This Week’s Sky at a Glance, April 21 – 29

Mars and Pleiades, April 21, 2017

Mars is passing the Pleiades. They appear closest together April 19–21. After that the cluster moves lower. (For clarity, the Pleiades are always slightly enlarged in these illustrations.)

Jupiter, Spica, Corvus at dusk, April 2017

Meanwhile on the other side of the sky, sail-shaped Corvus sits to the right of Spica and Jupiter.

Moon and Venus, April 23, 2017

As the crescent Moon wanes toward new it passes Venus in the dawn . . .

Moon, Mars, Pleiades, Aldebaran April 27-29, 2017

. . . and then the waxing crescent returns to the western evening sky.

Friday, April 21

• As night descends, look high in the west for Pollux and Castor lined up almost horizontally (depending on your latitude). These two stars, the heads of the Gemini twins, form the top of the enormous Arch of Spring. To their lower left is 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.

• The weak Lyrid meteor shower should peak in the hours before Saturday's dawn. The Moon will be only a waning crescent. The shower's peak usually lasts just a few hours, but this year the predicted timing (12h Universal Time April 22) is good for North America, especially the West. But don't expect more than one meteor every several minutes, at best.

Saturday, April 22

• The spring constellation Corvus, the Crow, perches in the southeast these evenings, about a fist and a half at arm's length to the right of Spica as shown here. But above Spica this year is brilliant Jupiter, hogging the show. Corvus is traditionally seen as ready to snatch sparkling Spica out of Virgo's hand. This year he has a brighter shiny to go for.

• As dawn begins to brighten Sunday morning, catch Venus and the waning crescent Moon low in the east.

Sunday, April 23

• Face north after nightfall, look very high, and you'll find the Pointers, the end stars of the Big Dipper's bowl. They're on the meridian pointing toward Polaris straight down below. From the Pointers to Polaris is about three fists at arm's length.

Monday, April 24

• 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 at some moment between about 8:30 and 10:00 p.m. daylight-saving time, depending mostly on how far east or west you live in your time zone. Can you time this event? Like everything star-related, it happens 4 minutes earlier every night.

Tuesday, April 25

• Jupiter's moon Europa reappears from eclipse out of Jupiter's shadow around 10:28 p.m. EDT.

Then at 11:17 p.m. EDT, Io starts crossing Jupiter's face.

And at 11:42 p.m. EDT, Io's shadow comes following behind.

Wednesday, April 26

• To the left of Jupiter by 30° (about three fists at arm's length) shines the second-brightest point in the area: Arcturus, pale yellow-orange. Whether you see it higher or lower than Jupiter depends both on the time and on how far north or south you live. The farther north you are, the more the advantage goes to Arcturus.

• New Moon (at 8:16 a.m. Eastern Daylight Time).

Thursday, April 27

• Can you detect the thin crescent Moon low after sunset? Look far below Aldebaran and Mars in the west-northwest in twilight, as shown above. The crescent is only about 1½ days old. Bring binoculars or a telescope!

Friday, April 28

• As twilight fades in the west, spot Aldebaran and Mars to the lower right of the crescent Moon, as shown above.

Saturday, April 29

• Now the curve of the crescent Moon points far down toward Aldebaran and Mars at dusk.

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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 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 guide to astronomy.

Pocket Sky Atlas, jumbo edition

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae. Shown above is the Jumbo Edition for easier reading in the night. Larger view. Sample chart.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, is the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

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, or the bigger Night Sky Observer's Guide by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (meaning heavy and expensive). And 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 on April 20, 2017

Jupiter rotates fast; Christopher Go took these images only about 20 minutes apart on April 20th. North is up. Note the Great Red Spot's dark center and edges and its wide, long turbulent wake. The North Equatorial Belt (above center) is very active in this region, with many small white outbreaks.

Mercury is out of sight in the glare of sunrise.

Venus (magnitude –4.7) shines low in the east as dawn brightens. In a telescope it's a crescent, thickening a little every morning.

Mars (magnitude +1.6, in Taurus) glows in the west in late twilight. Early in the week you'll find it to the lower left of similar-looking Aldebaran, as shown at the top of this page. By week's end Mars is to the right of Aldebaran (for skywatchers at mid-northern latitudes).

Jupiter (magnitude –2.4, in Virgo) shines like a beacon in the southeast at nightfall. It's highest in the south by 11 or midnight daylight saving time. Spica, just a trace bluer, hangs 9° lower left of it. In a telescope Jupiter is still 44 arcseconds across its equator.

Saturn (magnitude +0.3, in Sagittarius) rises around 11 or midnight and glows highest in the south before dawn, upper right of the Sagittarius Teapot. Redder Antares (magnitude +1.0) twinkles 18° to Saturn's right in the early-morning hours.

Uranus is hidden behind the glare of the Sun.

Neptune is still too low for observing before dawn.

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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) is Universal Time (UT, UTC, or GMT) minus 4 hours.

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"This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours."
— Neil deGrasse Tyson, 2014


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— John Adams, 1770


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