This Week's Sky at a Glance

Some daily events in the changing sky for February 17 – 25

by Alan M. MacRobert

Venus is the bright landmark for finding much more elusive Mercury and, on the 22nd, the 1-day-old hairline Moon. The visibility of Mercury and the thin Moon in bright twilight is exaggerated here; bring binoculars. (These scenes are drawn for the middle of North America. European observers: move each Moon symbol a quarter of the way toward the one for the previous date.)

Sky & Telescope diagram

Friday, February 17

This is the time of year when, after dinnertime, the landmark W pattern of Cassiopeia stands vertically on end high in the northwest.

Saturday, February 18

After dinnertime at this time of year, four carnivore constellations stand in a row from the northeast to south. They're all seen in profile with their noses pointed up and their feet (if any) to the right: Ursa Major in the northeast (with the Big Dipper as its brightest part), Leo in the east, Hydra the Sea Serpent in the southeast, and Canis Major in the south.

Sunday, February 19

We're in the dark of the Moon, which means it's deep-sky observing time. Check out a dozen winter planetary nebulae using your star atlas and Ted Forte's guided tour with pictures in the February Sky & Telescope, page 60.

Monday, February 20

Brilliant Sirius shines highest due south around roughly 8:30 or 9 p.m., depending on how far east or west you live in your time zone.

Have you ever seen Canopus, the second-brightest star after Sirius? In one of the many interesting coincidences that devoted skywatchers know about, Canopus lies almost due south of Sirius: by 36°. That's far enough south that it never appears above your horizon unless you live below latitude 37° N (southern Virginia, southern Missouri, central California). And even there you'll need a flat south horizon. Canopus transits the sky's north-south meridian just 21 minutes before Sirius does.

When to look? Canopus transits right when Beta Canis Majoris (Mirzim) does. That's the fairly bright star about three finger-widths to Sirius's right. When Mirzim is due south, look straight down from there.

Tuesday, February 21

Zenith star: Bright Capella crosses closest to straight overhead around 7 p.m., depending on how far east or west you live in your time zone. Capella goes exactly through your zenith if you live at latitude 46° north (the Oregon-Washington border, Montreal, central France). In you're south of there, Capella passes north of your zenith.

New Moon (exact at 5:35 p.m. EST)

Wednesday, February 22

An ultra-thin, one-day-old crescent Moon floats to the right of Mercury very low in the west a half hour after sunset, as shown above. Bring binoculars.

Thursday, February 23

The thickening crescent Moon hangs far lower right of Venus and over Mercury in bright twilight, as shown above.

Mark your calendar to catch the Moon's pairings with Venus and Jupiter on successive evenings. Their positions here are exact for the middle of North America. For clarity, the Moon is shown three times its actual apparent size.

Sky & Telescope diagram

Friday, February 24

Jupiter and Venus form a line pointing down to the crescent Moon during and after twilight, as shown here.

Saturday, February 25

The crescent Moon pairs spectacularly with Venus in the evening sky! They're just 3° apart as seen at the time of twilight for eastern North America. Then tomorrow the Moon pairs with Jupiter, not quite so closely.

Want to become a better amateur astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

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. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts effectively.

You'll also want a good deep-sky guidebook, such as Sue French's new Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the classic if dated Burnham's Celestial Handbook.

This Week's Planet Roundup

Mercury (about magnitude –1.2) is emerging into good evening view, moving higher in the afterglow of sunset every day. About 30 to 40 minutes after sunset, look for it far to the lower right of bright Venus. Next week Mercury will be at its evening best for 2012.

Venus (magnitude –4.2, in Pisces) is the brilliant “Evening Star” shining in the southwest during and after dusk. Don't confuse it with Jupiter higher to its upper left. Venus doesn't set now until more than two hours after dark. It will continue to move a bit higher, stay up later, and brighten slightly for the rest of the winter. In a telescope Venus is a brilliant white gibbous disk 17 arcseconds tall and 67% sunlit. It will reach half-lit phase (dichotomy) in mid- to late March.

Venus is almost always featureless blank white to visual observers. But digital stacked-video imaging can sometimes bring out the planet's subtle cloud patterns, especially in the near ultraviolet (UV). S&T's Sean Walker took these images through ultraviolet, blue, green, red, and near-infrared filters over a span of 9 minutes on February 9th shortly before sunset. He used the same equipment described for Mars below.

S&T: Sean Walker

Mars (about magnitude –1.0, in western Leo) rises bright fire-orange in the east during twilight. It's nearly as big and bright as it will become this year. It shines highest in the south, in best telescopic view, around 1 a.m.

In a telescope Mars has grown to about 13.5 arcseconds wide, essentially the same as the 13.9″ it will display when it's nearest to Earth around March 5th. Mars appears almost uniformly lit now, since it is getting very close to its March 3rd opposition.

Mars had grown to 13.4 arcseconds wide by February 19th, when S&T's Sean Walker took this image with a 12.5-inch Newtonian reflector and a DMK 21AU618.AS video camera, through red, green, blue, and near-infrared filters. Longer wavelengths penetrate to the surface better; shorter wavelengths highlight features in Mars's atmosphere.

South is up. The North Polar Cap is shrinking in the of the Martian northern-hemisphere spring. Dark Solis Lacus is near top, and Mare Acidalium is at lower left. The sunset terminator is along the left limb here (celestial west; the preceding limb).

S&T: Sean Walker

Jupiter (magnitude –2.2, in Pisces) shines high in the southwest at dusk, upper left of brighter Venus. The gap between Jupiter and Venus is narrowing by 1° per day: from 23° to 16° this week. They're on their way to a spectacular conjunction just 3° apart on March 13th. These are the two brightest planets in the sky. Both move lower toward the southwest as evening advances. Venus sets in the west around 9:30 p.m., Jupiter around 11.

In a telescope Jupiter has shrunk to 37 arcseconds wide, as Earth pulls ahead of it in our faster orbit around the Sun.

Saturn's rings are tipped a good 15° from our line of sight. South is up. Note the very pale light band in the north temperate region, the remnant of the dramatic, billowing white outbreak that attracted so much attention last year. Christopher Go took this image on January 21, 2012.

Christopher Go

Saturn (magnitude +0.5, in Virgo) rises in the east around 10 or 11 p.m. and shines highest in the south in the hours before dawn. Spica, a little fainter at magnitude +1.0 (and bluer), is 7° to Saturn's right or upper right. Saturn's rings are tilted a generous 15° from our line of sight, the most open the rings have appeared since 2007.

Uranus (magnitude 5.9, in Pisces) is getting low in the west right after dark (to the lower right of Venus).

Neptune is in conjunction, hidden behind the glare of the Sun.

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 that also depend on longitude (mainly Moon positions) are for North America. Eastern Standard Time (EST) equals Universal Time (also known as UT, UTC, or GMT) minus 5 hours.

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This Week's Sky at a Glance

Some daily events in the changing sky for February 17 – 25