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This Week's Sky at a Glance

by Alan M. MacRobert

Some daily events in the changing sky for April 25 – May 3.

Friday, April 25

  • Bright Sirius, the "Winter Star," still sparkles in the southwest during twilight. How late in the season can you keep it in view as it sinks away? Its last date of visibility is called its date of heliacal setting.

    Saturday, April 26

  • Mars is passing less than 5° south (lower left) of Pollux, its near-twin in brightness and color, tonight and for the next couple of nights.

    Sunday, April 27

  • As twilight fades this week, Vega, the "Summer Star," is just rising over the northeast horizon. How soon in the darkening evening can you first see it?

    Monday, April 28

  • Last-quarter Moon (exact at 10:12 a.m. Eastern Daylight Time).

    Tuesday, April 29

  • The red long-period variable star RS Herculis should be at maximum light (8th magnitude) around this date.

    Wednesday, April 30

  • A small telescope will always show Titan, Saturn's largest and wildest moon. This evening Titan is four ring-lengths to Saturn's west. A 6-inch telescope will begin to show the orange color of its smoggy atmosphere.

    Thursday, May 1

  • Use binoculars in twilight to spot the Pleiades just 2° upper right of Mercury, as shown below.

  • Happy May Day! In Celtic societies this was celebrated as Beltane, one of the four "cross-quarter days" marking the midpoint of each season.

    Friday, May 2

  • Now that it's May, the Big Dipper floats upside down nearly overhead in the north after dark. Its curving handle points around toward bright Arcturus, the "Spring Star," which dominates the high eastern sky. Arcturus is a yellow-orange giant, spectral type K1.5 III. To me it looks ginger-ale colored — or as Sky & Telescope columnist Fred Schaaf put it more elegantly, "champaign colored."

    Saturday, May 3

  • For observers in the tropics and the Southern Hemisphere, the annual Eta Aquarid meteor shower is getting under way. It runs for about five days. The time to watch is in the hours before dawn. (You can find the time of dawn's first light at any location worldwide using our online almanac.)


    Want to become a better amateur astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly foldout map in each issue of Sky & Telescope, the essential magazine of astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

    Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of maps; the standards are Sky Atlas 2000.0 or the smaller Pocket Sky Atlas) and good deep-sky guidebooks (such as Sky Atlas 2000.0 Companion by Strong and Sinnott, the even more detailed Night Sky Observer's Guide by Kepple and Sanner, or the enchanting though increasingly dated Burnham's Celestial Handbook). Read how to use them effectively.

    More beginners' tips: "How to Start Right in Astronomy".



    This Week's Planet Roundup

    Mercury (about magnitude –1) is rapidly emerging into evening view. Look for it above the west-northwest horizon as twilight fades. It's getting higher and easier every day. Don't confuse it with Aldebaran, far to its upper left.

    Mercury is often called "elusive," but by the end of this week it is plain and obvious as it passes south (lower left) of the Pleiades. See the sky scene above.

    Venus is lost in the glare of the Sun.

    Mars (magnitude +1.2, in Gemini) shines high in the west during evening. It forms a curved line with Pollux and Castor to its upper right or right. Watch as the line becomes less curved every day. It straightens out completely on May 4th.

    Compare Mars's color to that of Pollux, which is just about equally bright. Pollux is an orange giant of spectral type K0 III. To me, the tint of Mars looks slightly deeper.

    In a telescope Mars is a disappointing 5.9 arcseconds wide — a very tiny gibbous blob.

    Jupiter (magnitude –2.4, in eastern Sagittarius) rises around 1 or 2 a.m. daylight saving time and glares in the south-southeast by early dawn. The farther south you live, the higher you'll be able to observe it with your telescope before dawn gets too bright.

    Saturn (magnitude +0.5, in Leo) glows very high in the south to southwest during evening, just 2¼° from fainter Regulus (magnitude +1.4). Saturn and Regulus will remain nearly this close together for a month to come.

    Telescope users: can you see the new white storm on Saturn? How big a telescope will do it? See the picture caption to predict when the white spot will be turned into view (which happens at least twice a day).

    There's more to Saturn than you may realize! See our Saturn observing guide in the April Sky & Telescope, page 66. Saturn's rings now appear open by 10°, our best view of them until December 2010.

    Uranus and Neptune are low in the southeast before dawn.

    Pluto (magnitude 14.0, in northwestern Sagittarius) is highest in the south before dawn's first light.

    All descriptions that relate to your horizon or zenith — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America. Eastern Daylight Time (EDT) equals Universal Time (UT, UTC, or GMT) minus 4 hours.

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