This Week's Sky at a Glance, April 18 – 27

FRIDAY, APRIL 18

■ Right after dark, Orion still poses in the southwest in his spring orientation: striding down to the right, with his belt horizontal. The belt points left toward Sirius and right toward Aldebaran and, farther on, the Pleiades. Above Aldebaran shines bright Jupiter, as shown below.

SATURDAY, APRIL 19

■ After nightfall the Sickle of Leo stands vertical high in the south. Its bottom star is Regulus, the brightest of Leo. Leo himself walks horizontally westward. The Sickle forms his front leg, chest, mane, and part of his head. Off to the left, a long right triangle forms his hind end and long tail.

The farthest left of the three is his tail-tip, Denebola, Leo's second brightest star. How do we know the ancients saw it as Leo's tail? It's in the name. Many of our star names come from Arabic, and medieval Arab scholars translated these from ancient Greek and Latin designations, which often simply stated a star's placement in its constellation. "Denebola" is from the Arabic for "Tail of the Lion."

SUNDAY, APRIL 20

■ Find Procyon high above brilliant Sirius in the southwest right after dark. Look upper left of Procyon by 15° (about a fist and a half at arm's length) for the dim head of Hydra, the enormous Sea Serpent. His head is a group of 3rd- and 4th- magnitude stars about the size of your thumb at arm's length.

About a fist and a half lower left of Hydra's head shines Alphard, his 2nd-magnitude orange heart. The rest of Hydra zigzags (faintly) from Alphard all the way down to the southeast horizon. For more on Hydra and his hellish lore, see Stephen James O'Meara's "Alphard and the Golden Bough" in the April Sky & Telescope, page 45.

■ Last-quarter Moon tonight (exact at 9:36 p.m. EDT). The half-lit "backward" Moon doesn't rise until around 3 a.m. daylight-saving time. By the very beginning of dawn, roughly an hour and a half later, it's still low in the southeast: in dim Capricornus, about two fists lower left of the Sagittarius Teapot.

MONDAY, APRIL 21

■ The Lyrid meteor shower should be at its most active late tonight. It's usually fairly weak, with 15 or 20 meteors visible per hour even under ideal conditions. Your best time for a meteor watch will be from midnight or 1 a.m. until the Moon (a day past last quarter) rises around 3 or 3:30 a.m. daylight-saving time. See Bob King's See the Lyrid Meteor Shower the Night of April 21–22.

TUESDAY, APRIL 22

■ Bright Arcturus is climbing high in the east these evenings. Equally bright Capella is descending high in the northwest. They stand at exactly the same height above your horizon sometime between 8:30 and 10 p.m. daylight-saving time, depending mostly on how far east or west you live in your time zone.

How accurately can you time this moment for your location? Like everything constellation-related, it happens 4 minutes earlier each night (or to be more exact, 3 minutes 56 seconds earlier). This is the sort of measurement for which sextants were invented.

WEDNESDAY, APRIL 23

■ Capella is the brightest star high in the west-northwest during and after dusk. Its pale-yellow color matches that of the Sun, meaning they're both about the same temperature. But otherwise Capella is very different. It consists of two yellow giant stars orbiting each other every 104 days.

That part is common knowledge among starwatchers. But did you know that, for telescope users, Capella is accompanied by a distant, tight pair of red dwarfs? They're Capella H and L, magnitudes 10 and 13. Article and finder charts.

THURSDAY, APRIL 24

■ High above the Big Dipper late these evenings, nearly crossing the zenith, are three pairs of dim naked-eye stars, all 3rd or 4th magnitude, marking the Great Bear's feet. They're also known as the Three Springs (or Leaps) of the Gazelle, from early Arab lore. They form an east-west line that lies roughly midway between the Bowl of the Big Dipper and the Sickle of Leo. The line is 30° (three fists) long. See the evening constellation chart in the center of the April or May Sky & Telescope.

According to the ancient Arabian story, the gazelle was drinking at a pond — the big, dim Coma Berenices star cluster — and bounded away when startled by a flick of Leo's nearby tail, Denebola. Leo, however, seems quite unaware of all this, facing the other way.

Another version of the story sees Coma Berenices as Leo's extended tailtip, and the pond as formed by stars in Ursa Major.

■ If you know a spot with an open view of the east horizon, be there about 40 or 30 minutes before sunrise Friday morning to catch Venus over the thin crescent Moon, as shown in the second illustration below. Bring binoculars to try for much more difficult Saturn and Mercury. And know that a fourth planet lurks invisibly faint in their background: 8th-magnitude Neptune.

FRIDAY, APRIL 25

■ This is the time of year when, as the last of twilight fades away, the dim Little Dipper extends to the right from Polaris and slightly up. High above the Little Dipper's bowl, you'll find the Big Dipper's bowl.

SATURDAY, APRIL 26

■ As night descends, look high in the west for Pollux and Castor, the heads of the Gemini twins, lined up almost horizontally (depending on your latitude). Mars is off to their left.

Pollux and Castor form the top of the enormous Arch of Spring. To their lower left spot Procyon, the left end of the Arch. Farther to their lower right is the other end, formed by Menkalinan (Beta Aurigae) and then brilliant Capella. The whole thing sinks in the west through the evening.

SUNDAY, APRIL 27

■ Arcturus is the brightest star high in the east these evenings. Spica shines lower right of it by about three fists at arm's length. To the right of Spica by half that distance is the distinctive four-star constellation Corvus, the springtime Crow.

■ Vega, the Summer Star, the zero-magnitude equal of Arcturus, is now twinkling low in the northeast at nightfall. . . depending on your latitude. The farther north you are the higher Vega will be. If you're in the latitudes of the southern US, you'll have to wait until a bit later for it to appear.

■ New Moon (exact at 3:31 p.m. EDT).

This Week's Planet Roundup

Mercury and Saturn are barely detectable very deep in the glow of sunrise. But they're located under a bright marker, Venus. On the morning of April 19th find them positioned under Venus as shown below. Saturn is then 5½° from Venus and magnitude +1.2, while Mercury is 9° from Venus and mag +0.5. Bring binoculars. Good luck. And know that a fourth planet, Neptune, lurks invisibly faint behind them.

On subsequent mornings Venus stays at nearly the same height and brightness, Saturn creeps a little closer to it, and Mercury moves a bit left while brightening a trace to reach magnitude +0.3 on the morning of April 25th. That morning the thin Moon sits in their midst, as shown below.

Venus itself, magnitude –4.7, rises at the very beginning of dawn before the others. For skywatchers at mid-northern latitudes, Venus will continue to rise at the start of dawn for the rest of April and May.

In a telescope as dawn grows bright, Venus is a shrinking, thickening crescent. It will fill out to become half lit around the end of May. How far into broad daylight can you follow Venus with your scope?

Mars (magnitude +0.8, in Cancer) glows high in the southwest in the evening, left of Pollux and Castor (magnitudes +1.1 and +1.6, respectively). Mars continues to pull a little farther away from them every day.

For telescope users Mars has shrunk to only 7 arcseconds in diameter. Good luck making out any surface markings aside from (maybe) the North Polar Cap. At least you can see that Mars is definitely gibbous, 90% sunlit, as gibbous as it's going to get this season.

Jupiter (magnitude –2.0, in Taurus) shines bright white in the west in early evening, 42° lower right of Mars along the ecliptic. Jupiter forms a triangle with Taurus's two horntip stars above it: Beta and fainter Zeta Tauri.

About a fist below below Jupiter shines orange Aldebaran. Farther to Jupiter's lower right are the Pleiades. Jupiter sets in the west-northwest around 11 or midnight daylight-saving time.

In a telescope Jupiter has shrunk to only 34 arcseconds wide, about as small as it gets. For the daily doings of its Galilean moons see the April Sky & Telescope, page 51.

Uranus is lost in the western twilight.

Neptune is hidden in the glare of dawn behind Venus, Mercury, and Saturn.

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 and graphics that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time (EDT) is Universal Time minus 4 hours. UT is also known as UTC, GMT, or Z time.

Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For a more detailed 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.

For the attitude every new amateur astronomer needs, read Jennifer Willis's Modest Expectations Give Rise to Delight.

Once you get a telescope, to put it to good use you'll need a much more detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas, in either the original or Jumbo Edition. Both show all 30,000 stars to magnitude 7.6, and 1,500 deep-sky targets — star clusters, nebulae, and galaxies — to search out among them.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many, as well as many more deep-sky objects. It's currently out of print, but maybe you can find one used.

The next up, once you know your way around well, are the even larger Interstellarum atlas (with 201,000+ stars to magnitude 9.5, and 14,000 deep-sky objects selected to be detectable by eye in large amateur telescopes), and Uranometria 2000.0 (332,000 stars to mag 9.75, and 10,300 deep-sky objects). And read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet as to charts on paper.

You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner. The pinnacle for total astro-geeks is the new Annals of the Deep Sky series, currently at 11 volumes as it works its way forward through the constellations alphabetically. So far it's up to H.

Can computerized telescopes replace charts? Not for beginners I don't think, and not for scopes on mounts and tripods that are less than top-quality mechanically. Unless, that is, you prefer spending your time getting finicky technology to work rather than learning how to explore the sky. 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, "the sky never becomes a friendly place."

If you do get a computerized scope, make sure that its drives can be disengaged so you can swing it around and point it readily by hand when you want to, rather than only slowly by the electric motors (which eat batteries).

However, finding faint telescopic objects the old-fashioned way with charts isn't simple either. Do learn the essential tricks at How to Use a Star Chart with a Telescope.

Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the naked-eye heavens above. It's free.

"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
            — Carl Sagan, 1996

"Facts are stubborn things; and whatever may be our wishes, our inclinations, or the dictates of our passion, they cannot alter the state of facts and evidence."
            — John Adams, 1770