This Week's Sky At A Glance, September 9 – 17

FRIDAY, SEPTEMBER 9

■ Full Moon tonight (exactly full at 5:59 a.m. Saturday morning EDT). The Moon rises in the east-southeast just after sunset.

As the stars come out, you can see that the Moon is between bright Jupiter a couple of fists to its left or lower left, dimmer Saturn farther to the Moon's upper right (out of the frame below), and Fomalhaut, twinkling down to the Moon's lower right. The Moon forms a roughly equilateral triangle with those last two.

SATURDAY, SEPTEMBER 10

■ The Moon, a day past full, rises in early twilight with Jupiter now shining just one fist or less to its left, as shown above.

SUNDAY, SEPTEMBER 11

■ How soon in twilight can you see the big Summer Triangle? Face east and look straight up. Vega, the Triangle's brightest star, is passing the zenith (for skywatchers at mid-northern latitudes). Deneb is the brightest star a couple fists to Vega's northeast. Altair shines somewhat less high in the southeast.

MONDAY, SEPTEMBER 12

■ The Moon now shines well to Jupiter's lower left in mid- to late evening.

Look less far to the Moon's left or upper left for the two or three brightest stars of Aries. They're lying more or less horizontally.

TUESDAY, SEPTEMBER 13

■ The stars say the season is changing: We've reached the time of year when, right at nightfall, Cassiopeia has climbed as high in the northeast as the Big Dipper has sunk in the northwest. Cas highlights the northern sky in early evening during the chilly fall-and-winter half of the year. The Big Dipper takes over for the mild evenings of spring and summer.

Almost midway between the two stands Polaris. It's currently a little above the midpoint between them.

WEDNESDAY, SEPTEMBER 14

■ Spot the Big Dipper scooping low in the northwest. Examine Mizar, the middle star of its bent handle. Can you see its tiny close companion Alcor? Since this is the time of year when Vega is crossing the zenith in early evening, it's also when Alcor stands straight over Mizar. Because a line from Mizar through Alcor always points to Vega.

■ As dusk turns to night, Arcturus twinkles due west. It's getting lower every week. From Arcturus, the narrow, kite-shaped pattern of Bootes extends upper right by a little more than two fists at arm's length. Farther off to the right in the northwest, the Big Dipper is turning more and more level.

THURSDAY, SEPTEMBER 15

■ The Great Square of Pegasus is climbing the eastern sky. Look for it above bright Jupiter and maybe a bit left. It's balancing on one corner, as it always does when we see it in either the east or the west. Only when the Great Square is very high and more or less south does is sit level like a box.

■ Late these nights, ever-brightening Mars dominates the eastern sky, well outshining Aldebaran to its right as shown below. In the small hours of the morning Betelgeuse, essentially the same color as Mars and Aldebaran, will come up farther below them. (The dates in the illustration consider midnight to be part of the evening date.)

FRIDAY, SEPTEMBER 16

■ Now the late-rising Moon shines in line with Mars and Aldebaran to its right, as shown above. The Moon and Mars are about 4° or 5° apart as seen from the longitudes of the Americas. But this is still the closest pairing of the Moon and a bright planet visible in September from this part of the world.

SATURDAY, SEPTEMBER 17

■ Arcturus shines in the west these evenings after twilight fades out. About a half hour after nightfall is complete, brighter Jupiter shines at the same height in the east. How well can you time their exact balance?

Barely rising in the north-northeast when that happens is Capella (depending on your latitude; the farther north you are the higher it will be.) Capella and Arcturus are both magnitude 0.

Later in the evening, Arcturus and Capella shine at the same height. Again, the time of this will depend on both your latitude and longitude.

Watch for when it happens. Then turn and look low in the south-southeast. There will be 1st-magnitude Fomalhaut at about the same height too — exactly so if you're at latitude 43° north (Boston, Buffalo, Milwaukee, Boise, Eugene). Seen from south of that latitude, Fomalhaut will appear higher than Capella and Arcturus are. Seen from north of there, it will be lower.

■ Neptune is at opposition. It's magnitude 7.8 in eastern Aquarius. Equally bright (or faint) this week is the asteroid 3 Juno about 10° to Neptune's west. Using a telescope or large binoculars, find them both in late evening using the article and charts in the September Sky & Telescope, page 49. And some 20° west-southwest of Juno, brighter 4 Vesta shines on at about magnitude 6.3. Use the finder chart in the August Sky & Telescope, page 49.■ Last-quarter Moon (exact at 5:52 p.m. EDT). The Moon rises around 11 or midnight in the northeast, two fists lower right of Capella.

Once the Moon is well up, you can see that it makes a nearly equilateral triangle, 7° on a side, with the two horn-tip stars of Taurus: Beta and Zeta Tauri. They're to its upper right and right, respectively.

This Week's Planet Roundup

Mercury is hidden in the glare of the Sun.

Venus, magnitude –3.9,rises in mid-dawn about 45 minutes before sunrise. As dawn brightens, look for Venus very low in the east. How near to sunrise can you still hold it in view?

Mars, magnitude –0.3 in Taurus, clears the east-northeast horizon around 11 p.m. and gains altitude for the rest of the night. It's more than three times as bright as Mars-colored Aldebaran, magnitude +0.9, sparkling to the right or lower right of it. Look for the Pleiades farther above them.

By the beginning of dawn this array is very high in the south-southeast. This the best time to examine Mars in a telescope: when we see it highest through the thinnest, steadiest air. Mars is nearly 11 arcseconds in diameter and growing. It'll appear 17.2 arcseconds across when closest to Earth on December 1st.

Jupiter, a bright magnitude –2.9, glares very low due east by late twilight. After dark it dominates the low east, then the higher southeast, in an otherwise bland area at the Pisces-Cetus border. It stands highest in the south around 1 or 2 a.m. daylight-saving time. In a telescope Jupiter is 49 to 50 arcseconds wide across its equator, nearly its maximum possible, as it nears its September 26th opposition. Jupiter this year is close to the perihelion of its 12-year orbit around the Sun.

Saturn, magnitude +0.4 in eastern Capricornus, is a month past opposition. So as twilight fades you can spot it fairly well up in the southeast. It's at its highest and best in the south around 10 or 11 p.m.

Uranus, magnitude 5.7 in Aries, is west of Mars late at night.

Neptune, magnitude 7.8 at the Aquarius-Pisces border, is west of Jupiter.

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. (Universal Time is also called UT, 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 an easy-to-use 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 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 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. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And be sure to 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.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. 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."

Audio sky tour. Out under the evening sky with yourearbuds in place, listen to Kelly Beatty's monthlypodcast tour of the 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