Sun

We’re moving towards the longest nights of the year, and with that come the earliest sunsets. The earliest occurs at 4:14pm on December 8, and although nights will continue to get longer until solstice on the 21st, the Sun will be present for slightly longer times in our afternoons.

Following an 18.5-day stay in the non-zodiacal constellation Ophiuchus, the Sun enters Sagittarius on the 17th. 

Our season of winter begins at 4:20am on the 21st, when the Sun will be at its most southerly point on the ecliptic. We will experience our longest night, totaling 14 hours and 52 minutes.

In what is nothing more than a cosmic coincidence, the Sun crosses the galactic equator at 1:08pm on the 21st, just eight hours past solstice.

Moon

The Moon is new on the 1st, beginning Lunation 1261. 

The 6-day waxing crescent Moon is 4.1° west-southwest of Saturn on the 7th. First Quarter is at 10:27am on the 8th, in Aquarius. Late that evening, the Moon is then 3.2° west of Neptune.

On the morning of the 13th, the waxing gibbous Moon is 3.4° north of Uranus, and later that evening, the 97.2% illuminated Moon is 2.5° east of the Pleiades cluster.

The Full Cold Moon occurs at 4:02am on the 15th. The closest moonrise is at 3:17pm on the 14th. The Moon transits at 11:28pm. Note its position near Elnath (beta Tauri), and that this is the most northerly full Moon of 2024, at 76.0° above the southern horizon. Jupiter is positioned prominently nearby throughout the evening. The Moon sets at 7:43am.

Later on the 15th, the Moon is 3.2° north of the open star cluster M35 in Gemini.

The waning gibbous Moon is 2.5° southwest of Pollux on the morning of the 17th, and has a very close 0.1° conjunction with Mars at 5:00am on the 18th.

Early in the morning of the 20th, the Moon is 1.6° north of Regulus.

Last quarter Moon occurs at 8:28pm on the 22nd, in Virgo.

On the morning of the 24th, the waning crescent Moon will be 3.9° west-northwest of Spica, in Virgo. On the 28th, the 6.4% illuminated crescent will be 1.4° west of Antares, and 0.1° southwest of the globular cluster M4, both in Scorpius. This will also appear 9.5° to the southwest (to the right of) Mercury.

The 2.4% illuminated crescent Moon is 8.2° south-southeast (almost directly below) Mercury on the 29th.

The Moon is new at 5:27pm on the 30th, beginning Lunation 1262.

Mercury

After having a relatively favorable evening appearance during the past month, Mercury is at inferior conjunction on the 5th, and returns to the morning sky for a very favorable apparition.

By the 11th, Mercury rises over an hour before sunrise, and by the 15th, 90 minutes. 

Be sure to observe Mercury with a telescope to see its widening crescent phase. It is 50% illuminated on the 20th, and becomes gibbous thereafter.

Earliest Mercuryrise is at 5:22am on the 21st.

Mercury reaches greatest elongation at 22.0° west of the Sun on the 24th, and it rises a full hour and 45 minutes before sunrise.

Mercury is 8.2° northeast (directly to the left of) Antares on the 28nd.

Venus

Venus is the brightest starlike object in the evening sky this month. Beginning on the 1st, it will be visible for at least three hours after sunset.

One of the most beautiful sights in the sky, a pairing of the crescent Moon with Venus, occurs on the 4th, when the 13.8% illuminated Moon is just 2.6° south of the brilliant planet.

Although it will be a challenge to see, Pluto is 1.0° south of Venus on the 7th. What can be easily seen, however, is the globular cluster M75, which will be 0.8° northwest of Venus on the same evening.

Venus moves into Capricornus on the 6th.

On the 15th, Venus lies on a line extending through the western edge of the Summer Triangle, extending from Vega and through Altair.

Throughout December, Venus continues to gain apparent distance from the Sun, as well as elevation over the southwestern horizon. 

During the second half of the month, Venus is visible after sunset in the south-southwest, almost as far south of west as it can possibly get. It is also getting noticeably higher in elevation with each passing night. Our “Evening Star” is with us for 3.5 hours, and through a telescope it shows a distinctly flattened gibbous phase, as it makes its way towards half-illumination next month.

Venus crosses the line connecting Altair and Fomalhaut on the 26th. Note also how it is getting closer to Saturn.

By the end of December, Venus shines at a brilliant magnitude -4.4.

Mars

Mars rises at 10:00pm in early December and reaches its stationary point on the 7th. As such, the Red Planet makes a nice pairing with M44, the Beehive Cluster, for the next several days, lying within 2° of the cluster, making for a fine sight in binoculars and rich field telescopes.

From the second week of December onward, soon after Mars rises, you can also see four naked-eye planets in the sky simultaneously, starting from Venus low in the southwest, Saturn high in the southwest, Jupiter high in the southeast, and Mars. With Neptune and Uranus also visible telescopically, all of the planets except Mercury are visible at this time.

The Red Planet continues to brighten as we get closer to it, reaching -1.0 on the 21st.

The waning gibbous Moon is close to Mars on the 17th-18th, passing within just 0.1° north of it at 5:00am.

On the 25th it lies along the line extending from Sirius through Procyon, and is still located within a few degrees of the Beehive cluster, M44.

Jupiter

Jupiter reaches opposition on the 7th. The giant planet will be at its largest and brightest during this week, peaking at magnitude -2.8 and its fully-illuminated globe shows a diameter of 48.3 arcseconds. Of all the planets, only Venus can appear larger and brighter in our sky.

It is notable that in the days closest to Jupiter’s opposition, the shadows of the Galilean moons appear very close to the transiting moons themselves, and moons passing behind the planet will pass into shadow rather close to the limb of the planet itself.

Some examples of this can be seen at 10:12pm on the 5th, when a transit of Europa’s shadow begins; at 4:27am on the 7th, when Io and its shadow transit nearly simultaneously; at 6:52pm on the 7th, when Europa emerges from eclipse; and 4:00am on the 8th, when Io emerges from the eclipse. Watch the moons on subsequent nights to see the shadow offset increase.

At 2:18am on the 23rd, Io and Ganymede are both located very close – within 1 arcsecond – to Jupiter’s eastern and southwestern limb, respectively. This is a good test of your telescope’s resolving power, as well as atmospheric seeing conditions. Also, Ganymede’s shadow will be transiting the planet.

An east-to-west alignment of the four Galilean moons in order of their orbital radius occurs after 10:30pm on the 23rd.

Saturn

Saturn reaches its point of eastern quadrature on the 4th, indicating that the season to observe it is about ¾ over. Saturn is now setting before 11:30pm, and is still situated on a line running through the western edge of the Great Square of Pegasus and extending south to Fomalhaut.

From mid-December, Saturn becomes noticeably west of the meridian after twilight, meaning we only have a few hours to view it in the evening sky.

On the 23rd,  note the diamond formed by Venus in the west, Saturn, Diphda (beta Ceti) towards the east, and Fomalhaut in the south. Each segment of the diamond is approximately 26° in length. The diamond becomes distorted as Venus approaches Saturn.

The ring plane angle continues to narrow, closing from 6.2° in early December to 5.3° at the end of the month.

Uranus

Uranus is in western Taurus, and well-placed for observing all night in December. From the Pleiades cluster, locate a pair of 6th magnitude stars, 14 and 13 Tauri, about 4° to the south. Uranus is situated southwest of these stars; its distance ranges from 3.6° to 4.5° through December.

You can also locate it near the magnitude 6.5 star HD 21335. In early December, Uranus is 0.2° southeast of the star, and at the end of the month, it ranges to 0.7° southwest of this star.

Given that Uranus is still at its best viewing of the year, this is a good opportunity to locate its largest moons. With a sufficiently large telescope (12-inches or greater), or photographically, given a tracking mount and relatively long focal length, Titania and Oberon, which shine at magnitudes 13.8 and 14.0, and extend as far as 30 and 40 arcseconds from the planet, respectively, can be challenging, yet rewarding to hunt for.

Neptune

Neptune, in Pisces, is stationary on the 8th, and resumes its prograde (eastward) motion. At magnitude 7.7, it is 4.7° south-southwest of lambda Piscium, the southwesternmost star in the Circlet asterism, and 0.7° east-northeast of 20 Piscium.

Neptune reaches its point of eastern quadrature (90° east of the Sun) on the 18th. You will find it due south at the end of evening twilight,

Neptune sets at 11:15pm.

Minor Planets

Ceres is low in the southwest, shining at magnitude 9.2 in eastern Sagittarius. At the beginning of the month, it is just east of 62 Sagittarii, in the Herman’s Cross asterism.

In early December, Pluto is located less than 5° to the northeast of Ceres, and 1.8° south-southwest of globular cluster M75. At magnitude 14.5, Pluto is becoming difficult to observe as it sinks lower in the southwest after sunset.

Both dwarf planets will be too low in the southwest to be viewed later in the month. 

Asteroid 4 Vesta shines at 8th magnitude and is moving eastward through Virgo. It is within 3.3° south-southwest of Heze (zeta Virginis) from the 14th-16th.

Comet C/2023 A3 Tsuchinshan-ATLAS is best viewed in early December, as bright moonlight will not interfere with observing the fading comet, estimated to be around 8th magnitude. It is moving eastward in Aquila as it gains distance. Its long tail is a distant memory, but the comet’s nucleus and coma may still be observed in binoculars and small telescopes. 

On the 1st, it passes between the open clusters NGC 6755 and NGC 6756. On the 7th, the comet is within 0.5° south of 22 Aquliae, and is now over 1.5 au from Earth. It is about 2.0° south of the planetary nebula NGC 6781 on the 9th, and 2.0° north of delta Aquilae on the 13th-14th.

A lesser known asteroid is visible in our evening sky. 15 Eunomia reaches opposition on the 13th. It is 1.3 au away and shines at magnitude 8.1. It can be found in Auriga, 5.0° north of Elnath, and within the same binocular field of view as the open clusters M36 and M38.

On the 17th-19th, it  can be seen near 16 Aurigae, in the Leaping Minnow asterism, 5.0° north-northwest of Elnath and 3.6° east of Hassaleh (iota Aurigae). 

The Geminids, typically one of the year’s best-performing meteor showers, is active all week, and peaks on the 12th-13th. At peak, this shower, which consists of particles from asteroid 3200 Phaethon and appears to originate from a point 2° east of Castor, can produce 100 meteors per hour, however, the nearly full Moon will interfere with viewing the 2024 show.

The Ursids, a low rate yet fairly reliable shower, peaks on the 21st-22nd. About 5-10 meteors per hour may be seen originating from the circumpolar constellation Ursa Minor. The 56.6% gibbous Moon rises at 11:02pm.