OUT THERE 



THE SKY IN APRIL 



By Joe Rao 



(Continued from page 70) 



turn reflected so much sunlight, that 

 the cloud of comet dust obscured the 

 impact crater from view. By the time 

 the cloud cleared, the Deep Impact 

 mother ship had already flown too far 

 away from the comet to get a clear look 

 at the crater bottom. So A'Hearn and 

 his collaborators still cannot give an ex- 

 act measure of the crater's depth. 



But the gigantic plume from the im- 

 pact has led to another important pre- 

 liminary conclusion. The internal struc- 

 ture of Tempel 1 is downright flimsy. 

 The projectile from Deep Impact didn't 

 hit a rock or an ice cube or even some- 

 thing in between. Instead it whacked in- 

 to a loosely packed ball, assembled slow- 

 ly out of sand and snowflakes over bil- 

 lions of years. The particles did not get 

 cemented together by melting and reso- 

 lidification, and apparently are barely 

 held together by gravity. 



So what? Consider this: If a killer 

 comet bound for Earth were equally 

 flimsy, we earthlings probably couldn't 

 use a single powerful rocket to push or 

 pull it off its collision course. Instead, 

 the comet would break apart, leaving 

 most of its deadly mass still hurtling to- 

 ward our hapless home. 



A few days after the historic colli- 

 sion, the Deep Impact mother ship 

 was still fully operational. So a big ques- 

 tion loomed: Should the trajectory of 

 the spacecraft be adjusted to fly by 

 Comet Tempel 1 again, in 2011, to get 

 a fresh look at the crater after the dust 

 has settled? After weighing the options, 

 A'Hearn's team decided instead to aim 

 the space probe toward another comet, 

 85P/Boethin, for a rendezvous in 2008. 

 If all goes as planned, a little more than 

 two years from now astronomers will 

 once more get a close-up view of an icy 

 cosmic dirtball. Since the Deep Impact 

 impactor is nothing but interplanetary 

 vapor now, though, we'll all have to do 

 without the Fourth of July fireworks. 



Charles Liu is a professor of astrophysics at the 

 City University of New York and an associate 

 with the American Museum of Natural History. 



On April 8, one day after passing aphe- 

 lion (its farthest point from the Sun), 

 Mercury reaches its greatest western 

 elongation. One might hope for good 

 viewing, since the planet attains its 

 greatest possible angular separation 

 from the Sun — twenty-eight degrees. 

 But for observers at mid-northern lat- 

 itudes, that morning's apparition is the 

 poorest of the year, because Mercury 

 is well to the south of the Sun and hugs 

 the eastern horizon for much of the 

 month. On the 8th, Mercury shines 

 at magnitude 0.3. Because the planet 

 rises less than fifty-five minutes before 

 the Sun, however, it is soon lost in the 

 glare of sunrise. 



Venus is the brightest morning "star" 

 this month, but it blazes low in the 

 east-southeast during the first light of 

 dawn. As the sky brightens and Venus 

 rises, the planet appears to fade and 

 shrink. This month, it gets a little low- 

 er and less brilliant each week. On the 

 morning of the 1 8th Venus has a close 

 encounter with Uranus, the seventh 

 planet from the Sun. With Venus as a 

 guide, use binoculars to find a small, 

 greenish-blue "star" just 0.3 degree 

 below and slightly to the right of 

 Venus. That "star" is Uranus. On the 

 24th Venus and the waning crescent 

 Moon pair up in dawn's early light. 



As evening twilight ends, Mars is in 

 the western sky, nearly halfway be- 

 tween the horizon and zenith (the 

 point directly overhead). This month 

 the Red Planet continues to fade and 

 shrink as it speeds eastward from the 

 constellation Taurus, the bull, and in- 

 to the constellation Gemini, the 

 twins, by the 14th. Two evenings lat- 

 er it is midway between the orange 

 star Aldebaran, in Taurus, and the yel- 

 low star Pollux, in Gemini. A fat cres- 

 cent Moon hovers well above Mars 

 on the evening of the 3rd. 



As April begins, Jupiter rises in the east- 

 southeast about an hour and a quarter 

 after evening twilight. The planet is 



about two and a half degrees east of the 

 star Alpha Librae. Each night, Jupiter 

 moves a bit farther to the west; on the 

 night of the 24th and 25th it passes one 

 degree north of the star, the second of 

 three conjunctions the pair has in 2006. 



Saturn, in the constellation Cancer, the 

 crab, rides high in the south-south- 

 west as darkness falls and doesn't set 

 until well past midnight all month. In 

 a telescope the rings are tilted twenty 

 degrees toward Earth. After this 

 month, you won't see the rings tipped 

 this far again until 2014! In fact, in 

 three years the rings will be edge-on 

 to our line of sight. On the evening of 

 the 6th, Saturn is situated below a 

 waxing gibbous Moon. 



The Moon waxes to first quarter on 

 the 5th at 8:01 a.m. and to full on the 

 13th at 12:40 p.m. It wanes to last 

 quarter on the 20th at 11:28 P.M. and 

 to new on the 27th at 3:44 P.M. 



On the 1st, observers across much of 

 eastern North America can watch as 

 a waxing crescent Moon occults, or 

 crosses in front of, the main body of 

 the Pleiades star cluster, in Taurus. 

 With binoculars or a low r -power tele- 

 scope, watch between about 7:20 and 

 9:40 P.M. as each jewel-like star in the 

 Pleiades abruptly winks out behind 

 the Moon's dark side and, later, sud- 

 denly reappears from behind the sun- 

 lit crescent. The Moon's dark portion 

 will likely be dimly lit by earthshine, 

 giving it a mottled blue-gray and yel- 

 low-white cast. For observers in the 

 central and western states, the Moon 

 edges just to the east of the cluster as 

 darkness falls. 



Daylight saving time returns for much 

 of Canada and the United States on 

 Sunday, the 2nd. Remember to 

 "spring ahead," and set clocks for- 

 ward one hour. 



Unless otherwise noted, all times are giv- 

 en in eastern daylight time. 



74 



NATURAL HISTORY April 2006 



