The Sky This Week from December 6 to 13: Jupiter reaches opposition

Friday, December 6
Asteroid 15 Eunomia is making its way through central Auriga this month, passing numerous deep-sky objects along the way. 

By 9 P.M. local time this evening, the asteroid is nearly 50° high in the eastern sky. It’s skimming close to 5th-magnitude Phi (φ) Aurigae, just 1° east of the star tonight. Now magnitude 8.2, Eunomia is easily within reach of binoculars or any small scope, and will fit neatly within the same field of view as the star in either. 

As a bonus, the 6th-magnitude open cluster M36 is just 1° east of Eunomia, or 2° east of Phi Aur, with the main-belt world sandwiched between them. Spanning some 12’, M36 is one of Auriga’s three bright open clusters, along with 6th-magnitude M37 (3.7° to its southeast) and 7th-magnitude M38 (2.3° to its northwest). Together, the three form a rough line running northwest to southeast, with M36 in the middle. 

Additionally, just 0.3° south-southwest of Eunomia this evening is NGC 1931, sometimes referred to as a miniature Orion Nebula. It is composed of a reflection nebula some 4’ across surrounding a small, 10th-magnitude open cluster containing five bright stars.

Sunrise: 7:08 A.M.
Sunset: 4:34 P.M.
Moonrise: 11:37 A.M.
Moonset: 10:00 P.M. 
Moon Phase: Waxing crescent (30%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 12 P.M. local time from the same location.

Saturday, December 7
Mars stands stationary against the stars of Cancer at 4 P.M. EST today, pausing its motion some 2° northwest of M44, the Beehive Cluster. After today, the Red Planet will begin moving retrograde, or westward, against the background stars. 

But the real showstopper tonight is Jupiter, which reaches its best Northern Hemisphere opposition in a decade this afternoon at 4 P.M. EST. Rising roughly at sunset and visible until around sunrise, you’ll find the magnitude –2.8 planet 50° high in the east by 9 P.M. local time and more than 70° above the southern horizon by midnight, when it reaches its highest point. Once Venus sets about three hours after the Sun, Jupiter is the brightest point of light in the sky, outshining even the night sky’s brightest star, magnitude –1.4 Sirius. 

The giant planet is located in central Taurus the Bull, to the upper left of the constellation’s brightest star, the magnitude 0.9 red giant Aldebaran. Through binoculars or a telescope, Jupiter’s 48”-wide disk shows off incredible detail, particularly its alternating light and dark horizontal cloud bands. Around 9 P.M. EST, the planet’s huge, prominent Great Red Spot is located roughly in the middle of the disk, moving from east to west as the planet rotates. 

Also visible this evening are Jupiter’s four Galilean moons. The evening starts with only three moons visible, as Europa is passing behind Jupiter as the planet rises for the eastern half of the U.S. At that time, Ganymede stands alone to Jupiter’s east, with Io (closest) and Callisto (farthest) to its west. Europa appears just off the planet’s northeastern limb shortly before 7 P.M. EST, then continues to move away from the planet to the east, while Io — on the other side of the planet — approaches the northwestern limb over the next several hours. 

Less than 10 minutes before 2 A.M. EST (on the 8th for the eastern two-thirds of the U.S.), Io disappears in an occultation that takes over two hours; it reappears at the northeastern limb just after 4 A.M. EST, now on the 8th for the entire continental U.S. With Jupiter’s long, dark shadow stretched almost exactly behind the planet from our point of view, the moons both disappear and reappear right at the limb at opposition.

Sunrise: 7:09 A.M.
Sunset: 4:34 P.M.
Moonrise: 12:04 P.M.
Moonset: 11:12 P.M. 
Moon Phase: Waxing crescent (40%)

Sunday, December 8
The Moon passes 0.3° north of Saturn at 4 A.M. EST. You can catch the pair in the evening sky tonight, after our satellite has reached First Quarter phase at 10:27 A.M. EST this morning. 

An hour after sunset, Saturn and the Moon both hang in Aquarius in the south, now some 8.5° apart. Saturn glows at 1st magnitude, about 2° to the lower left of fainter, 4th-magnitude Phi Aquarii. The planet outshines all the stars around it, and even those farther away — the closest luminary of similar magnitude is magnitude 1.2 Fomalhaut in Piscis Austrinus, hanging below Saturn and much closer to the horizon. 

Through a telescope, Saturn’s stunning rings surround its 17”-wide disk. Its brightest moon, 8th-magnitude Titan, stands some 2.3’ west of the planet, while several 10th-magnitude moons cluster closer to the planet and rings. But there’s also a “stranger” wandering in from the northeast: 11th-magnitude Iapetus, heading for inferior conjunction north of the planet in just two days. We’ll come back then for another look. 

Just over the border in Pisces is distant Neptune, its 8th-magnitude glow visible only with binoculars or a telescope. You can find it 4.7° below (south-southeast of) magnitude 4.5 Lambda (λ) Piscium, one of the stars in Pisces’ Circlet asterism. The ice giant planet reached its stationary point at 6 A.M. EST this morning.

Sunrise: 7:10 A.M.
Sunset: 4:34 P.M.
Moonrise: 12:28 P.M.
Moonset: —
Moon Phase: Waxing gibbous (51%)

Monday, December 9
Moving along from Aquarius into Pisces, the Moon passes 0.8° north of Neptune at 4 A.M. EST. By evening, when the region is visible, our satellite has moved to a position some 9° east of the planet. 

Our satellite now hangs above the constellation Cetus the Whale, home to the famous variable star Mira. Located in the center of the constellation (about 12.9° southwest of magnitude 2.5 Menkar), Mira’s brightness varies between visible and invisible to the naked eye, reaching as bright as 2nd or 3rd magnitude and dropping as faint as 10th. Its period is just under a year — some 330 days — and right now, observers are reporting it around 9th magnitude. That means when you look at this region with the naked eye, you won’t see the star! But under magnification, the bright pinpoint of Mira’s light will appear. 

Mira is a red giant that undergoes periodic changes in temperature, which we see here on Earth as changes in brightness. It is the archetype Mira variable, a class of red giant stars that exhibit similar changes in temperature and brightness on roughly the same timescale. These aging stars are fusing hydrogen not in their cores, but in a shell around their core, as that core has already been depleted of hydrogen to convert into helium for energy.

Sunrise: 7:11 A.M.
Sunset: 4:34 P.M.
Moonrise: 12:52 P.M.
Moonset: 12:23 A.M.
Moon Phase: Waxing gibbous (63%)

Tuesday, December 10
Iapetus reaches inferior conjunction, standing 45” north of the ringed planet tonight. You’ll find Saturn in the same spot as earlier this week, the brightest point of light in the constellation Aquarius, high in the south after sunset.

Orbiting every 79.3 days, Iapetus is tidally locked, meaning that it rotates in the same amount of time as it orbits and the same side always faces Saturn. From Earth, we see the moon appear to brighten and darken as it orbits, thanks to its strange terrain: One side of the moon is much brighter than the other. When Iapetus is at western elongation, it appears brightest to us, around 10th magnitude. At eastern elongation, it is faintest at 12th magnitude. In the middle, at superior and inferior conjunction, its magnitude, too, is in the middle of these two extremes.

Tonight, at inferior conjunction, Iapetus is roughly magnitude 11. Much brighter is 8th-magnitude Titan, which now stands far west of Saturn, while 10th-magnitude Rhea, Tethys, and Dione sit closer to the gas giant. Rhea is east of the planet, while Tethys and Dione are to the west. Several fainter moons are also nearby, including Mimas and Enceladus. 

Just before 11 P.M. EST, as Saturn is quite low in the Eastern time zone, Tethys and then Mimas disappear in quick succession behind the planet’s northwestern limb. By the time both moons reappear, well over two hours later after passing behind the planet and through its dark shadow, Saturn is setting or has already disappeared even for those on the West Coast.

Sunrise: 7:12 A.M.
Sunset: 4:35 P.M.
Moonrise: 1:17 P.M.
Moonset: 1:35 A.M.
Moon Phase: Waxing gibbous (73%)

Wednesday, December 11
With a brightening Moon in the evening sky, let’s turn to the morning hours for our target today. The bright planet Mars stands near the gorgeous Beehive Cluster in the predawn sky, some 60° high in the southwest two hours before sunrise. 

Both are visible to the naked eye: Mars shines at magnitude –0.7, easy to spot amid the fainter stars of Cancer. The Beehive, also cataloged as M44, sits just to Mars’ left, a little over 2° away. That puts both within a single field of view of binoculars or a telescope’s finder scope. To their lower right are the brightest stars in Gemini the Twins: Castor on the right and Pollux on the left. 

M44 shines at magnitude 3.7 and appears to the naked eye as a fuzzy glow or — if your eyesight is particularly keen — perhaps a few scattered points of light. It has been known since antiquity and is sometimes also called Praesepe or the Manger. Dozens of stars are visible under low magnification, while larger telescopes will show hundreds. It stretches nearly 100’ across, nearly as large as the Pleiades in Taurus. 

Although Mars sometimes passes through this cluster, it won’t do so this month — you might recall the Red Planet reached its stationary point earlier this week, so it is now moving away from the Beehive, rather than toward it.

Sunrise: 7:12 A.M.
Sunset: 4:35 P.M.
Moonrise: 1:44 P.M.
Moonset: 2:49 A.M.
Moon Phase: Waxing gibbous (83%)

Thursday, December 12
The Moon reaches perigee, the closest point to Earth in its orbit, at 8:20 A.M. EST. At that time, our satellite’s distance will be 227,025 miles (365,361 kilometers). This is the first month since September when this point has not lined up with the Full phase — in August, September, October, and November, the Moon was Full near or at perigee, resulting in 2024’s four Super Moons. Now that the cycle is broken, the next Super Moon won’t occur until October next year.

With Jupiter still large and bright in the sky, let’s return to the king of planets to watch the icy moon Europa transit the disk together with its shadow overnight. Jupiter hangs in Taurus, shining brightly between the Bull’s two horns, visible all night and standing high in the south around local midnight. The waxing gibbous Moon is nearby but, although it is bright, it shouldn’t drown out Jupiter’s blazing light.

The transit begins at 12:31 a.m. EST (the 13th in the Eastern time zone only), as Europa slips onto Jupiter’s disk from the southeast. Also visible are Io (alone to the west) and Ganymede and Callisto to the planet’s east; Callisto lies slightly closer to Jupiter than Ganymede. 

Less than 20 minutes later, Europa’s shadow appears behind it, not trailing by much — because Jupiter is just past opposition, we are still viewing it with the Sun nearly directly behind us. The two travel across the cloud tops together for a little over two hours, until Europa exits the disk at the southwestern limb around 3 A.M. EST (now the 13th across the U.S.). Again, its shadow follows, disappearing within 20 minutes.

Sunrise: 7:13 A.M.
Sunset: 4:35 P.M.
Moonrise: 2:15 P.M.
Moonset: 4:06 A.M.
Moon Phase: Waxing gibbous (91%)

Friday, December 13
The Moon passes 4° north of Uranus at 5 A.M. EST, both in Aries the Ram. 

By early morning, the pair is sinking in the west but the bright gibbous Moon still casts its light across the sky, unfortunately affecting the Geminid meteor shower, which peaks today as well. 

Despite the poor conditions, it’s worth trying to catch a few of the brightest fireballs before dawn, as the shower is expected to produce some 150 meteors per hour. Although the apparent rate will be badly diminished, the radiant — the point from which Geminid meteors appear to originate on the sky — is some 80° high around 2 A.M. local time, offering the best chances to spot the at least a few bright shooting stars.

The Geminids’ radiant lies near the bright star Castor in Gemini the Twins, high in the south in the early-morning hours. Because meteors in the shower seem to streak away from this point, the best way to catch shooting stars with the longest bright trains is to look slightly away from this point, some 40° to the left or right of it in the sky. Early morning is also best because this is when your location on Earth is rotating into the stream, producing more meteors.

Next year, the Geminids will be much less affected by moonlight. And to close out this year, you might have better luck observing the smaller Ursid meteor shower later this month, which peaks on the 22nd. Although the Ursids’ rate is much lower, there will be no Moon in the late-evening sky, offering better views on the peak — we’ll highlight that shower later this month if you’re hoping to catch it.

Sunrise: 7:14 A.M.
Sunset: 4:35 P.M.
Moonrise: 2:54 P.M.
Moonset: 5:26 A.M.
Moon Phase: Waxing gibbous (96%)

Sky This Week is brought to you in part by Celestron.