Sky at a Glance | April 30th, 2010

Friday, April 30

  • Spica, Saturn, Regulus, Mars, Pollux, Castor, and Capella form a long, ragged line across the evening sky this spring, counting from left to right all the way around from southeast to northwest.

  • By 11 or midnight tonight, the waning Moon lofts up above the southeastern horizon. Look for Antares and other stars of Scorpius twinkling to its upper right. By dawn Saturday morning the Moon is to their left, as shown below.

    Moon crossing Scorpius as dawn begins
    If you're up when dawn is just beginning, the Moon lights the way to an early preview of the summer constellation Scorpius.

    Saturday, May 1

  • Happy May Day! This was traditionally celebrated as one of the four "cross-quarter days" midway between the solstices and equinoxes. However, the timing is a bit off; the actual midpoint of spring this year (Northern Hemisphere spring) comes on May 5th.

  • In late twilight, use binoculars to spot the 4th-magnitude star Kappa1 Tauri about ¼° lower left of dazzling Venus (at the time of twilight for North America). Look even closer to the lower left of Kappa1 for 5th-magnitude Kappa2.

    Sunday, May 2

  • Can you still find Orion at dusk? It's about halfway between bright Venus low in the west and bright Sirius low in the southwest. As Orion sinks away each spring, its topmost remaining bright star is Betelgeuse. Look for orange-red Betelgeuse above the midpoint between Venus and Sirius. Orion's horizontal belt is well below Betelgeuse and (depending on your latitude) a bit to the left.

    Monday, May 3

  • As the stars come out these evenings, the bright "Spring Star" Arcturus appears halfway up the sky due east. The bright "Summer Star" Vega is just rising low in the northeast.

    Tuesday, May 4

  • At dusk, use binoculars to look for the 4th-magnitude star Tau Tauri less than ½° left of Venus (at the times of dusk for North America). A telescope or even firmly mounted binoculars will show that this star is a wide double, with a 7th-magnitude companion 63 arcseconds southwest of (i.e. below) the primary.

  • A small telescope will always show Titan, Saturn's largest moon. Tonight and tomorrow, Titan is four ring-lengths to Saturn's west.

    Wednesday, May 5

  • Last-quarter Moon late tonight (exact at 12:15 a.m. Thursday morning Eastern Daylight Time).

    The Virgo cluster region of the sky
    The realm of the Virgo Cluster galaxies, as seen by Northern Hemisphere observers on spring evenings. Click the image for a view into the boxed region, on the Virgo-Coma Berenices border.
    Courtesy Akira Fujii.

    Thursday, May 6

  • Amateur observers usually assume that even the brightest galaxies of the Virgo Cluster are beyond the reach of binoculars. But are you sure? If you have a good dark sky and at least 50mm binos, pick a moonless night (such as this week) and give it a try using Gary Seronik's Binocular Highlight map and column in the May Sky & Telescope, page 45.

    Friday, May 7

  • It's May, so the Big Dipper floats upside down high overhead toward the north on starry evenings. Leo stands on the meridian high in the south — now with yellow-gold Mars to its right and yellow-white Saturn to its lower left.

  • The Eta Aquarid meteor shower, usually the year's best for the Southern Hemisphere, is underway for several days. This year the waning Moon adds some light to the sky during the pre-dawn meteor-watching hours. The shower's radiant point is near the Water Jar of Aquarius. Few of the meteors are visible from north temperate latitudes.

    Saturday, May 8

  • Sunday morning in early dawn, look east to spot Jupiter below the waning crescent Moon.


    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 must have a detailed, large-scale sky atlas (set of charts). The standards are the
    Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger Sky Atlas 2000.0 (stars to magnitude 8.5); and the even larger and deeper Uranometria 2000.0 (stars to magnitude 9.75). And read how to use your charts effectively.

    You'll also want a good deep-sky guidebook, such as Sky Atlas 2000.0 Companion by Strong and Sinnott, or the more detailed and descriptive Night Sky Observer's Guide by Kepple and Sanner or the classic if dated Burnham's Celestial Handbook.

    Can a computerized telescope take their place? I don't think so — not for beginners, anyway, and especially not on mounts that are less than top-quality mechanically. As Terence Dickinson and Alan Dyer say in their 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

    Jupiter's dark North Equatorial Belt is currently wide and massive, in contrast to its almost nonexistent South Equatorial Belt — whose absence leaves the Great Red Spot floating free in whiteness. Note too "Red Spot Junior" (Oval BA) adding a trace of color upper right of center. South is up.


    Christopher Go in the Philippines took this image on April 24th at 21:16 UT, when the central-meridian longitude was 174° (System II). The Great Red Spot is at about System II longitude 150°. Here's a list to print out of all the Great Red Spot's predicted transit times for the rest of 2010.

    Christopher Go

    Mercury is hidden in the glare of the Sun.

    Venus (magnitude –3.9, in Taurus) shines brightly in the west-northwest during twilight. Look for Aldebaran (magnitude +1.0) to its left early in the week, and lower left of it later. Capella (magnitude +0.1) shines much higher to Venus's upper right.

    Mars, dimming farther away into the distance at magnitude +0.8 now, is in Cancer high in the southwest to west during evening. In a telescope Mars is tiny, shrinking from 7.3 to 6.9 arcseconds in diameter this week.

    Jupiter (magnitude –2.1) shines low before dawn; look east about an hour before your local sunrise time. No other point there is nearly so bright!

    Saturn (magnitude +0.8, in the head of Virgo) is high in the south to southwest during evening. In a telescope Saturn's rings are tilted a mere 1.9° from edge-on, practically at the minimum inclination of 1.7° that they will display from mid-May through early June. Note the fine black shadow-line they cast on Saturn's disk! Now is also a good time to try for the more difficult of Saturn's moons with your telescope; see the May Sky & Telescope, page 61.

    Titan and its shadow transiting Saturn
    Titan and its shadow were leaving Saturn's face when Christopher Go in the Philippines took these shots on May 1, 2010. North is up. Click image for video of the transit.


    Four more Titan transits are coming up, all visible from the Pacific. Lasting up to a few hours, they are centered on May 17 at 8.6 hours UT; June 2, 7.2h UT; June 18, 6.2h UT; and July 4, 5.5h UT.


    Also in these images, note the dark reddish South Equatorial Belt, the wide, bright Equatorial Zone — and the striking black line of the rings' shadow on the globe. We see the shadow because, on the date of the pictures, the rings were tilted 1.9° to Earth's line of sight but 4.0° to the incoming sunlight. This disparity allows us to see the rings' shadow on the globe. This situation began in mid-March and has increased since then, making the shadow-line grow wider and more prominent.

    Alan MacRobert

    Uranus and Neptune are still low before sunrise. Wait another few weeks.

    Neptune, however, is passing a historic milestone this season. For the first time since it was discovered in 1846, Neptune has completed a full circuit of the sky and has returned very close to the point (near the Aquarius-Capricornus border) where Johann Galle first spotted it from Berlin Observatory on September 23rd of that year — following a prediction by Urbain Le Verrier in France that a new planet ought to be there, based on gravitational perturbations of Uranus. See The Return of Neptune.

    Pluto (magnitude 14, in northwestern Sagittarius) is highest in the south before dawn.

    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 (also known as UT, UTC, or GMT) minus 4 hours.


    "The universe is full of magical things patiently waiting for our wits to grow sharper."
    — Eden Phillpotts, "A Shadow Passes," 1918


    "Science is built up of facts, as a house is with stones. But a collection of facts is no more a science than a heap of stones is a house."
    — Henri Poincaré (1854–1912)


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