Skylights: December 2023

The Sun spends December moving along the most southerly section of the ecliptic, bringing the longest nights to the northern hemisphere. If you’re familiar with the twelve zodiacal constellations, you may be surprised to learn that the Sun lies within a constellation that is not a member of this set, Ophiuchus, the Doctor, through December 18, when it enters into Sagittarius.

On the 1st, the Sun is at the same right ascension as, and 4.4° north of Antares at noon. 

The earliest sunset of the year occurs at 4:14pm EST on the 8th.

Solstice occurs at 10:27pm EST on the 21st, marking the start of winter for the northern hemisphere, and summer in the southern hemisphere. At a declination of -23.4°, the Sun will now appear to move slowly north.

Although it appears to coincide with solstice, the Sun’s crossing of the galactic equator at about 7:00am on the 22nd is owed to the unrelated orientation of our planet and solar system at this point in cosmic history.

Look 3.0° south of the waxing gibbous Moon about an hour after it rises on the 1st to find the open cluster M44, the Beehive, in Cancer. Two nights later, it rises in a position 3.2° north of Regulus, in Leo.

Last quarter Moon occurs on the 5th, in Leo.

When it is going through its waning crescent phase, it passes 2.7° northwest of Spica, in Virgo, on the 8th, then 3.6° south-southwest of brilliant Venus on the 9th.

New Moon occurs just after noon on the 12th, marking the beginning of Lunation 1249.

The Moon is well south of the ecliptic as it passes through its waxing crescent phases, and as such, you’ll notice the position of the young Moon unusually far to the southwest.

On the 17th, the 5-day crescent Moon joins Saturn, 2.5° to its north. First quarter occurs on the 19th.

On its way to full, the waxing gibbous Moon is 5.0° west of Jupiter on the 22nd, and just 2.0° southwest of the Pleiades cluster just before moonset on the morning of the 24th.

The Full Cold Moon occurs at 7:33pm EST on the 26th. It rises in the east-northeast at 3:45pm EST, when the Sun is still above the horizon in the west-southwest. This makes for a photogenic moonrise as the foreground subjects are still in sunlight while the Moon is visible low over the horizon.

The Moon transits the meridian at three minutes before midnight on the 26th-27th at an elevation of 76°, the most northerly Full Moon of 2023. It is notable also that about half of the Moon’s disk lies in the constellation Auriga, with a slightly larger proportion of its lower disk lying within Gemini.

The Moon sets the following morning at 8:07am.

Following 2023’s final full Moon, the waning gibbous passes 2.2° southwest of Pollux in the morning hours of the 28th, then 2.3° north of Regulus on the last morning of the year.

Mercury is an evening planet in December, but remains rather low, and sets no later than 75 minutes after sunset, which occurs on the 7th, four days after its greatest elongation of 21° east of the Sun.

Mercury passes two of the more prominent globular clusters in Sagittarius in December, although you’ll need a clear southwestern horizon and medium-high magnification in a telescope in order to see them through diminishing twilight. On the 7th, M28 is just 0.5° to the right of Mercury, and on the 12th, M22 is 0.2° above the planet, and 0.2° to the left of it the following night.

While watching Mercury pass these objects, even if you are unable to see the clusters, you should notice the planet’s changing phase. On the 8th, it exhibits a quarter Moon phase, just over 7 arcseconds in size. On the 12th and 13th, you’ll see Mercury as a slightly larger crescent.

The nights of the 18th and 19th may be the last time before inferior conjunction on the 22nd that you’ll be able to see Mercury 30 minutes after sunset. If you’re able to see it, you’ll notice that it has become a very thin crescent, nearly 10 arcseconds across.

During the last week of December, Mercury returns to the morning sky, and quickly gains elevation needed to easily track it going through its widening crescent phases. By the 28th, Mercury is rising at least an hour before the Sun, and this morning may be your first opportunity to see Mars since it was in conjunction last month, as the Red Planet lies 3.7° south of Mercury, although it is still very low and dimly shining through twilight at a distance of 2.434 AU.

At the beginning of December, Venus rises at 3:11am EST, nearly four hours before the Sun, and lies just a few degrees to the northeast of Spica, in Virgo. At just under 1 AU from Earth, Venus is moving away from us and is steadily closing its apparent elongation angle from the Sun. A view through a telescope reveals a 68% gibbous phase just over 17 arcseconds large. 

The 26-day crescent Moon joins Venus on the 9th, rising nearly simultaneously with the brilliant planet, the two objects being just 2.7° apart over the eastern horizon.

As the month progresses, Venus rises about two minutes later each morning, crossing from Virgo into Libra on the 11th.

At the end of the month, Venus still rises three hours before sunrise, and shows a 78% gibbous phase that will have shrunk to 14 arcseconds as the planet increases its distance to 1.178 AU.

Mars, having passed superior conjunction last month, spends much of December closely trailing the Sun through Ophiuchus. By month’s end, it will have crossed into Virgo, and rises only 50 minutes before sunrise.

December is the best time to take a telescopic tour of our outer solar system. If you haven’t taken the opportunity to view all four outer planets during one night, give it a try sometime this month, as Jupiter, Saturn, Uranus, and Neptune are all high in the sky and easy to view, lying within an 80° arc across the ecliptic from southwest to southeast.

The December night sky is dominated by Jupiter, the king of planets, which is the brightest starlike object visible. Only Venus, which is rising when Jupiter sets, is brighter.

Jupiter remains high in the sky on December evenings. It ends its westward retrograde motion, in Aries, on the 31st.

Keep watching Jupiter’s moons throughout December for some interesting arrangements.

On the 14th, from 5:52pm until 7:22pm EST, Jupiter appears to have only two moons, as Io and Europa are both behind the planet and its shadow. A similar arrangement occurs on the 21st, from 7:42pm until 10:00pm EST, and on the 28th, from 9:30pm to 10:13pm. However, on this occasion, Europa goes into Jupiter’s shadow within four minutes of emerging behind the planet’s globe. Jupiter then retains its “two moon” appearance for another two hours and 22 minutes. Watch closely beginning at 12:39am on the 29th, as Europa and Io both emerge from shadow within nine minutes of each other.

In the early evening of the 24th, all four Galilean moons are visible, in close arrangement and in order of orbital distance, extending to the west of Jupiter.

On the 26th, the moons are arranged in close pairs on both sides of Jupiter, from west to east, Io, Ganymede, Europa, and Callisto.

Saturn is still in our evening sky during December, but it is now setting by 10:30pm in early December, and just 8:45pm by the end of the month.

The 5.2-day crescent Moon is just 2.5° south of Saturn on on the 17th.

Uranus, residing in Aries, is high in the southeast and ready for viewing immediately following evening twilight.

By now you’re probably familiar with finding Uranus by starting with Botein (delta Arietis), and the trapezoidal asterism it is the western point of, and noticing the blue-green planet about 2.5° to the south-southwest.

Since Uranus is on the more northerly portion of the ecliptic, it culminates quite high in the sky when it crosses the meridian, meaning that its light passes through a minimum amount of our atmosphere to reach us. And with the Moon out of view during the first half of the month, this is an ideal time to try spotting Uranus without binoculars or a telescope.

Neptune is ideally positioned high in the south during the early evening hours through the first half of December, and can be easily observed without interference from bright moonlight. 

Located 5° south of lambda Piscium, in the Circlet asterism of Pisces, Neptune spends the first ten days of December in Aquarius, during which it ends its retrograde motion, becoming stationary on the 6th, and resuming its eastward, prograde motion. On the 16th, Neptune reaches its point of eastern quadrature.

Ceres passed superior conjunction in November, and has entered the morning sky, though it is best to wait until the latter third of December, when it rises before twilight begins. Looking at the magnitude 8.9 dwarf planet through a thicker slice of Earth’s atmosphere will require a telescope. On the 30th, Ceres lies in Ophiuchus, 8° northeast of Antares.

Ceres is 3.633 AU away.

Pluto remains visible low in the southwest in early December, but by the 21th, it sets before the end of astronomical twilight. If you’re up for the challenge of locating it, it can be found 1.3° south-southwest of the globular cluster M75, in Sagittarius.

Asteroid 2 Pallas, at magnitude 9.6, is moving eastward through Virgo, crossing into Serpens on the 27th. On the 31st, when the large asteroid is 3.075 from Earth, it passes just 1.1° south of globular cluster M5.

The brightest asteroid, 4 Vesta, reaches opposition in Orion on the 21st. At a distance of 1.583 AU, Vesta shines at magnitude 6.4, placing it within reach of 50mm binoculars. Vesta is fairly easy to locate: It passes just a few arcminutes south of NGC 2175, a cluster in nebulosity. It then passes 8’ north of χ2 Orionis on the 14th, and 20’ north of χ1 Orionis on the 23rd.

Asteroid 9 Metis reaches opposition at a distance of 2.101 AU in Gemini on December 22. At magnitude 8.4, it is within the same binocular field of view, about 3° to the north of open cluster M35 throughout most of December.

The final month of the year gives us the longest nights and earliest sunsets. One of the benefits of this is that one can be under the stars at 6pm, spend a few hours observing, and still be in bed well before midnight.

The pointer stars of the Big Dipper are at their lower culmination in the north during early December evenings, a conspicuous indicator that we’re about to roll into a new year as the northern sky’s most familiar asterism continues to rotate into position east of the meridian.

The Summer Triangle, which has hovered high overhead for the past few months, is now moving lower in the west, with Altair, and then Vega, dropping out of view during the mid-evening hours, leaving only Deneb, with its host constellation Cygnus, the swan, pointed nose-down, over the northwestern horizon. 

Still in the early evening, the Great Square of Pegasus is high in the south, and Cassiopeia nearly overhead. The Great Andromeda Galaxy, the most distant object that can be viewed without optical aid at 2.5 million light years, crosses the zenith at about 7:30pm in early December, before bright moonlight washes out its distant glow.

In the east, the entire Winter Hexagon is within view by 9:00pm, giving indication that the change of season is upon us. 

Another way to tell that we’re about to enter a new year is the transit of Sirius at about midnight at the end of December.

Lastly, one of the year’s most reliable meteor showers, the Geminids, appears mid-month. Conditions for this shower are favorable for several reasons: The radiant, the point from which the meteors appear to originate near Castor, is above the horizon soon after twilight ends. Also, because this shower is. Also, the peak nights of December 13 and 14, host a young waxing crescent Moon low in the southwest, which will not be a factor in brightening the sky.

The Geminids originate from 3200 Phaethon, a roughly 6-kilometer asteroid whose perihelion brings it to within 0.140 AU of the Sun, during which it actively sheds meteoric material. Phaethon completes an orbit every 1.43 years, guaranteeing a frequently replenished supply of Geminids.