Special to CosmicTribune.com, December 16, 2024

Excerpts from weekly Sky&Telescope report.

SUNDAY, DECEMBER 15

■ The Summer Triangle is finally sinking low in the west, and Altair is the first of its stars to go (for mid-northern skywatchers).

Start by spotting bright Vega, magnitude zero, the brightest star in the northwest right after dark. The brightest one above Vega is Deneb. The Triangle’s third star, Altair, is farther to Vega’s left or lower left. How late into the night, and into the advancing season, can you keep Altair in view?

■ Algol overhead should be at its minimum brightness for about two hours centered on 9:58 p.m. EST.

■ The Moon forms a line with Jupiter and Aldebaran to its right, as shown. For most of the night in most places, the line will gently curve. But the Moon will make an exact straight line with Jupiter and Aldebaran in late evening for most of eastern and central North America, and earlier in the evening for most of western North America.

MONDAY, DECEMBER 16

■ The Moon, now waning gibbous, rises around the end of twilight and is reasonably high by an hour later. It forms a triangle with Pollux and Castor about 5° to its right. Watch the triangle change shape through the night as the Moon moves eastward along its orbit.

TUESDAY, DECEMBER 17

■ Late this evening, the bright white waning Moon shines in the east just above fire-colored Mars. They rise about an hour after dark and are nicely up an hour later. These are currently the closest major objects in the solar system, 1.3 light-seconds and 5.9 light-minutes from your eyes.

■ Jupiter’s inner moon Io starts crossing Jupiter’s face at 7:05 p.m. EST, followed by its tiny black shadow (much more visible) 15 minutes later. They enter from Jupiter’s celestial-east side. They depart from Jupiter’s other side at 9:16 and 9:33 p.m. EST, respectively. Io is Jupiter’s fastest-moving major satellite.

Meanwhile, Jupiter’s (no longer so) Great Red Spot should cross Jupiter’s central meridian around 7:21 p.m. EST.

WEDNESDAY, DECEMBER 18

■ Have you ever watched a Sirius-rise? Find an open view right down to the east-southeast horizon, and watch for Sirius to come up about two fists at arm’s length below Orion’s vertical Belt. Sirius rises around 8 p.m. now, depending on your location.

About 15 minutes before Sirius-rise, a lesser star comes up barely to the right of where Sirius will appear. This is Beta Canis Majoris or Mirzam. Its name means “the Announcer,” and what Mirzam announces is Sirius. You’re not likely to mistake them; the second-magnitude Announcer is only a twentieth as bright as the King of Stars soon to make its grand entry.

When a star is very low it tends to twinkle slowly, and often in vivid colors. Sirius is bright enough to show these effects well, especially in binoculars.

THURSDAY, DECEMBER 19

■ Now the late-night waning Moon pairs up with Regulus. Look east after about 10 p.m.

■ Last year, this year, and next, we see the orbital plane of Jupiter’s moons tilted a bit to our line of sight. So from our viewpoint, the moons can deviate from their usual nearly straight line east and west of the planet.

For instance, tonight around midnight EST, when Jupiter is high for all of North America, Ganymede, Callisto, and Europa form a surprising isoceles triangle east of the planet, while Io looks on from Jupiter’s western side as shown below.

For four other odd configurations this winter, see Joe Rao’s More Unusual Jovian Satellite Lineups.

A simulation of tonight’s Jovian satellite triangle at midnight EST, courtesy Joe Rao. North is up.

FRIDAY, DECEMBER 20

■ Sirius and Procyon in the balance. Sirius, the Dog Star, sparkles low in the east-southeast after dinnertime. Procyon, the Little Dog Star, shines to its left in the east about two fist-widths at arm’s length from Sirius.

But directly left? That depends. If you live around latitude 30° (Tijuana, Austin, New Orleans, Jacksonville), the two canine stars will be at the same height above your horizon soon after they rise. If you’re north of that latitude, Procyon will be higher. If you’re south of there, Sirius will be the higher one. Your eastern horizon tilts differently with respect to the stars depending on your latitude.

Saturn continues to close in toward Venus during and after twilight. The triangle they form with 1st-magnitude Fomalhaut with be an exact right triangle on December 22nd.

SATURDAY, DECEMBER 21

■ This is the shortest day of the year in the Northern Hemisphere; the longest day in the Southern Hemisphere. The solstice occurs at 4:20 a.m. EST, when the Sun reaches its farthest southern declination and begins its six-month return northward.

SUNDAY, DECEMBER 22

■ How well do you really know the Orion’s Belt region in binoculars? Can you piece out Orion’s S, which begins and ends with the belt’s two westernmost stars?

The Belt marks the rough center portion of the Orion OB1 Association, called OB1b. Are you aware of Orion OB1a and OB1c on either side of it? Binoculars are all you need.

■ Last-quarter Moon (exact at 5:18 p.m. EST). The Moon rises around midnight, under the hind feet of Leo.

  Dec. 15-22: Solstice on the 22nd, The Sky