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This Week's Sky at a Glance

by Alan M. MacRobert

Some daily events in the changing sky for April 18 – 26.

Spica and Corvus in the southeast
Spica (left) and Corvus (stars at right) glitter at the same height in the southeast shortly after dark in late April.
Akira Fujii
Friday, April 18

  • There's a bright Moon this evening. Look to its lower left for Spica, and look to its lower right for the fainter four-star pattern of Corvus, the Crow, barely visible through the moonlight. They're shown (moonless) at right. The Crow is supposedly eyeing shiny Spica, intending to pluck it from Virgo's hand. He's paying no attention to the vastly shinier prize of the Moon.

    The bright star far off to the Moon's left, dominating the eastern sky, is ginger-ale-colored Arcturus.

    Saturday, April 19

  • Full Moon tonight (exact at 6:25 a.m. Sunday morning Eastern Daylight Time.) This evening Spica is above the Moon.

    Sunday, April 20

  • Bright Sirius, the "Winter Star," still sparkles in the southwest during twilight in late April. How late in the season can you keep it in view as it sinks away?

    Monday, April 21

  • The red long-period variable stars S and RS Herculis should be about at their maximum brightnesses (8th magnitude) this week.

    Tuesday, April 22

  • A small telescope will always show Titan, Saturn's largest and wildest moon. Tonight Titan is four ring-lengths to Saturn's east. A guide to identifying all six of Saturn's satellites that are sometimes visible in amateur scopes is in the April Sky & Telescope, page 58.

  • It you're up before dawn Wednesday morning, the red supergiant Antares is that orange star shining upper left of the Moon.

    Wednesday, April 23

  • Arcturus, the "Spring Star," dominates the high eastern sky during evening. By mid-evening Vega, the "Summer Star," is rising low in the northeast. Arcturus and Vega are equally bright. However, Vega will look dimmer when it's very low due to atmospheric extinction, absorption of light by the thick layers of Earth's atmosphere near the horizon. This is the same effect that dims the setting Sun.

    Thursday, April 24

  • Once you've got Arcturus and Vega identified, try for the much dimmer star patterns between them. A third of the way from Arcturus to Vega is the little semicircle of Corona Borealis, the Northern Crown. Two thirds of the way is the Keystone of Hercules. If light pollution is a problem try again at a later hour, or a later month, when everything that's now in the east will be higher.

    Friday, April 25

  • Mars, Pollux, and Castor form an eye-catching arc of three "stars" high in the west after dark. To their lower left is Procyon. Now draw a line from Procyon left to the Saturn-Regulus pair high in the south. Look almost halfway along this line, and just a bit below, to see if you can spot a biggish bunch of faint stars. This is the head of Hydra, the Sea Serpent, a distinctive asterism of spring.

    The rest of Hydra winds dimly all the way left across the sky to its tail at the southeastern horizon.

    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.

  • Take a look out a south-facing window during the Sunday dawn, and there will be Jupiter above the waning Moon, as shown below. What the heck — set the alarm early and greet them both outside with your telescope! Life should be an adventure.

    South view in early dawn
    Early risers will see the waning Moon passing the Sagittarius Teapot and bright Jupiter. (These scenes are drawn for the middle of North America. European observers: move each Moon symbol a quarter of the way toward the one for the previous date.)
    Sky & Telescope diagram


    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

    Jupiter is getting high enough now in early dawn to show detail well in a telescope . The North Equatorial Belt (brown band just above center) remains wide and very dark. The South Equatorial Belt (just below center) has divided into northern and southern halves. A South Equatorial Belt Disturbance has created the irregular white markings near the left (following) limb. Note the very different colors of the belts in the in the northern and southern hemispheres. The Equatorial Zone, after being remarkably dark last year, has returned to its normal bright state. Christopher Go took this extremely sharp image on April 3, 2008. The time was 20:20 UT, and the System II central-meridian longitude was 32°. North is up (but remember that many telescopes will show south up).
    Christopher Go
    Mercury and Venus are lost in the glare of the Sun.

    Mars (magnitude +1.1, in Gemini) shines high in the southwest to west during evening. It forms a skewed triangle with Pollux and Castor above it. The triangle grows flatter every day.

    Compare Mars's color to that of Pollux, which now its equal in brightness. Pollux is an orange giant star of spectral type K0 III. To me, the tint of Mars looks slightly deeper.

    In a telescope Mars is only 6.0 arcseconds wide — just a very tiny gibbous blob.

    Jupiter (magnitude –2.3, in eastern Sagittarius) rises around 2 a.m. daylight saving time and glares in the south-southeast by dawn. The farther south you live, the higher you'll be able to observe it — and thus the sharper the view of it in your telescope — before dawn gets too bright.

    Saturn with white spot, April 23, 2008
    A white storm has broken out in Saturn's South Temperate Zone. It shows plainly in this stacked-video image, taken by Sean Walker through a 12.5-inch reflector at 0:42 UT April 23rd in excellent atmospheric seeing. The System II central-meridian longtude was 57°. "The white spot was visible in an eyepiece at over 500x, particularly through a green filter," he writes. North is up.

    To find when this view will repeat: For your date, look up Saturn's System II central-meridian longitude in this table (it's the "CMII" column). That's the value for 0:00 Universal Time (UT or GMT) on your date. To this value, add 33.8° for each whole hour since 0:00 UT, and 0.56° for each minute, for the time you plan to observe. See what you get. Find a good Saturn-observing time and date that results in something around 57°, and plan to be out then with your scope.

    S&T: Sean Walker
    Saturn (magnitude +0.5, near Regulus in Leo) glows very high in the south to southwest during evening, just 2¼° from fainter Regulus (magnitude +1.4). They'll remain this close for a month to come.

    Telescope users: there's more to Saturn than you may realize. See our Saturn observing guide in the April Sky & Telescope, page 66.

    Uranus and Neptune are low in the southeast before dawn.

    Pluto (magnitude 14.0, in northwestern Sagittarius) is highest in the south just 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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