This Week's Sky at a Glance, May 10 – 19

FRIDAY, MAY 10

■ 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. Capella is about two fists to the right of the crescent Moon this evening, as shown during bright twilight below.

Arcturus, Vega, and Capella appear so bright because each is at least 60 times as luminous as the Sun, and they're all relatively nearby. They're 37, 25, and 42 light-years from us, respectively.

At some point shortly after dark, you'll find Vega and Capella standing at exactly the same height above the horizon. For how long will their heights be indistinguishable? That's the sort of thing sailors used a sextant for. Their lives could depend on it.

SATURDAY, MAY 11

■ This is the time of year when Leo the Lion starts walking downward toward the west, on his way to departing into the sunset in early summer. Right after dark, spot the brightest star fairly high in the west-southwest. That's Regulus, his forefoot.

Regulus is also the bottom of the Sickle of Leo: a backward question mark about a fist and a half tall that outlines the lion's leading foot, chest, and mane.

SUNDAY, MAY 12

■ The thickening crescent Moon shines just left of Pollux this evening. About twice as far to their right, Castor lies nearly on the same line. For skywatchers in the Central time zone, the lineup is perfect about an hour after full darkness. The exact lineup happens later at night for the Eastern time zone; earlier if you're west of Central.

MONDAY, MAY 13

■ After dark, use binoculars or a good finderscope to look 3° lower left of the thick crescent Moon. That's about half the width of a typical binocular's field of view. Can you make out the Beehive star cluster, M44, through the moonlight? The loose-scattered cluster is roughly ½° wide.

The Moon is 1.3 light-seconds from you. The Beehive is 600 light-years in its background.

TUESDAY, MAY 14

■ The Moon shines to the right of Regulus at nightfall, as shown below. Notice that the Moon's terminator is not yet perfectly straight. The Moon is exactly first quarter about 12 hours later for the time zones of the Americas, at 7:48 a.m. EDT Wednesday morning the 15th.

WEDNESDAY, MAY 15

■ Now the evening Moon, about 12 hours past first quarter, shines closer over Regulus as shown above.

THURSDAY, MAY 16

■ 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 the Earth's surface is much younger.

Next is Arcturus, very high in the southeast these evenings, which most people have surely seen whether they know it or not; 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 telescope users have globular clusters. Most are older still, at least in part. For instance, white dwarfs in the familiar M4 in Scorpius have been dated at 12.7 ±0.7 billion years.

But what about 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 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 on 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, and it has very likely remained intact since the Big Bang's first millionth of a second. And what a history it has been through since!

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, for many cities, are very 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. 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 the Virgo 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 is now directly to its left.

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This Week's Planet Roundup

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

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

Mars and Saturn, both about magnitude +1.2, rise just before dawn. In early dawn, about 75 to 60 minutes before sunrise, look for them low in the east-southeast. Saturn is the easy one. Find Mars well to Saturn's lower left. They widen from 21° apart (about two fists at arm's length) on the morning of May 11th to 26° on May 18th.

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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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