■ The Big Dipper glitters high in the northeast these clear 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. It's three fists at arm's length away.

And, maybe you 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.

And if you follow that arc on by about another Dipper length, you speed to Spica (three fists at arm's length). It's still pretty low until late, but that's where to look for it.

And from Spica, do you know you can continue the curve to Corvus? (by one and a half fists.) Corvus is a compact constellation of four 2nd- and 3rd-magnitude stars, now to Spica's right once they have time to rise.

Back to the Dipper. Follow the Pointers the opposite way from Polaris, and you'll land in Leo (four or five fists).

Draw a line diagonally across the Dipper's bowl from where the handle is attached, continue about four fists farther on, and you go to Gemini (about four fists).

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 cruise to Capella (about five fists).


■ First-quarter Moon (exact at 12:11 a.m. tonight Eastern Daylight Time). Just below the Moon is 2nd-magnitude Beta Tauri, El Nath. Much farther below are Aldebaran, and then lower, the Pleiades. Bright Jupiter shines beneath the Pleiades.

Looking wider, the Moon is nearly midway between Capella, two fists at arm's length to its upper right, and Betelgeuse, two fists to the Moon's lower left.

And by the way, Betelgeuse IS fading again! As you can see with the naked eye, by judging it against Aldebaran and Procyon. See Bob King's Is Betelgeuse Fading Again? with a chart showing comparison-star magnitudes.


■ The Moon, a day past first quarter, is approaching Castor and Pollux high overhead, as shown below.

Moon passing Castor and Pollux, March 17-19, 2024
The waxing gibbous Moon passes under the heads of Gemini. They're over Procyon at nightfall.

■ Look for Arcturus, the Spring Star, 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; they appear single to the unaided eye).

But for northerners who can never see Canopus or Alpha Cen, Arcturus is outshone by Sirius alone, which now glitters in the south-southwest after dark. However, Vega and Capella are very close on the heels of Arcturus brightness-wise.


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

The "twin" heads of the Gemini figures are fraternal twins at best. Pollux is visibly brighter than Castor, and it's pale orange compared 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. A scale model: If Pollux were a basketball, Castor A and B would be a tennis ball and a baseball about a half mile apart from each other.

Moreover, Castor A and B are each closely orbited by an unseen red dwarf — a dim marble in our scale model just a foot or so from the tennis ball and the baseball.

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.


■ This is the time of year when Orion declines in the southwest after dark, with his Belt turning 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 9:10 this evening (daylight-saving time). . . more or less.

■ The equinox comes at 11:06 p.m. EDT, when the Sun crosses the equator heading north. Astronomical spring begins in the Northern Hemisphere, fall in the Southern Hemisphere. And no, eggs don't balance any better on equinox day than they usually do! Hoaxers just laugh at people who believe them.


■ The waxing gibbous Moon shines high in the southeast after dark. It forms the long end of a long isosceles triangle (two sides equal) with Regulus and slightly fainter Algieba (Gamma Leonis).


■ Now the Moon forms a much smaller and flatter triangle with Regulus and Algieba, still nearly isosceles.


■ Arcturus, the "Spring Star," now rises above the east-northeast horizon just around the time the stars come out. How soon can you spot it? Once Arcturus is nicely up, look for the narrow Kite asterism of Boötes extending two fists to its left. The left end of the Kite is bent slightly up. How much of it can you pick out through the moonlight?


■ The signature fall-and-winter constellation Cassiopeia retreats down after dark. Look for it in the north-northwest. It's standing roughly on end.

For skywatchers at mid-northern latitudes and farther north, Cassiopeia is circumpolar, never going away completely. By 1 or 2 a.m. daylight-saving time it will be at its lowest in the north, lying like a not quite horizontal W.


■ If you haven't spotted Mercury yet this season, look for it lower right of Jupiter as twilight fades, as shown below. Jupiter is magnitude –2.1. Mercury this evening is magnitude –0.1, meaning one sixth as bright.1 And that's not counting the extra atmospheric extinction that affects low objects.

Jupiter and Mercury in the western twilight, March 24, 2024
Jupiter and Mercury in the western twilight are currently 23° apart, or about two fists at arm's length. They remain at nearly this separation for several days before and after.

This scene is drawn for a skywatcher at latitude 40° north. Farther south of there, Mercury will be more nearly under Jupiter. North of there, Mercury will be farther to the right. Since we're looking due west, the difference in their tilt will match your difference in latitude from 40°.

■ Full Moon tonight (exactly full at 3:oo a.m. EDT Monday morning the 25th). The Moon rises due east around sunset. By 9 p.m. or so, once the Moon is shining high in the southeast, look lower left of it by two fists at arm's length for Spica.

About three fists left of the Moon shines brighter Arcturus.

This Week's Planet Roundup

Mercury is very low due west in evening twilight. The best time to look might be about 45 minutes after sunset, depending on the clarity of your air. Mercury is very far below Jupiter and a somewhat to the right. It's brighter than it often is, but watch it fade this week from magnitude –1 to 0.

Venus, magnitude –3.8, is disappearing way down into the dawn. You can try with binoculars barely above your east-southeast horizon 15 or 20 minutes before sunrise.

Mars, magnitude +1.3, is also deep in the sunrise, not as low as Venus but only a hundredth as bright.1

Jupiter, magnitude –2.1 in Aries, is the bright white "star" in the west in twilight, and lower later. In a telescope Jupiter has shrunk to only 35 arcseconds wide, and the seeing has also been getting worse as Jupiter moves lower every week. It's almost as distant and small as we ever see it.

Jupiter with Red Spot, March 5, 2024
Jupiter shrinking into the distance, imaged on March 5th by Christopher Go. South here is up. "Conditions were very unstable today because I had to image during twilight," he writes. "Seeing was bad because of the hot roof of my house. The Great Red Spot has become very round now rather than an oval shaped spot!"

We've adjusted the contrast of the image to approximate Jupiter's visual appearance. To get a better idea of Jupiter as seen in a telescope at high power, stand well back from your screen and maybe squint a bit.

Saturn is out of sight very deep in the sunrise.

Uranus, magnitude 5.7 in Aries, is 5° above Jupiter. Right after dark, use the finder charts for it in last November's Sky & Telescope, pages 48-49.

Neptune is hidden in conjunction with 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 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.

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.

Pocket Sky Atlas cover, Jumbo edition
The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6, and hundreds of telescopic galaxies, star clusters, and nebulae among them. Shown here is the Jumbo Edition, which is in hard covers and enlarged for easier reading outdoors by red flashlight. Sample charts. More about the current editions.

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 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 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 computerized telescopes replace charts? Not for beginners I don't think, especially not 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 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 the drives can be disengaged so you can swing it around and point it readily by hand if you want, rather than only slowly by the electric motors.

However, finding faint telescopic objects the old-fashioned way with charts isn't simple either. Learn the 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."
             John Adams, 1770

1. Okay, how do you convert a magnitude difference into the brightness ratio? Here we go.

In 1856, astronomers formalized the stellar magnitude system so that 5 magnitudes is a brightness ratio of exactly 100 to 1. So, one magnitude is a brightness difference of the fifth root of 100. Which is 2.512 for all practical purposes.

So here's the formula to use: If Δm is the magnitude difference, then

brightness difference = 2.512Δm

…which is just a few taps on your scientific calculator.


Image of misha17


March 15, 2024 at 1:49 pm

On the same page as Bob King's article about Betelgeuse, is an article about the penumbral lunar eclipse that will occur late Sun Mar 24th / early Mon Mar 25th

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March 18, 2024 at 10:17 pm

When there is a halo around the Sun, you may notice 2 bright spots on either side of the Sun. They are called "sun dogs" are are positioned parallel to the horizon. Around sunset or sunrise they lie north and south of the Sun.

By coincidence a halo's radius is 22 degrees, and Sun's solstice points are 23 degrees north and south of the Celestial Equator.

Because of this, around the time of the equinoces - when the Sun lies near the CE - the sun dogs at sunrise or sunset will be close to the same places where the sun rises or sets at the solstices.

For the next couple of weeks sun dogs seen near sunset will be the ghost of the Sun of Christmas Past, and the spirit of the Sun of Summer yet to come.

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mary beth

March 18, 2024 at 11:19 pm

Very interesting, thank you!

So the sun dogs at the autumnal equinox will be the ghosts Sun of Summer Past and Christmas Coming I suppose.

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March 19, 2024 at 2:16 pm

That's the idea, but I borrowed the "Past/Yet to come" concept from Dickens' "A Chistmas Carol", although Scrooge also adressed it as, "Ghost of the Future"

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March 21, 2024 at 8:56 am

Early morning skies at my location this morning were lovely near 0530 EDT. Scorpius easy to see and Antares in south. I am tempted to break out my telescope and enjoy some early spring views of M4 globular cluster and other targets in Scorpius. This morning temperature 0C here with NW winds 7 knots or so.

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mary beth

March 21, 2024 at 11:31 am

Hope you get set up soon, wishing for good weather for you!

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