May 2007

by Deane Morrison

Watching that brilliant evening star come out in the west, it's clear that the night belongs to Venus. Shining at full wattage, our sister planet lingers for three-and-a-half hours past sunset. On May Day Venus glides between the horns of Taurus, and on the 19th the planet and a crescent moon appear spectacularly close to each other--so close, in fact, that you could use the moon to find Venus in the daytime by looking near the lower, or left, cusp of the crescent. Two nights later, Venus forms the point of a nearly perfect isosceles triangle with the Gemini twin stars Castor and Pollux.

Mercury, the other inner planet, can't match Venus for brilliance. Being the closest planet to the sun, Mercury can never get very far from it in the sky. No doubt the ancients named this planet after the messenger of the gods because of its habit of quickly popping in and out of the morning and evening sky, as if it were at the beck and call of planets with more staying power. Look for it after mid-month low in the west-northwest, below and to the right of Venus.

Two other bright planets ornament the evening sky. Jupiter, rising ever earlier in the evening, is in Ophiuchus, just east of Scorpius. By the end of May the king of planets will clear the horizon in twilight. Earth is about to zip between Jupiter and the sun, at which point the planet will be up all night. Saturn, still bright, is being carried close to Venus along with its companion stars. Through a telescope you can see the shadow Saturn casts on its rings, which gives the lovely golden planet an especially strong 3-D appearance.

Mars, the sole bright morning planet, rises about two hours before dawn. Mars is slowly brightening as Earth chases it around the sun. We'll finally overtake this speediest of outer planets on Christmas Eve.

May is favored with two full moons this year, and both occur close to the time of moonrise or moonset. This means we get a double dose of very round, softly lit moons that seem quite large due to being so near the horizon. The first, at 5:09 a.m. on the 2nd,

comes about a half-hour before moonset. The second, at 8:04 p.m. on the 31st, arrives just over an hour before moonrise.

The full moon of May is called the flower moon, corn planting moon or milk moon. With two full moons this month, it's probably safest just to call the second one a blue moon, as the second full moon in a calendar month is commonly known. Whatever we call it, it will close out the month in style, rising just below the huge red star Antares, in Scorpius, and not far from Jupiter.

The bright star in the south on May evenings is Spica, representing an ear of grain held by the Virgo virgin. Above Spica, a very bright star anchors the kite-shaped constellation Bootes. This is Arcturus, one of the most brilliant stars in the sky.

Arcturus has long been known to be following an unusual orbit around the center of our Milky Way galaxy. While most stars we see are orbiting in the plane of the galactic disk, Arcturus is plunging through the disk at a steep angle. There are two possible explanations, says University of Minnesota astronomy professor Terry Jones. Either Arcturus was born outside the disk of the Milky Way, or the giant star is an immigrant to our galaxy, one of many stars that began life in a much smaller galaxy that was torn apart by the Milky Way's tremendous gravity.

The star's name means "follower of the bear," and it can be found by extending the curve of the Big Dipper's handle. The Big Dipper, of course, is the most prominent feature of Ursa Major, the great bear.