Sky at a Glance | April 13th, 2012

Venus and company in twilight
Venus stays high in the same place as seen at the same time from night to night this week. But its starry background is slowly sliding to the lower right.
Alan MacRobert

Friday, April 13

  • Watch the triangle of Venus, Aldebaran and the Pleiades shift shape from night to night this week.

  • Last-quarter Moon (exact at 6:50 a.m. on the 13th EDT).

    Saturday, April 14

  • Summer Star preview: stay up till 10 or 11 and look northeast, and you'll have a preview of bright Vega, the "Summer Star" in little Lyra, climbing into good view.

  • Tonight is the 100th anniversary of the sinking of the Titanic, and we're proud of the attention our April cover story is receiving: "Did the Moon Sink the Titanic?" Astro-historians Don Olsen, Russell Doescher, and Roger Sinnott make the case that the unusually high spring tides several months earlier, caused by the unusually close Moon and Sun, floated an abnormal number of icebergs away from the shores of northern Canada — to reach the North Atlantic by the night of April 14–15. Short summary: Titanic's Celestial Connections.

    Sunday, April 15

  • Saturn is at opposition, opposite the Sun. It rises around sunset, shines highest in the middle of the night, and sets around sunrise. Telescope users: watch for the Seeliger effect, described under Saturn in "This Week's Planet Roundup" below.

  • Mars ends its retrograde (westward) motion for the year and resumes heading east against the background stars. Watch it pull away from Regulus in the coming weeks: slowly at first, then faster.

    Monday, April 16

  • As twilight fades, Look for bright Sirius in the southwest, Orion's horizontal Belt off to the right (with Betelgeuse above it, Rigel below it), and Aldebaran and Venus farther to the right in the west.

    Tuesday, April 17

  • The eclipsing variable star Algol is at minimum brightness, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 10:44 p.m. EDT (9:44 p.m. CDT). This is the last chance till August to see Algol at minimum from the latitudes of the U.S.

    Wednesday, April 18

  • This is the time of year when the dim Little Dipper juts to the right from Polaris (its handle-end) at nightfall. The much brighter Big Dipper curls over high above it, "dumping water" straight down into it.

    Thursday, April 19

  • Bright Vega is now visible low in the northeast as early as 9:30 or 10 p.m. (depending on where you live). Bright Arcturus dominates the high sky due east. Look a third of the way from Arcturus to Vega for dim Corona Borealis, the Northern Crown, with its one modestly bright star, Alphecca. Look two-thirds of the way from Arcturus to Vega for the dimmer Keystone of Hercules.

    Friday, April 20

  • Capella, Venus, and Aldebaran form an almost perfectly straight line in the western sky this evening and Saturday evening, in that order from upper right to lower left.

    Saturday, April 21

  • The weak but unpredictable Lyrid meteor shower should reach maximum activity in the hours before dawn Sunday morning. There will be no Moon. You might see a dozen Lyrids per hour under ideal dark-sky conditions before the beginning of dawn.

  • New Moon (exact at 3:18 a.m. EDT on the 21st).


    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 map in the center of each issue of Sky & Telescope, the essential magazine of astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

    Pocket Sky Atlas
    The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but that's less than one star in an entire telescopic field of view, on average. By comparison, Sky Atlas 2000.0 plots 81,312 stars to magnitude 8.5, typically one or two stars per telescopic field. Both atlases include many hundreds of deep-sky targets — galaxies, star clusters, and nebulae — to hunt among the stars.
    Sky & Telescope

    Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts effectively.

    You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the classic if dated Burnham's Celestial Handbook.

    Can a computerized telescope replace charts? I don't think so — not for beginners, anyway, and especially not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability). As Terence Dickinson and Alan Dyer say in their invaluable Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."


    This Week's Planet Roundup

    Mercury (magnitude +0.4) is deep in the glow of sunrise. It's having a very poor dawn apparition just above the eastern horizon.

    Venus (magnitude –4.6; in Taurus) is shining the highest and brightest it ever appears in the evening sky during its 8-year cycle of repeating apparitions. Venus comes into easy view high in the west soon after sunset. It doesn't set in the northwest until around 11 or even midnight daylight saving time (depending on where you live). You can see Venus through the clear blue sky of day if your eye lands right on it; look for it 44° (4 or 5 fist-widths at arm's length) to the Sun's celestial east-northeast.

    Look high to Venus's upper right at dusk for Capella, to its lower left for Aldebaran, and to its lower right for the Pleiades. Far below Venus in twilight is Jupiter.

    The best time to examine Venus in a telescope is late afternoon or around sunset. It's now a thick crescent 30 arcseconds tall and 40% sunlit, waning and enlarging week by week as it swings toward Earth.

    Mars on April 13, 2012
    Christopher Go took this image of Mars, now gibbous, on April 13th (at 12:06 UT). The planet was 11.5 arcseconds wide. South is up here, and celestial east is to the right (the following side; Mars's morning side). Two of the Tharsis shield volcanos are poking through the clouds near the morning terminator. Also note the much-shrunken North Polar Cap breaking up, and the dark collar that has been laid bare around it.

    Mars (magnitude –0.4) shines fire-orange in Leo. Regulus is 4° to Mars's right in early evening, and fainter Gamma Leonis is 7° above it. Look for them high in the south.

    Mars in a telescope is a small 11.4 arcseconds wide, fading and shrinking as Earth pulls ahead of it in our faster, inside-track orbit around the Sun.

    Jupiter (magnitude –2.0) is sinking ever lower toward the sunset, far below Venus. It's rounding toward the far side of the Sun, which is why a telescope shows it a disappointingly small 33 arcseconds wide. In addition, Jupiter appears increasingly fuzzy at its ever-lower altitude.

    Saturn away from and close to opposition
    Yes, the Seeliger effect is for real. Christopher Go took these images on February 23 and April 13, 2012. The latter was two days before Saturn's opposition. The brightening of the rings with respect to the globe shows that the rings are made of solid grains (which backscatter light) and Saturn's cloudtops are not. South is up.

    Saturn (magnitude +0.2, in Virgo) is at opposition this week, on April 15th. It rises in twilight and glows highest in the south around 1 a.m. Shining 5° to its right is Spica: fainter, bluer, and twinklier.

    Keep careful watch on Saturn and its rings in a telescope. With Saturn at or near near opposition, notice the Seeliger effect: a temporary brightening of the rings with respect to the globe. This happens because the solid particles making up the rings backscatter sunlight (reflect it back in the direction it came from) more effectively than the planet's cloudtops do. Compare how the rings and globe look now with with how they look a week or more past opposition.

    Uranus is hidden low in the dawn.

    Neptune (magnitude 7.9, in Aquarius) is barely emerging into view low in the east-southeast before dawn's first light.


    All descriptions that relate to your horizon — 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 (also known as UT, UTC, or GMT) minus 4 hours.


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