This Week's Sky at a Glance, May 2 – 11

FRIDAY, MAY 2

■ Look for the Moon forming a gently curving line with Pollux and Castor this evening, as shown below. Mars watches on from their upper left.

■ Although it's May, wintry Sirius still twinkles very low in the west-southwest at the end of twilight. It sets soon after. How much longer into the spring can you keep Sirius in view at all? In other words, what will be its date of "heliacal setting" as seen by you?

■ Meanwhile summer is seven weeks away, but the Summer Triangle is beginning to make its appearance in the east, one star after another. The first in view is Vega. It stands out brightly low in the northeast after nightfall.

Next up is Deneb, lower left of Vega by two or three fists at arm's length. Deneb takes about an hour to appear after Vega does, depending on your latitude.

The third is Altair, which shows up far to Vega's lower right by about 11 or midnight.

SATURDAY, MAY 3

■ Now the waxing Moon shines next to Mars. They're only couple degrees apart. Meanwhile, Mars is less than 1° to the right of the Beehive star cluster, as binoculars or a telescope at low power will reveal.

SUNDAY, MAY 4

■ Mars skims across the northern edge of the Beehive cluster this evening and tomorrow evening.

■ First-quarter Moon (exact at 9:52 a.m. Eastern Daylight Time). By evening for North America the Moon's terminator will be showing a slight bit of convexity.

After dark the Moon will be under the Sickle of Leo, with Regulus to its left or upper left and Mars (similar to Regulus in brightness but different in color!) farther to the Moon's right or lower right.

■ The Eta Aquariid meteor shower should be at its peak in the hour or so before the beginning of Monday's dawn (i.e. from about 3 to 2 hours before sunrise. Also, the Moon will have set by then.) This tends to be the best meteor shower of the year for Southern Hemisphere observers, but it's poorer for those of us living at mid-northern latitudes due to the placement of its radiant (at the head of Aquarius, the Water Jar asterism). From north of about 40° latitude, almost no Eta Aquariids are seen.

MONDAY, MAY 5

■ The waxing gibbous shines near Regulus, magnitude 1.4. The star is 3° or 4° to the Moon's right early in the evening, then lower right of it as the night grows late.

TUESDAY, MAY 6

■ As night descends, look high in the west for Pollux and Castor lined up almost horizontally (depending on your latitude) with Mars upper left of them. The two stars, the heads of the Gemini twins, 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 2nd-magnitude Menkalinan (Beta Aurigae) and then brilliant Capella. The whole thing sinks in the west through the evening.

Modern skywatchers are not alone in seeing the Arch of Spring as one big asterism. Extend it down past Procyon to add Sirius, and you've got the Hawai‘ian Canoe-Bailer of Makali‘i.

WEDNESDAY, MAY 7

■ A gigantic asterism you may not know about is the Great Diamond, some 50° tall and extending over five constellations. It now stands upright in the southeast to south after dusk.

Start with Spica, its bottom. Upper left from Spica is bright Arcturus. Almost as far upper right from Arcturus is fainter Cor Caroli, 3rd magnitude. The same distance lower right from there is Denebola, the 2nd-magnitude tailtip of Leo. And then back to Spica. Robert H. Baker may have been the first to name the Great Diamond, in his 1954 book When the Stars Come Out.

The bottom three of these stars, the brightest, form a nearly perfect equilateral triangle. We can call this the Spring Triangle to parallel to those of summer and winter. The first to name it such was probably Sky & Telescope columnist George Lovi, writing in the March 1974 issue.

THURSDAY, MAY 8

■ Whenever the stick figures of the Gemini twins stand upright in the west, as they do after dark for much of the spring, you know that the two-star pattern of Canis Minor lower left of them (Procyon and much fainter Beta CMi) lies horizontal or nearly so. And that the Big Dipper hangs vertically by its handle high in the northwest.

FRIDAY, MAY 9

■ Three zero-magnitude stars shine after dark in May: Arcturus high in the southeast, Vega much lower in the northeast, and Capella in the northwest. They appear so bright because each is at least 60 times as luminous as the Sun and because they're all relatively nearby: 37, 25, and 42 light-years from us, respectively.

■ Spica, less bright than Arcturus at 1st magnitude, shines a couple degrees lower left of the Moon this evening, as shown below.

SATURDAY, MAY 10

■ Now Spica shines about 10° upper right of the bright evening Moon, as shown above.

SUNDAY, MAY 11

■ What is the oldest thing you have ever seen? For everyone in the world it's at least the Sun and other objects of the solar system, age 4.6 billion years. (Everything on or just under Earth's surface is much younger.)

Next is Arcturus, which most people have surely seen whether they know it or not, since it's one of the brightest stars in the sky. It's a Population II orange giant, age about 7 billion years, just passing through our region of the Milky Way.

Amateur astronomers have globular clusters. Most are older still, at least in part. White dwarfs in familiar M4 in Scorpius have dated it (or at least some population of it) at 12.7 ±0.7 billion years.

But individual stars that you can observe? For that you want Bob King's article In Search of Ancient Suns with its finder charts. Assigning dates to individual local stars from the first eras after the Big Bang is still iffy; astronomers have to work from the near-absence of heavy elements in their spectra. But a 6th-magnitude star in Boötes and a 7th-magnitude star in Libra, both in binocular range, await you these May and June nights. They probably date from about 12½ billion and at least 13 billion years ago, respectively. These will probably be the oldest things you have ever seen or ever will. The Big Bang itself is well dated at 13.8 billion years.

One could pick nits. Pick a proton, any proton right in front of you. That assemblage of three quarks has very likely remained intact since the Big Bang's first millionth of a second. And what a history it has been through, its integrity undisturbed, since then!

This Week's Planet Roundup

Mercury is sinking out of sight into the bright glow of sunrise.

Venus and Saturn, on the other hand, are getting slightly higher in the eastern dawn. They both rise around the very first sign of morning twilight, nearly two hours before sunrise. But Venus, magnitude –4.7, shines 250 times brighter than Saturn! Which is a weak magnitude +1.3. Binoculars will help locate Saturn 4½ ° to the right of Venus on the morning of May 3rd, widening to 8° right of Venus by May 10th, as shown in the two frames below.

How different they look in a telescope! Venus is a dazzling crescent about 33% sunlit. Saturn is a vastly dimmer little ball with only 1/250 of Venus's surface brightness. That's mostly because Saturn is 13 times farther from the illuminating Sun than Venus is, and partly because Venus's albedo is 65% compared to Saturn's 47%. (Albedo is how much of the incoming light an object reflects.)

Both will be blurred and shimmery in a telescope in the low-altitude seeing. So you may see nothing of Saturn's rings, which are presented nearly edge-on this year. Look for traces of a hairline needle stuck diagonally (i.e. celestial east-west) through Saturn's globe. That's them.

Mars (magnitude +1.0, in Cancer) glows high in the southwest in the evening. It's the orange dot a little more than a fist-width upper left of Pollux and Castor (which are similar to it at magnitudes +1.1 and +1.6, respectively). It's about two fists above Procyon (mag +0.4).

In a telescope Mars has shrunk to only 6 arcseconds in diameter, hardly more than a fuzzy blob. Maybe you can see that it's slightly gibbous, 90% sunlit.

Jupiter (magnitude –2.0, in Taurus) shines bright white low in the west in early evening, 46° lower right of Mars along the ecliptic. Jupiter continues to form a shrinking triangle with Taurus's two horntip stars, Beta and fainter Zeta Tauri. They're upper right and closer left of it, respectively. The triangle will flatten to a straight line on May 15th.

Jupiter sets in the west-northwest around 11 p.m. daylight-saving time.

In a telescope Jupiter has shrunk to only 33 arcseconds wide, about as small as it ever gets. For the daily doings of its Galilean moons see the May Sky & Telescope, page 51.

Uranus is lost in the sunset.

Neptune, a mere 8th magnitude, lurks hidden in the dawn in the background of Venus 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 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