This Week's Sky at a Glance, August 15 – 24

FRIDAY, AUGUST 15

■ Last-quarter Moon tonight (exactly last-quarter at 1:12 a.m. Saturday morning EDT). The Moon rises around 11 or midnight daylight-saving time, with the Pleiades following about 20 minutes later. As the Moon gets higher, look for the Pleiades about 6° to its lower left. By the beginning of dawn the Moon and Pleiades are very high in the southeast, now level with each other and only about 3° or 4° apart.

SATURDAY, AUGUST 16

■ Different people have an easier or harder time seeing star colors, especially subtle ones. To me, the tints of bright stars stand out a little better in a sky that's neither black nor light-polluted gray, but the deep blue of late twilight.

For instance, the two brightest stars of summer are Vega, overhead after dark, and Arcturus shining in the west. Vega is white with just a touch of blue. Arcturus is a yellow-orange giant. Do their colors stand out a little better for you in late twilight? What about deeper orange Antares, lower in the south-southwest? Could this be a color-contrast effect of seeing yellow, orange, or orange-red stars on a blue background?

SUNDAY, AUGUST 17

■ Whenever Vega crosses nearest your zenith, as it does soon after dark now, the Sagittarius Teapot is at its highest due south. The Sagittarius area is famously rich in deep-sky objects. A sampling of major ones:

Two hours later, when Deneb passes the zenith, it's the turn of boat-shaped Capricornus to stand at its highest due south.

MONDAY, AUGUST 18

■ Titan casts its shadow on Saturn tonight. Every 15 years, Saturn's largest moon repeatedly crosses Saturn's face from Earth's viewpoint — and, more visibly, casts its very tiny black shadow onto Saturn' face. A series of these events is now under way. They will continue every 16 days until October.

Tonight Titan's shadow crosses Saturn from 5:52 to 10:00 UT August 19th (UT date). In EDT that's from 1:52 a.m. to 6:00 a.m. In PDT it's from 10:52 p.m. to 3:00 a.m.

Wherever you are, Saturn rises in the evening and is highest in the hours before dawn. So all of North America again gets a chance at this shadow transit. Titan its shadow will be crossing Saturn's high northern latitudes. See Bob King's Titan Shadow Transit Season Underway.

But if you're used to seeing shadows of Jupiter's moons on Jupiter, be warned: Saturn is twice as far away, and although Titan is large, it's not twice as large as Jupiter's moons.

■ And as dawn begins on Tuesday morning the 19th, the waning crescent Moon tops the Jupiter-Venus-Mercury stack as shown below.

TUESDAY, AUGUST 19

■ August is prime Milky Way time whenever the Moon is out of the evening sky like it is this week. Once evening darkness is complete, the Milky Way runs up from between Sagittarius and Scorpius in the south-southwest, tilts leftward across Aquila, continues left through the big Summer Triangle very high, then tapers on down through Cassiopeia to Perseus low in the north-northeast.

■ On Wednesday morning the 20 the crescent Moon, Venus, Jupiter, Pollux and Castor all fit in a circle 12° wide, as shown above. See if you can take a good pic of them, and maybe Mercury and Procyon below them, early in the eastern dawn before the sky brightens too much. Find a spot with good foreground scenery, frame the group, and brace your phone or camera firmly.

Have you tried taking such skyscapes yet? Why not start now? Yes, you have an alarm clock. Plan to be out and set to shoot at least an hour before sunrise.

WEDNESDAY, AUGUST 20

■ The W of Cassiopeia, still tilted, is nicely up in the northeast these evenings, above Perseus. The upper right-hand side of the W is its brightest side. Watch Cas rise higher and straighten upright through the next two hours and, equivalently, the next month. The rule to remember for the turning of the sky: Two hours later equals one month later. (Why? Answer in footnote 1 at the bottom of this page.)

THURSDAY, AUGUST 21

■ The brightest star high in the southeast these moonless evenings is Altair, with little orange Tarazed above it by a finger-width at arm's length.

A little more than a fist-width to Altair's left is delicate Delphinus, the Dolphin, leaping left.

Above Altair, slightly less far, is smaller, fainter Sagitta, the Arrow. It too points leftward. You'll need a nice dark sky. Or binoculars.

FRIDAY, AUGUST 22

■ As summer wanes and Arcturus moves down the western sky, the kite pattern of Boötes that sprouts from Arcturus leans over to the right. The kite is narrow, slightly bent, and 23° long: about two fists at arm's length. Arcturus is its bottom point from which the stubby tail hangs down.

The Big Dipper poses to itsright in the northwest at about the same height as Arcturus, as shown below.

SATURDAY, AUGUST 23

■ New Moon (exact at 2:07 a.m. on this date EDT).

■ As August proceeds and nights begin to turn chilly, the Great Square of Pegasus looms up in the east, balancing on one corner. Its stars are only 2nd and 3rd magnitude. Your fist at arm's length fits inside it. Saturn glows down to its lower right.

From the Square's left corner, the backbone of the constellation Andromeda extends to the lower left: three stars in a slightly curving line (including the corner) about as bright as those forming the Square.

This whole giant pattern was named "the Andromegasus Dipper" by the late Sky & Telescope columnist George Lovi. It's shaped sort of like a giant Little Dipper with an extra-big bowl. It's currently lifting its contents upward.

SUNDAY, AUGUST 24

■ The actual Little Dipper, meanwhile, is tipping over leftward in the north. It's only 40% as long as the Andromegasus Dipper, and most of it is much fainter. As always, you'll find that it's oriented more than 90° counterclockwise compared to Andromegasus. It's dumping its contents out.

This Week's Planet Roundup

Mercury, low in the dawn, nearly triples its brightness this week: from magnitude +0.5 on Saturday morning the 16th to –0.6 a week later on Saturday the 23rd. Look for it low in the east-northeast in early twilight. Jupiter and Venus point down to it all week, for instance as shown under August 18 above.

Venus and Jupiter, both in Gemini, shine in the east before and during dawn. They're the two brightest planets, currently magnitudes –3.9 and –2.0 respectively.

Venus and Jupiter went through their conjunction on the 12th, just under 1° apart. On Saturday morning the 16th they're still only 4° apart, with Jupiter on top. They widen to 11° apart by the 23rd. All this time they point down to little Mercury.

Both bright shiners are about at their farthest from Earth, so they're not much in a telescope.

Mars, a weak magnitude 1.6 in western Virgo, still glimmers very low in the west during evening twilight. Binoculars will help. Mars sets at twilight's end. It too is about at its farthest from Earth.

Don't confuse orange Mars with brighter, whiter, twinkly Spica to its upper left. Mars and Spica are 19° apart on Friday evening the 15th, narrowing to 14° by Friday the 22nd.

Saturn (magnitude +0.7, in Pisces) rises due east around the end of twilight. It's lower right of the Great Square of Pegasus, which now stands on one corner diamond fashion. The Square's lower left side points diagonally down almost straight to Saturn.

But the best time to observe Saturn with a telescope is in early morning hours when it's high toward the south. We see Saturn's rings almost edge-on this year, and the Sun shines on them from nearly our direction too. So the rings and their shadow combine to form a black line along Saturn's equator.

Uranus (magnitude 5.7, in Taurus near the Pleiades) rises around 11 or midnight and is very high in the east before the beginning of dawn. In a telescope it's a tiny but definitely non-stellar dot 3.6 arcseconds wide.

Neptune, a telescopic "star" at magnitude 7.8, lurks a little more than 1° from Saturn. Use the finder chart for Neptune with respect to Saturn in the June Sky & Telescope, page 51. With a pencil, put a dot on the path of each of the two planets for your date. Neptune is just 2.3 arcseconds wide.

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 (which many observers find more versatile) 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

1. Why, in the ever-changing constellation panorama, does one month later equal two hours later?

Because the direction in space you look when you face into the night sky depends on two things: Earth's own 24-hour rotation, and where Earth's night side — the side away from the Sun — faces during the Earth's 12-month orbit around the Sun. So, looking one month later turns your night-direction view as much as looking two hours later on any given night.