopic viewers can observe Venus becoming larger and its crescent thinner as it swings around from the far side of the Sun toward the Earth.

Mars rises within about a half hour of the first sign of morning light. Look for the orange first-magnitude object low in the east as dawn brightens. It moves eastward this month, and crosses from the dim stars in the constellation Aquarius, the water-bearer, into the similarly dull constellation Pisces, the fish, on the 9th. It also passes to the south of the Great Square of Pegasus.

Jupiter is the brilliant light pushing its way up into the southeast sky during the evenings. The giant planet rises around 10:45 P.M. LDT as May begins. By month's end it is rising around 8:30 P.M. and is already above the horizon as darkness falls, shining at magnitude -2.5. To its right or lower right is Antares, the red first-magnitude heart of the constellation Scorpius, the scorpion. The entire "fishhook" of the scorpion is extracting itself almost straight up from the horizon to Jupiter's right.

Saturn appears as a yellowish-white "star" of magnitude 0.4, about eleven degrees west (to the lower right) of Regulus, the brightest star in the constellation Leo, the lion. Saturn is more than halfway up in the southwest sky as darkness falls. It sets around 2:45 A.M. LDT on the 1st and a couple of hours earlier by month's end. At midmonth, a thirty-power telescope shows the famous ring system tilted about fifteen degrees from our line of sight.

The Moon is full on the 2nd at 6:09 A.M. Our satellite wanes to last quarter on the 10th at 12:27 A.M. and to new on the 16th at 3:27 P.M. The Moon waxes to first quarter on the 23rd at 5:03 P.M. A second full moon takes place on the 31st at 9:04 P.M. The second full moon in a calendar month is sometimes referred to as a "blue moon."

Unless otherwise noted, all times are eastern daylight time.