SKY MAP: The sky as seen at 5:30 a.m. on May 4 from mid-northern latitudes.
This week will be especially interesting for skywatchers because of a fine array of bright planets in our evening sky. In fact, four of the five bright naked-eye planets are now readily visible beginning about 45 minutes after sundown.
Venus is the most obvious. It is bright enough to show through the blue sky soon after sunset. Even though its greatest elongation from the Sun won't be until June 9, Venus reaches the summit of its current evening apparition this month, appearing at its greatest height in the evening twilight for the year 2007.
Venus now shines at a dazzling magnitude of ?4.2 (more than 13 times brighter than Sirius, the brightest star in the sky) and stands nearly 40-degrees above the western horizon at sunset (your clinched fist held at arm's length measures roughly 10-degrees in width; so 40-degrees is roughly "four fists" up from the horizon).
Venus is now staying up very late, well past 11 p.m. for many locations. On Saturday, May 19, Venus and the crescent Moon will make for a stunning sight for North Americans, as the two objects will descend down the western sky side-by-side, only about one degree apart. I wouldn't be at all surprised if local media outlets receive a bevy of phone calls that evening all asking what that "strange light" (or UFO) is to the left of the Moon.
Jupiter lights up
Meanwhile, another brilliant light is pushing its way up into the southeast sky during May evenings: Jupiter. This week the giant planet rises around 9:30 p.m. local daylight time; by month's end, it's rising closer to 8:30 p.m. and is already above the horizon as darkness falls. Jupiter shines at a brilliant magnitude of ?2.6 (about one-fourth as bright as Venus). To its right or lower right is Antares, the red 1st-magnitude heart of Scorpius, the Scorpion. The full Moon will be passing to the south of Jupiter during the dawn hours of June 1.
Consider some of the contrasts between Venus and Jupiter: Venus is a sister Earth in size, but so shrouded in dense, hot clouds of carbon dioxide that its brilliance in our sky is largely due to its high reflectivity (about 76 percent). Jupiter on the other hand, is an entirely different type of planet?gigantic, surrounded by a thick atmosphere composed chiefly of methane and ammonia, and icy cold. Ordinarily it appears second only to Venus in brightness, its remoteness being compensated by its great size. Its surface area is 130 times that of Venus.
And have you ever wondered how the ancient Romans happened to name Jupiter after the most powerful of their gods, even though they knew nothing of the planet's physical characteristics?
Spot Saturn, too
Sitting roughly in between Jupiter in the southeast and Venus in the northwest is the ringed beauty of our solar system, Saturn.
Saturn appears as a moderately bright yellowish-white "star" very close to the border between the zodiacal constellations of Leo and Cancer, and about 11-degrees west (to the lower right) of Leo's brightest star, Regulus. Saturn can be found more than halfway up in the southwest sky as darkness falls during May. It will set this week at around 1:30 a.m. local daylight time; about an hour earlier by month's end.
A telescope magnifying 30-power or more will reveal that the famous ring system is tilted about 17-degrees from our line of sight. The nearly first-quarter Moon will lie to the left of Saturn as darkness falls on May 22.
Last but not least
Finally, late May provides us with an especially favorable apparition of Mercury for Northern Hemisphere observers.
This orange-yellow-hued planet currently rivals all the stars in brightness save for the two brightest, Sirius and Canopus. It can easily be picked up in binoculars shining through the Sun's bright afterglow. Just look for it far to the lower right of Venus near the west-northwest horizon; Mercury will be the brightest starlike object down there.
Mercury has its best evening apparition of the year during these next two weeks (for observers in the Northern Hemisphere), as an easy naked-eye object and setting near the close of evening twilight. Although slowly fading, the little planet gains altitude rapidly day by day. By May 27 it climbs to within 22-degrees of Venus. Then, for about the next ten days these two planets will seem to stay almost fixed in their respective positions above the dusk horizon.
On June 2, Mercury will stand at greatest elongation, 23-degrees east of the Sun. During the following week it will rapidly fade as it circles back into the glow of the setting Sun.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.