This Week's Sky at a Glance, May 24 – June 2

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. Vega and Deneb 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 your last chance at Mercury this apparition. Bring binoculars!

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 across.

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.

MONDAY, MAY 27

■ With the Moon gone from the evening sky, can you see the big, dim Coma Berenices star cluster? Does your light pollution really hide it, or do you just not know exactly where to look? It's southwest of the zenith these evenings, 2/5 of the way from Denebola (the tip of Leo's tail) to the end of the Big Dipper's handle (the tip of Ursa Major's tail). Its brightest members form an inverted Y. The entire cluster is about 4° or 5° wide — a big, dim glow in a fairly dark sky. It nearly fills a binocular view.

TUESDAY, MAY 28

■ 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, now around 11 or midnight, drop your gaze down from there!

WEDNESDAY, MAY 29

■ The last-quarter Moon rises late tonight, around 1 or 2 a.m. daylight-saving time. By dawn on Thursday May 30th the Moon is high in the southeast, with Saturn hardly more than a fist-width to its left, as shown below. (Exact last-quarter phase is at 1:13 p.m. Thursday afternoon EDT.)

THURSDAY, MAY 30

■ For much of the spring at mid-northern latitudes, the Milky Way lies right down out of sight all around the horizon. But watch the east now. The rich Cepheus-Cygnus-Aquila stretch of the Milky Way starts rising up all across the east late these nights, earlier and higher every week. A hint for the light-polluted: It runs horizontally under Vega, along the bottom of the Summer Triangle.

FRIDAY, MAY 31

■ Vega is the brightest star on the northeastern side of the sky. Arcturus is the brightest high overhead. A third of the way from Arcturus to Vega is dim Corona Borealis, the semicircular Northern Crown. Its one moderately bright star is Alpha Cor Bor, a.k.a. Gemma or Alphecca, magnitude 2.2. But within the next year or so, astronomers expect there may suddenly be two! Because T Coronae Borealis, a famous recurrent nova, shows signs (a gradual dimming) of being about to blow again for its first time since 1946. If it does, if could match the brightness of Alphecca.

Normally T CrB simmers along uneasily at about 10th magnitude. Its explosive rise may take only about a day. Use the finder chart with Bob King's article from 2016; the chart there is detailed enough for identifying T with a telescope even at 10th magnitude. But no guarantees anything will happen anytime soon! That time, nothing did.

SATURDAY, JUNE 1

■ Constellations seem to twist around fast when they pass your zenith — if you're comparing them to the direction "down." Just a week and a half ago, the Big Dipper floated horizontally in late twilight an hour after sunset (as seen from 40° north latitude). Now it's angled diagonally at that time. In just another week and a half it will be hanging straight down by its handle!

The farther north you are, the quicker the Dipper seems to gyrate. If, that is, you can judge your zenith point accurately enough to make "up" and "down" meaningful that close to it.

SUNDAY, JUNE 2

■ To many people, "Cassiopeia" means "Cold." Late fall and winter are when this landmark constellation stands high overhead. But even on hot June evenings, it still lurks low (as seen from mid-northern latitudes). As twilight fades out, look for it down near the north horizon: a wide, upright W. The farther north you are the higher it'll appear, but even as far south as San Diego and Atlanta it's completely above the horizon.

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This Week's Planet Roundup

Mars and Saturn are up in early dawn. The highest and easiest is Saturn, in the southeast. It's magnitude 1.2, fairly modest, but there's nothing else that bright anywhere near it. Its background is dim Aquarius.

Look for Mars far lower left of Saturn, by roughly three fists at arm's length. It's magnitude 1.1, and again, nothing else there is as bright. Mars lies in dimmer Pisces.

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

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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 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.

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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 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.

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. 

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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.

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"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

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