This Week’s Sky at a Glance, May 18 – 26


Moon, Venus, Gemini May 17-19, 2018

Watch the waxing crescent Moon climb past Venus through Gemini as it orbits eastward around Earth away from our line of sight to the Sun.

Friday, May 18

• The Arch of Spring spans the western sky in late twilight, adorned this evening and tomorrow by the waxing crescent Moon. Pollux and Castor form the top of the Arch: they're lined up roughly horizontally in the west-northwest as shown here, about three finger-widths at arm's length apart. Look far to their lower left for Procyon (just outside the view here), and farther to their lower right for Menkalinan and then bright Capella. Venus blazes below the Arch, off center to the right.

• And then just as darkness becomes complete, an observing challenge: Get Venus in binoculars or a low-power, wide-field scope. Look upper left of it by about 1.6°. Can you make out the dim glow of the star cluster M35? Have you ever seen an M object this close to the horizon? It'll slide closer past Venus on Saturday and Sunday nights.

Saturday, May 19

• The Moon hangs in dim Cancer, left of Castor and Pollux almost in line with them as shown above. After nightfall is complete, use binoculars to look for the Beehive Star Cluster, M44, about 6° above the Moon (as seen from North America). That's roughly the width of the field of view in typical 8x binoculars.

Sunday, May 20

• Jupiter's Great Red Spot should cross the planet's central meridian around 9:17 p.m. Eastern Daylight Time. Farther west, Jupiter is too low and the sky too bright.

• But then Jupiter's largest moon, Ganymede, crosses the planet's face from 9:07 to 10:26 p.m. Pacific Daylight Time, followed by its tiny black shadow from 10:05 to 11:50 p.m. PDT. Convert these to other time zones, but check whether Jupiter will be in good view for you then.

For all Great Red Spot transits and all phenomena of Jupiter's satellites this month, complete for observers worldwide, see the May Sky & Telescope, page 50.

Monday, May 21

• First-quarter Moon (exact at 11:49 p.m. Eastern Daylight Time). The Moon shines very near Regulus during evening for the Americas.

• This evening the asteroid 201 Penelope should occult a 9.9-magnitude star in Virgo for telescope users along a narrow path from the Oregon coast through Arizona and into Mexico. Map, details, finder charts.

Tuesday, May 22

• Now and for months to come, Jupiter stays within less than 2° or 3° of 3rd-magnitude Alpha Librae (Zubenelgenubi): a fine, wide double star for binoculars. Its two components, magnitudes 2.8 and 5.1, are a generous 231 arcseconds apart. Nevertheless they form a real, gravitationally bound pair; they're both measured to be 77 light-years away.

• Shining in the east-northeast after dark is Vega, the brightest and currently highest star of the Summer Triangle. But with summer still a month away (astronomically speaking), the Triangle's final star doesn't rise above the eastern horizon until about 10 or 11 p.m. That's Altair, the Triangle's lower right corner. The third star is Deneb, sparkling less far to Vega's lower left.

Vega in the eastern sky

Vega far outshines the rest of Lyra's more delicate, 3rd and 4th-magnitude stars. The whole group climbs the eastern sky these evenings and shines near the zenith just before dawn.
Bob King

Wednesday, May 23

Vega is the brightest star in the east-northeast after dark. The other main stars of its constellation, Lyra, currently dangle down from it, as shown above.

• Look 14° (about a fist and a half at arm's length) to Vega's upper left for Eltanin, the nose of Draco the Dragon. Closer above and upper left of Eltanin are the three fainter stars of Draco's stick-figure head, also called the Lozenge. Draco always points his nose to Vega, as if sniffing for it.

Thursday, May 24

• Constellations seem to twist around fast when they pass your zenith — if you're comparing them to the direction "down," i.e. directly away from the zenith. Right now, as the stars come out, the Big Dipper still floats almost horizontal practically overhead to the north (seen from 40° N latitude). But in just two or three weeks, as seen in the same stage of twilight, the Dipper will be hanging straight down by its handle!

Moon, Spica, Jupiter, May 25-27, 2018

Using the Moon as your starting point, how early can you pick out Jupiter, then Spica, then 3rd-magnitude Alpha Librae? This frame is about 25° wide. (So as you can see, the Moon is shown about 3 times its actual apparent size.)

Friday, May 25

• The waxing gibbous Moon crosses the sky tonight in the company of Spica below it, as shown at right.

Saturday, May 26

• Tonight the Moon forms a flat triangle with Spica and brighter Jupiter, which are more or less on opposite sides of it, as shown here. This is their arrangement in bright twilight. The whole scene rotates counterclockwise as it moves westward through the night.


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, and hundreds of telescopic galaxies, star clusters, and nebulae among them. 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

Mars on April 20, 2018

Gibbous Mars on April 20th, imaged by Damian Peach remotely with the 1-meter Chilescope. South is up. In the south, the darkest band across the vast dark area is Mare Erythraeum, with Solis Lacus barely off its right-hand end. Lower right of center, not far from the sunset terminator, that looks like a pretty passable "canal" from Oxia Palus to the left end of Niliacus Lacus.

Jupiter with Great Red Spot on May 1, 2018

Jupiter's Great Red Spot side on May 1st . . .

Jupiter on May 23, 2018

. . . and on May 23rd. South is up. Like a giant-eyed white snake chasing its tail around the planet, a strong white band in the South Tropical Zone extends from the Great Red Spot westward (preceding) almost all the way around to the snake's nose. In the last three weeks, the nose has been catching up to the tail. Will this continue until they merge? Christopher Go in the Philippines has long been imaging Jupiter with his 14-inch scope at every opportunity.

Mercury (about magnitude –0.6) is sinking lower toward the end of a very poor apparition deep in the glow of sunrise. Using binoculars, look for it about 15 or 20 minutes before sunrise barely above the east horizon.

Venus (magnitude –3.9, near the feet of Gemini) shines brightly in the west-northwest during evening twilight and just after. It's as high in twilight as it will get all this spring and summer (for mid-northern skywatchers). In a telescope — look early! — Venus is a little disk 13 arcseconds wide and slightly gibbous: 84% sunlit.

Mars (magnitude –0.9, in Capricornus) rises around midnight or 1 a.m. daylight-saving time, about 24° lower left of Saturn. By early dawn in glares balefully orange in the south. Mars is brightening on its way to an unusually close opposition in late July. Already it's 13 or 14 arcseconds wide — its apparent diameter at its poorest oppositions!

What detail can you see on Mars in your scope? You'll want a Mars map that identifies what features are facing Earth at your time and date, such as our free Mars Profiler.

Jupiter (magnitude –2.5, in Libra) is two weeks past opposition. It shines in the southeast as twilight fades, and it's the brightest point in the sky once Venus sets below the opposite horizon.

Jupiter remains 44 arcseconds across for the rest of May. It's highest in the south, presenting the sharpest views in a telescope, by midnight daylight-saving time. It sets in the west-southwest during dawn. See our telescopic guide to observing Jupiter in the May Sky & Telescope, page 48.

Saturn (magnitude +0.3, just above the Sagittarius Teapot) rises about an hour after dark. It stands highest in the south before dawn begins, about 24° to the right of brighter Mars.

Uranus is hidden in the glow of dawn.

Neptune (magnitude 7.9, in Aquarius) is low in the east-southeast before the beginning of dawn.


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 (also called UT, UTC, GMT, or Z time) minus 4 hours.


"Remember to look up at the stars and not down at your feet. Try to make sense of what you see and wonder about what makes the universe exist. Be curious."
— Stephen Hawking, 1942–2018


"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


"Objective reality exists. Facts are often determinable. Vaccines save lives. Carbon dioxide warms the globe. Bacteria evolve to thwart antibiotics, because evolution. Science and reason are not a liberal conspiracy. They are how we determine facts. Our survival depends on our ability, and willingness, to do so."
— Alan MacRobert, your Sky at a Glance editor


"Facts are stubborn things."
— John Adams, 1770


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