This Week's Sky at a Glance, March 18 – 26

■The Moon is barely past full. Once it's well up late this evening, it forms a nearly right triangle with Arcturus about three fists to its left and Spica half that distance below it (for North America).

SATURDAY, MARCH 19

■The bright waning gibbous Moon rises around twilight's end. Once it's well up, look for Spica about 4° to its lower right (for North America) glimmering through the moonlight.Brighter Arcturus shines about three fists to their upper left.

SUNDAY, MARCH 20

■Spot Arcturus very low in the east-northeast after nightfall and higher in the east later in the evening. By modern measurements Arcturus is visual magnitude –0.05, making it the fourth-brightest nighttime star. It's bested only by Sirius, Canopus, and Alpha Centauri (counting the combined light of Alpha Cen A and B; the pair appears single to the unaided eye).

For northerners who can never see Canopus or Alpha Cen, Arcturus is bested by Sirius alone. However, Vega and Capella are very close on its heels.

■Speaking of spring, it begins today! (in the Northern Hemisphere; fall in the Southern Hemisphere). The equinox is at 11:33 a.m. EDT. This is when the center of the Sun's disk crosses the equator — both Earth's equator and, equivalently, the celestial equator — heading north for the season.

■Venus is at its greatest elongation, 47° west of the Sun for the next couple days. But it's still not very high in the dawn. The reason? At this time of year, the ecliptic lies relatively low to the eastern dawn horizon for skywatchers at mid-northern latitudes. So that 47°-longline from the Sun to Venus lies at a low angle, not straighter up.

MONDAY, MARCH 21

■This is the time of year when Orion declines in the southwest after dark, with his Belt roughly horizontal. But when does Orion's Belt appear exactly horizontal? That depends on where you're located east-west in your time zone, and on your latitude.

How well can you time this event? If you're near your time zone's standard longitude, expect it around 9:15 this evening (daylight-saving time). . . more or less.

TUESDAY, MARCH 22

■In a very dark sky, the realistic stick figure of Canis Major is fairly plain to see — the Big Dog is seen in profile prancing to the right on his hind legs, with Sirius on his chest as his shiny dog tag. But for many of us, only his five brightest stars show well through the light pollution. These form the unmistakable Meat Cleaver.

Sirius and Murzim (to its right) are the wide top end of the Cleaver, with Sirius sparkling on its top back corner. Down to Sirius's lower left is the Cleaver's other end including its short handle, formed by the triangle of Adhara, Wezen, and Aludra. The Cleaver is chopping toward the lower right.

■Just under Canis Major's tail, or the Meat Cleaver's handle, are two big, loose open clusters for binoculars: Collinder 140 and 132. They're about 1° and 1½° across, respectively, and they stand out only rather subtly from the starry background.But they're both real! Cr 140 is about 1,300 light-years away, and Cr 132 is a few hundred light-years farther. Piece them out using the chart in Matt Wedel's Binocular Highlight column in the March Sky & Telescope, page 43.

■ The waning gibbous Moon rises in the early hours of Wednesday morning, among the stars of upper Scorpius. Best view: just before the first glimmer of dawn Wednesday, about 1½ to 1¾ hours before sunrise. The Moon is due south then, hanging just 2° above the orange-red supergiant Antares (for North America).

WEDNESDAY, MARCH 23

■Pollux and Castor in Gemini pass nearly overhead around nightfall this week if you live in the world's mid-northern latitudes. They go smack overhead as seen from near latitude 30° north: Austin, Houston, and the US Gulf Coast, as well as northernmost Africa, Tibet, and Shanghai.

The "twin" heads of the Gemini figures are fraternal twins at best. Pollux is visibly brighter than Castor and pale orange to Castor's white. And as for their physical nature, they're not even the same species.

Pollux is a single orange giant. Castor is a binary pair of two much smaller, hotter, white main-sequence stars, a fine double in amateur telescopes. If Pollux were a basketball, Castor A and B would be a tennis ball and baseball about a half mile apart.

Moreover, each Castor star is closely orbited by an unseen red dwarf — a marble in our scale model just a foot or so from each of the two bright primaries.

And a very distant tight pair of red dwarfs, Castor C, is visible in amateur scopes as a single, 10th-magnitude speck 70 arcseconds south-southeast of the main pair. In our scale model, they would be a pair of marbles about 3 inches apart at least 10 miles from Castor A and B. Space is big.

THURSDAY, MARCH 24

■Last-quarter Moon tonight (exactly last-quarter at 1:37 a.m. Friday morning Eastern Daylight Time). The Moon rises as late as around 3 a.m. local time depending on your location. It's in or near the Teapot of Sagittarius. Catch the Moon and the Teapot higher in the south-southeast just before the beginning of dawn: about 1½ to 1¾ hours before sunrise. By then the Teapot is resting level. It's about the size of your fist at arm's length.

■And then once dawn begins to brighten, catch Venus in the east-southeast forming a nearly isosceles triangle with Saturn lower left of it and Mars to Venus's right. The long side of the triangle, the Saturn-Mars side, is 7° long.

FRIDAY, MARCH 25

■The Big Dipper glitters softly high in the northeast these evenings, standing on its handle. You probably know that the two stars forming the front of the Dipper's bowl (currently on top) are the Pointers; they point to Polaris, currently to their left or lower left.

And, you may know that if you follow the curve of the Dipper's handle out and around by a little more than a Dipper length, you'll arc to Arcturus, now rising in the east.

But did you know that if you follow the Pointers backward the opposite way, you'll land in Leo?

Draw a line diagonally across the Dipper's bowl from where the handle is attached, continue far on, and you go to Gemini.

And look at the two stars forming the open top of the Dipper's bowl. Follow this line past the bowl's lip far across the sky, and you come to Capella.

SATURDAY, MARCH 26

■In early dawn Sunday, the waning Moon shines in the southeast. Look left of it forthe evolving triangle of Venus and fainter Saturn and Mars, as shown below.

Come Monday morning the Moon will be under the three planets, and they will form a new, narrower isosceles triangle with Mars at its narrow point.

This Week's Planet Roundup

Mercury, Jupiter, and Neptune are out of sight in the glare of the Sun.

Venus, magnitude –4.5, is the bright "Morning Star" shining low in the southeast during dawn.

Mars, vastly fainter at magnitude +1.2, continues to hang 4° or 5° to the lower right or right of Venus. Look for Mars early before dawn gets too bright. It currently shines with less than 1% of Venus's light.

Challenge question!Why is Mars so much dimmer than Venus now? Four reasons combine to make it so, despite two other factors working in the opposite direction. Points for each one of the six that you get!Answers at the bottom of this page.

Saturn, magnitude +0.8, glows in early dawn lower left of Venus as shown above. Bring binoculars; Saturn is faint, perhaps appearing almost the equal of Mars considering the greater atmospheric extinction at Saturn's lower altitude.

Its separation from Venus closes from 9° on the morning of March 19th to just 3° on the 26th. Saturn will pass 2° under Venus at their conjunction on the mornings of the 28th and 29th, with Mars still nearby.Saturn is still way too low for decent telescopic viewing, but better times are coming. Saturn will reach opposition in August, and it will remain in fine evening view through September despite its southerly declination in Capricornus this year.

And plan to examine its globe as carefully as you can. Saturn is entering the season in its 30-year "year" when massive white spots — gigantic, continent-size thunderhead clouds — tend to burst upward into view from the deeper, hidden atmospheric layers below. Read all about it in Bob King's Will Saturn Sprout Spots This Observing Season?

Uranus (magnitude 5.8, in Aries) is low in the west right after dark.

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

* Answers to why Mars appears so much fainter than Venus.It's a combination of six factors:

– Mars is a smaller planet. It's only a little more than half the physical diameter of Venus, which means it has only a little more than 1/4 as much surface area to reflect sunlight with.

– Mars is much less reflective (has a lower albedo). The dusty brown Martian surface reflects only 15% of the sunlight that falls on it, while Venus's white cloudtops reflect 65%.

– Mars is currently twice as far from the Sun as Venus is. So the sunlight that strikes its it only one fourth as intense as sunlight at Venus.

– And, Mars is currently 3 times farther from Earth than Venus is.Three times farther makes a thing appear 9 times fainter.

On the other hand, Venus suffers at present from two effects of its phase:

– First, its globe currently appears only about 50% illuminated (at "dichotomy") as seen from Earth, while Mars is 93% sunlit: gibbous close to full.

– Second, the side of Venus we see is therefore more or less grazingly sunlit, while the nearly full face of Mars is hit by light coming almost face-on.