This Week's Sky at a Glance, May 17 – 26

By: Alan MacRobert May 17, 2024 0

FRIDAY, MAY 17

■ Tonight the dark limb of the waxing gibbous Moon occults Beta Virginis, magnitude 3.6, for telescope users across most of North America. The occultation happens after midnight in the Eastern and Central time zones, and late evening farther west. If you're near the West Coast the Moon will miss the star.

Map and timetables. The first two tables, with predictions for many cities, are long. The first gives the times of the star's disappearance behind the Moon's dark limb; the second its reappearance out from behind the Moon's bright limb (much less observable). Scroll to be sure you're using the correct table; watch for the new heading as you scroll down. The first two letters in each entry are the country abbreviation (CA is Canada, not California). The times are in UT (GMT) May 18th. UT is 4 hours ahead of Eastern Daylight Time, 5 hours ahead of CDT, 6 ahead of MDT, and 7 ahead of PDT.

For instance: Use the first table to see that for New York City, Beta Vir disappears on the dark limb at 1:33 a.m. May 18th EDT, when the Moon is 18° high in the west (at azimuth 256°).

SATURDAY, MAY 18

■ The waxing gibbous Moon shines brightly in the south after dark. But not so brightly as to hide Corvus, the Crow, a little more than a fist directly under it. Cover the Moon with your hand to make Corvus easier. Its four main stars are all between magnitude 2.6 and 3.0. They're within 8° of each other, smaller than your fist at arm's length.

SUNDAY, MAY 19

■ The Moon continues to illuminate Virgo's part of the sky. Early this evening look about 3° or 4° lower left of the Moon for Spica, Virgo's brightest star. By midnight the Moon moves closer to Spica, which then will be directly to its left.

MONDAY, MAY 20

■ Now Spica shines upper right of the Moon. Much farther to their upper left (three fists) is brighter Arcturus.

■ The Arch of Spring still spans the western sky in late twilight, but it's getting lower. Pollux and Castor form its top: They're lined up roughly horizontally in the west-northwest, about three finger-widths at arm's length apart. Look far to their lower left for Procyon, and farther to their lower right for 2nd-magnitude Menkalinan and then bright Capella.

TUESDAY, MAY 21

■ Have you ever seen Alpha Centauri? At declination –61° our brilliant, magnitude-zero neighbor is permanently out of sight if you're north of latitude 29°. But if you're at the latitude of San Antonio, Orlando, or points south, Alpha Cen skims just above your south horizon for a little while late these evenings.

When does this happen? Just about when Alpha Librae, the lower-right of Libra's two brightest stars, is due south over your landscape. At that time, drop your gaze down from there!

WEDNESDAY, MAY 22

■ The Moon this evening is ½ day before full (it'll be exactly full at 9:53 a.m. EDT Thursday morning), so "full Moon" applies equally to both this evening and tomorrow evening.

The Moon each month takes three or four days to walk the wide gap from Spica to Antares. Tonight after dark you'll see the Moon to the right of the three-star head of Scorpius, with Antares bringing up their rear to the lower left. Cover the glary Moon with your fingertip to help see the stars.

THURSDAY, MAY 23

■ Full Moon occults Antares. Tonight the brilliant round Moon passes right over Antares for the southeastern U.S., Central America, northeastern South America, the Caribbean, and parts of west and central Africa. Elsewhere the Moon performs a near miss.

Aldebaran about to be occulted by the waning gibbous Moon in daylight, Oct. 2, 2015. Bob King / Sky & Telescope — That's not Antares but Aldebaran, similar in brightness and color, about to be occulted in blue-sky daylight on October 2, 2015. Photographer Bob King wrote, "I took this picture with an older iPhone through a 10-inch scope, shortly before the Moon occulted Aldebaran an hour after sunrise." The Moon was waning gibbous.

Once again, map and timetables for the occultation. See the timetable instructions under May 17 above. For instance, telescope users in Miami will see Antares disappear at 9:12 p.m. EDT when the Moon is just 6° above the southeast horizon. Then Antares will reappear from behind the Moon's other side at 10:18 p.m. EDT, when the Moon has climbed to 18° high.

Wherever the occultation happens, the star will disappear smack on the Moon's bright edge, then will reappear from behind the extremely thin "dark crescent" of night on the other edge. So yes, in both cases you will definitely need a telescope.

Antares is a double star; its companion, magnitude 5.4, lurks a mere 2.7 arcseconds west of Antares A. The blazing moonlight is likely to overwhelm Antares B, but see if you can detect it emerging from behind the Moon's dark limb just a few seconds before the magnitude-1.1 primary pops out.

FRIDAY, MAY 24

■ Vega is the brightest star shining in the east-northeast after dark. Look lower left of it, by about two fists at arm's length, for Deneb, less bright. Those stars are two thirds of the Summer Triangle.

So where's the third? It's Altair. With summer still four weeks away (astronomically speaking), it stays below the eastern horizon until somewhat after dark. Watch for it to clear the horizon three or four fists at arm's length to Vega's lower right.

■ Have you tried for Mercury in the dawn? It's very low in the east a half hour before sunrise, as indicated below. This is about your last chance at Mercury this apparition. Bring binoculars.

Mercury barely rising in the east far lower left of Mars, May 25, 2024 — Mercury has been brightening but sinking lower in the dawn. Modest Mars, less low, guides the way. This morning they're 25° apart.

SATURDAY, MAY 25

■ Capella sets low in the northwest not very long after dark, depending on your latitude. That leaves Vega and Arcturus as the brightest stars in the evening sky. Vega shines in the east-northeast. Arcturus is very high toward the south.

Right after full dark and before the Moon rises, look a third of the way from Arcturus to Vega for semicircular Corona Borealis, with 2nd-magnitude Alphecca as its one moderately bright star.

Two thirds of the way from Arcturus to Vega is the dim Keystone of Hercules, now lying almost level.

Use binoculars or a telescope to examine the Keystone's top edge. A third of the way from its left end to the right is 6th-magnitude M13, one of Hercules's two great globular star clusters. In binoculars it's a tiny glowing cotton ball. A 4- or 6-inch scope begins to resolve some of its speckliness. Located 22,000 light-years away far above the plane of the Milky Way, it consists of several hundred thousand stars in a swarm about 140 light-years wide.

SUNDAY, MAY 26

■ Back to Vega getting high in the east-northeast. Look for its faint little constellation Lyra, the Lyre, hanging down from it with its bottom canted to the right. The most familiar part of Lyra consists of a little equilateral triangle (Vega is one corner) with a larger parallelogram attached to its bottom corner. The whole thing is 7½° long, about four finger-widths at arm's length.

The bottom two stars of the parallelogram, Beta and Gamma Lyrae, are the two brightest stars of the pattern after Vega. Gamma is the one farthest from Vega.

Most of the time those two are almost indistinguishable in brightness: Gamma is visual magnitude 3.25 and Beta is 3.4. But Beta is a famous eclipsing variable, one of the first discovered. Look up at these two enough times, and sooner or later you will catch Beta very obviously dimmer than Gamma, at its minimum brightness of mag 4.3. More often you're likely to catch it somewhere in between, when the difference is apparent but not so striking.

■ The waning gibbous Moon rises around midnight. Once it's well up, cover it with your fingertip to help reveal that it's in the middle of the Sagittarius Teapot.

This Week's Planet Roundup

All seven planets other than Earth remain near our line of sight to the Sun.

Mercury, Mars and Saturn can be found low during dawn. The easiest is Saturn, fairly well up in the east-southeast as dawn begins. It's magnitude 1.2, and nothing else that bright is anywhere near it.

Far lower left of Saturn is Mars, about the same brightness but harder to see in the brightening dawn. They widen from 25° apart (a little more than two fists at arm's length) on the morning of May 18th to 30° (three fists) on May 25th.

Mercury is far lower left of Mars. Those two are 20° apart on the 18th and 25° apart by the 25th. Use binoculars to try for Mercury through bright twilight a half hour before sunrise. Good luck.

Venus, Jupiter, Uranus, and Neptune are veiled by the Sun's glare.

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.

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), which shows all stars to magnitude 7.6.

Pocket Sky Atlas cover, 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 here is the Jumbo Edition, which is in hard covers and enlarged for easier reading outdoors by red flashlight. Sample charts. More about the current editions.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. (Sadly, it's currently out of print.) The next up are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to mag 9.75). And read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet, the preferred tools for many observers these days, as it does 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 top of the hill for total astro-geeks is the Annals of the Deep Sky series, currently at 10 volumes as it slowly works forward through the constellations alphabetically. So far it's only up to F.

Can computerized telescopes replace charts? Not for beginners I don't think, and not for any scopes on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. Unless, that is, you prefer spending your time getting finicky technology to work rather than learning the sky. 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."

If you do get a computerized scope, make sure 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. 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."
— John Adams, 1770