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EI2GYB > ASTRO 17.03.23 21:21l 146 Lines 10289 Bytes #999 (0) @ WW
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Subj: This Week's Sky at a Glance, March 17 - 26
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This Week's Sky at a Glance, March 17 - 26
By: Alan MacRobert March 17, 2023
FRIDAY, MARCH 17
Ý Spring arrives this week (the equinox is on March 20th), so Orion stands upright high in the south-southwest as the stars come out. He's just starting his long spring tilt and departure down toward the west - as the hours pass in the evening, and as the weeks pass with the advancing season.
Ý By nightfall this week, the Big Dipper is high in the northeast and beginning to tip left. Look left of its center, by about three fists at arm's length, for Polaris in the dim Little Dipper. Other than Polaris, all you may see of the Little Dipper through light pollution are the two stars forming the outer edge of its bowl: Kochab (similar to Polaris in brightness) and below it, fainter Pherkad. Find these two "Guardians of the Pole" to Polaris's lower right by about a fist and a half at arm's length. They're 2nd and 3rd magnitude.
Now is the time of year when the Guardians line up exactly vertically around the end of twilight.
SATURDAY, MARCH 18
Ý On the traditional divide between the "winter" and "spring" sky is the dim constellation Cancer, now very high toward the south. It's between Gemini to its west and Leo to its east.
Cancer holds something unique: the Beehive Star Cluster, M44, in its middle. The Beehive shows dimly to the naked eye if you have little or no light pollution. With binoculars it's easy even under mediocre conditions. Look for it a little less than halfway from Pollux in Gemini to Regulus in Leo. And see Fred Schaaf's "Sighting the Beehive" in the March Sky & Telescope, page 45.
SUNDAY, MARCH 19
Ý The "twin" heads of the Gemini figures are fraternal twins at best. Pollux is visibly brighter than Castor and pale orange. 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 smaller, hotter, white main-sequence stars, a fine double in amateur telescopes. If Pollux were a basketball, white Castor A and B would be a brilliant tennis ball and baseball about a half mile apart.
Moreover, each Castor star is closely orbited by an unseen red dwarf - dim marbles in our scale model, only 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 only about 3 inches apart at least 10 miles from Castor A and B.
Double stars come in a vast range of true separations - from nearly a light-year apart down to actually touching.
MONDAY, MARCH 20
Ý Spring begins today (in the Northern Hemisphere) at 5:42 p.m. EDT (21:42 UT). That's the moment of the equinox, when the center of the Sun crosses the equator (both Earth's equator and the celestial equator) heading north for the warm months.
The equinox is when the Sun is up for 12 hours (very nearly) and down for 12 hours (very nearly). It's also when the Sun rises and sets due east and due west (very nearly).
Okay, why "very nearly"? If you've ever looked up your sunrise and sunset times on the equinox day, you've seen that they are not 12 hours apart. Because...
1) Atmospheric refraction at the horizon bends the apparent position of the Sun up by about half a degree. This slightly advances the time of sunrise and delays sunset, and thus also shifts the places where these happen on the horizon slightly north.
2) Astronomers consider the Sun's position to be the position of the Sun's center, but "sunrise" and "sunset" mean the visibility of the Sun's top edge, which is ¬ø higher. This adds slightly to the time shifts and north shifts.
TUESDAY, MARCH 21
Ý The first-discovered asteroid, 1 Ceres, is at opposition in Coma Berenices. It glimmers at a binocular-friendly magnitude 6.9, and it's nice and high by late evening. See the article and finder charts in the March Sky & Telescope, page 48.
And on the night of March 26-27, Ceres crosses the face of the 10th-magnitude spiral galaxy M100. See March 26 below.
Ý New Moon (exact at 1:23 p.m. EDT).
WEDNESDAY, MARCH 22
Ý Look very low in the west in twilight for the thin crescent Moon paired with departing Jupiter, as shown below. Binoculars help.
THURSDAY, MARCH 23
Ý The Moon shines below Venus this evening, as shown above.
Ý After dark, Sirius sparkles brightly in the south-southwest. Lower left of Sirius, by about one fist, is the triangle of Adhara, Wezen, and Aludra, from right to left. They form Canis Major's hind foot, rear end, and tail, respectively. Or, alternatively, the lower end and handle of the Meat Cleaver.
Just left or upper left of them, forming a 3rd- and 4th-magnitude arc, are the three uppermost stars of the constellation Puppis. No, it's not a puppy, despite following right behind the Big Dog. It's the Poop Deck (stern) of the giant ancient constellation Argo Navis, the ship of Jason and the Argonauts. These three are the only stars of Argo you can see fairly easily from mid-northern latitudes.
Just 1.5ø upper right of the middle of the three, binoculars on a dark night will show the 6th-magnitude open cluster M93. It's elongated northeast-southwest.
FRIDAY, MARCH 24
Ý Now the thickening crescent Moon shines above Venus during and after twilight.
Ý 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'll 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 crash into Capella.
SATURDAY, MARCH 25
Ý Higher above Venus now, the waxing crescent Moon shines just a couple degrees from the Pleiades.
Ý The signature fall-and-winter constellation Cassiopeia retreats down after dark. Look for it fairly low in the north-northwest. It's standing roughly on end.
But for skywatchers at mid-northern latitudes Cassiopeia is circumpolar, never going away completely. By 1 or 2 a.m. daylight-saving time it's at its lowest due north, lying not quite horizontally.
SUNDAY, MARCH 26
This evening Ceres, 7th magnitude, crosses the northernmost spiral arm of the 10th-magnitude galaxy M100 in Coma Berenices, as shown below. Unlike how galaxies look in images, their dense inner regions are generally much more visible in telescopes than their low-surface-brightness disc and arms.
This Week's Planet Roundup
Mercury is hidden in the glow of sunset. But it's heading up for a good evening apparition in early April.
Venus (magnitude -4.0, in Aries) is the "Evening Star" shining brightly in the west during and after dusk. It finally sets about 2 hours after dark. Telescopically, Venus is a shimmering little gibbous ball 13 arcseconds in diameter and 82% sunlit.
Mars crosses from the horns of Taurus toward the feet of Gemini this week, moving east against the stars. Look for it high in the southwest in early evening, lower in the west later. It's upper left of Venus by about two fists at arm's length.
Mars continues to fade: from magnitude +0.7 to +0.8 this week. It's very similar to Mars-colored Aldebaran (+0.9) some 18ø below it, and Mars-colored Betelgeuse (currently +0.4) about the same distance to Mars's lower left. They form a big, temporary Orange Triangle, equilateral or nearly so.
In a telescope Mars is now a little blob only 7 arcseconds wide, generally too small to show any detail visually. But notice its gibbous shape: 90% sunlit.
Jupiter is heading down deep into the afterglow of sunset. Look for it about two fists lower right of Venus. Binoculars may help, especially later in the week.
Saturn is deep in the dawn glow, very low in the east-southeast. Try binoculars about 40 minutes before sunrise.
Uranus, magnitude 5.8 in Aries, is in the west right after dark above Venus. Uranus finder charts are in last November's Sky & Telescope, page 49.
Neptune is hidden low in the sunrise.
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 sometimes called UTC, GMT, or Z time.
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