Some daily events in the changing sky for April 17 – 25.

Comet Yi-SWAN isn't much at magnitude 8½, but it's far north, crossing from Cassiopeia into Perseus (near the Double Cluster) this week. It's low in the northwest right after dark, and low in the northeast right before the first light of dawn. The farther north you are the better. See our AstroAlert with the comet's positions for plotting on your star atlas.

Looking southeast in early dawn

Watch the Moon pass Jupiter in the dawn early in the week. . .

Sky & Telescope diagram

Friday, April 17

  • Last-quarter Moon (exact at 9:36 a.m. Eastern Daylight Time).
  • Venus is passing 6° north (upper left) of faint Mars, low in the east before Saturday's sunrise.

    Saturday, April 18

  • During dawn Sunday morning, the waning crescent Moon hangs only 1° to 3° from Jupiter (at the time of dawn for North America), as shown at right.

    Sunday, April 19

  • Not far from bright Saturn in Leo is the famous small-telescope galaxy trio of M65, M66, and NGC 3628. Do you know where to find them? See Ken Hewitt-White's "Suburban Star-Hop" column and map in the April Sky & Telescope, page 58.

    Monday, April 20

  • At this time of year Vega, the Summer Star, rises above the northeast horizon during evening twilight. How early in the evening can you first see it? If your horizon in that direction is obstructed, here's a tip: If you know the constellation Draco above it, the Dragon's nose points right down to Vega's location.

    Tuesday, April 21

  • The Lyrid meteor shower should be at its most active from about midnight until dawn Wednesday morning. The shower is usually weak, but some years have brought a surprise upsurge.

    . . .and three days days later the waning Moon passes Venus. (These scenes are drawn for the middle of North America, at latitude 40° north, longitude 90° west. European observers: move each Moon symbol a quarter of the way toward the one for the previous date. The blue 10° scale is about the size of your fist held at arm's length. For clarity, the Moon is shown three times actual size.)

    Sky & Telescope diagram

  • A dawn spectacle! Early in Wednesday's morning twilight, look low in the east for Venus close by the waning crescent Moon, as shown at right. Then, the Moon occults (covers) Venus in twilight or broad daylight for North America except the East. See the article in the April Sky & Telescope, page 56, or online.

    Wednesday, April 22

  • The Big Dipper floats in a diagonal position high in the northeast when the stars come out. In only about an hour, the Dipper levels out horizontally (as you face north-northeast). The farther north you are, the faster this transition occurs.

    Thursday, April 23

  • As spring advances, bright Sirius is getting lower in the southwest at dusk. How late in the season can you keep it in sight? In other words, what will be Sirius's date of heliacal setting at your latitude? Start keeping watch now.

    Friday, April 24

  • New Moon (exact at 11:23 p.m. EDT)

    Looking west-northwest in bright twilight

    On what date can you first see the waxing crescent Moon? The visibililty of objects in bright twilight is exaggerated here.

    Sky & Telescope diagram

    Saturday, April 25

  • After sunset, an elusive, extremely thin young crescent Moon displays its early-springtime upright smile (as seen from mid-northern latitudes). Look for it just above the west-northwest horizon, to the lower right of Mercury, as shown here at lower right. Bring binoculars.

    Will this sighting set your personal young-Moon record? The crescent will be only about 21 hours from new at viewing time from the East Coast of North America, and 24 hours from new as seen from the West Coast. Have you ever seen a crescent that thin? (Calculate from the exact time of new Moon under Friday above.)

  • As early dawn begins on Sunday morning, aim your telescope at Jupiter in the southeast. The star 44 Capricorni intrudes directly between Jupiter and Ganymede, the planet's brightest satellite.

    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 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 charts; 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 classic Burnham's Celestial Handbook). Read how to use them effectively.

    Can a computerized telescope take their place? 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 and a curious mind." Without these, they note, "the sky never becomes a friendly place."

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

    This Week's Planet Roundup

    There's a new white outbreak, still pointlike in this April 11th image, in Jupiter's dark North Equatorial Belt (just below center). South is up. The North Equatorial Belt remains dark reddish, but the outbreak is spreading white material left and right along the belt's middle. The South Equatorial Belt is wider and double, with busy turbulence all along its middle. The central-meridian longitude here is 238° (System II).

    Christopher Go

    Mercury (about magnitude –0.5) is having its best evening apparition of the year. Look for it low in the west about an hour after sunset. As the sky gets darker, the Pleiades glimmer into view above it. Watch the Pleiades sink lower toward Mercury every day.

    Venus (magnitude –4.7) shines low in the dawn. Look for it above the horizon due east about 60 to 40 minutes before sunrise. (Don't confuse it with Jupiter, which is much higher and far to the right in the southeast.)

    In a telescope, Venus is a thickening and shrinking crescent. The best telescopic views come in full morning daylight, when Venus is higher in steadier air.

    Mars (only magnitude +1.2) is very low in the sunrise glow. It's 5° or 6° to the lower right of Venus early in the week, and about 4° directly below Venus late in the week. Use big binoculars. Good luck.

    Jupiter (magnitude –2.2, in Capricornus) shines in the southeast before and during dawn.

    Saturn's rings remain thin for all of f2009. Christopher Go imaged Saturn and three of its moons during excellent seeing on March 25th. Dione and Tethys are on the left, and fast-moving little Enceladus is just above the rings' tip on the right. North is up; celestial east is left.

    Christopher Go

    Saturn (magnitude +0.7, in Leo) shines high in the southeast at dusk and highest in the south around 9 p.m. Regulus, a little less bright, sparkles 16° (roughly one and a half fist-widths at arm's length) to Saturn's upper right in early evening, and directly to its right later.

    In a telescope, Saturn's rings are 4° from edge on. They'll close to become exactly edge-on next September 4th, when, unfortunately, Saturn will be out of sight practically in conjunction with the Sun.

    Uranus (6th magnitude) is hidden low in the sunrise glow, in the background of Venus and Mars.

    Neptune (8th magnitude) is also in the glow of dawn, in the background of Jupiter.

    Pluto (14th magnitude, in northwestern Sagittarius) is in the south-southeast before the first light of 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 (known as UT, UTC, or GMT) minus 4 hours.

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