The sky, March 18-24: The rise of ‘spring star’ Arcturus

Special to, March 19, 2024

Excerpts from weekly Sky&Telescope report.


■ 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).

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.

■ 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).


■ 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.

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