Skylights: December 2022

December 2022  :  Jim Hendrickson

December brings the solstice, and with it, the longest nights of the year. The final month of 2022 gives us a favorable opposition of Mars, which, although not the closest, always brings excitement and enthusiasm for planetary observers, as it returns only once every 26 months.

The Red Planet is at its closest to Earth early on the 1st, at a distance of 0.544 AU. That’s 4.5 light minutes away. Through a telescope, you will see a 17 arcsecond globe with surface features that rotate slightly slower than Earth. About 40 minutes later each night, you will see the same longitude of Mars facing Earth. 

During early December, Mars is just 1° north of open cluster NGC 1746, a large and loose binocular-visible cluster in Taurus. Notice how dramatic Mars’s motion is in our sky, moving about 1/2° westward each night. By the end of the month, it will be located just north of Davis’s Dog, a large and bright, yet often overlooked asterism located close to both the more prominent Hyades and Pleiades clusters. The asterism was first noted by the late John Davis, a prolific deep-sky observer from western Massachusetts, who enjoyed attending many of the local conventions, including AstroAssembly.

The Full Cold Moon occurs on the 7th. This month’s full Moon is notable for two reasons: The first is that it is the most northerly full Moon of 2022. This is because the full Moon closest to the winter solstice will be near the northernmost region of the ecliptic, opposite the Sun, which is at the southernmost point.

The December full Moon is best experienced late at night from an area located away from artificial lights. Close to midnight, the high angle of the moonlight illuminating your surroundings is evocative of the sunlight experienced at noon near the June solstice. This effect is further enhanced by the lack of foliage, and a freshly-fallen snow will give the night an almost  daylight-like appearance. This is the best time to take a moonlit night hike.

The second reason that this months’ full Moon is noteworthy is that during the late evening of December 7, it will pass very close to Mars, which will be at opposition just a few hours later. For observers north of a line that extends from southern New Hampshire and through central and western Massachusetts, the Moon will actually occult Mars. At Seagrave Observatory, the closest the two objects will appear to each other is 25 arcseconds, or just 1.5 times the apparent diameter of Mars at the time. This will occur at about 11:30pm, and should be a spectacular sight.

The Sun is traversing the most southerly position of the ecliptic this month, and it is notable that it spends a significant amount of time in a constellation that is not part of the set of twelve, known as the zodiac. From the final day of November, through the 18th of December, the Sun transits the constellation Ophiuchus, the serpent-bearer. From there, it enters Sagittarius, where it will spend the next month.

Solstice occurs at 16:30 EST on the 21st. At the most southerly point of the Sun’s path across our sky, it lies 23.4° south of the celestial equator, and at 11:44 EST on the 21st, it climbs no higher than 24.7° in our sky, as seen from Seagrave Observatory. On the longest night, we have 14 hours and 53 minutes of night (between sunset on the 21st and sunrise on the 22nd), although true astronomical darkness (when the Sun is greater than 18° below the horizon) is actually 11 hours and 31 minutes.

In an event of cosmic coincidence, the Sun appears to cross the galactic equator just eight hours after the solstice, though it doesn’t eclipse the exact center, as its point of passage is 6.4° north of Sgr A*. If we could see the sky behind the Sun, the Lagoon Nebula, Messier 8, would be less than 1° to the southeast of the Sun.

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

Mercury and Venus have been in our evening sky for several weeks, but have been hovering too low in twilight to be easily observed. The second half of December brings our two innermost planets back into good viewing position.

The 3.4% illuminated crescent Moon joins the pair on the 24th. Look for it 6.3° southeast (to the left) of Venus, and 4.2° south-southeast of Mercury.

Mercury reaches its greatest elongation on the 21st, when it will be 20° east of the Sun. For a few days on either side of greatest elongation, Mercury remains visible for over an hour after sunset.

Venus appears closest to Mercury on the 26th, when it will be 1.5° south of the innermost planet. They will then swap positions on the 30th, when Venus begins to set later than Mercury, which dives rapidly towards the Sun for its next inferior conjunction on January 7. The final week of December and first few days of January is the best time for viewing Mercury with a telescope, as it exhibits a rapidly changing phase, and increasing size.

If you’re up for a real challenge, Pluto is just 1° south-southeast of Venus on the 31st.

While the prime season for viewing Saturn has passed, the ringed planet remains high enough in the sky during the early evening hours for telescopic observation. You’ll notice how its eastward motion is bringing it back towards Nashira and Deneb Algedi (gamma and delta Capricorni), a position it was in back in August.  The 4-day crescent Moon passes 4.6° southeast of Saturn on the 26th.

Now over 10 AU away from Earth, Saturn has grown noticeably smaller in a telescope, with its globe being about 16 arcseconds across - slightly smaller than that of Mars through much of December.

The 4-day-old, 17% illuminated waxing crescent Moon passes 4.5° to the southeast of Saturn on the 26th.

Jupiter is visible high in the south during twilight, and remains the most prominent star-like object in the evening sky. By mid-month, Jupiter sets before midnight, and on the 21st, it reaches quadrature, 90° east of the Sun, in heliocentric longitude.

The 9-day gibbous Moon passes 2.2° south of Jupiter on the night of the 1st-2nd.

Our outermost planet, Neptune, is an 8th magnitude, bluish point of light about 7° west of Jupiter along the ecliptic, and can be found in binoculars and telescopes near the midpoint of the southeastern segment of a 1.5 diamond asterism just 5 south of the Circlet asterism of Pisces. Neptune ends its retrograde motion on the 4th, resuming its eastward motion along the ecliptic. On the 28th, the 6-day crescent Moon is 2.8° southeast of Neptune, and 7.5° southwest of Jupiter.

Uranus, in Aries, remains well-positioned for observing through December. It is already high in the southeastern sky as twilight fades, so you do not need to wait long to observe it with a telescope. It can be located in a field of similarly bright stars about midway between Bharani (41 Arietis) and Menkar (alpha Ceti). Uranus is a blue-green star-like point that is visible in binoculars or small telescopes. The waxing gibbous Moon is 4° to its east on the 5th.

The morning sky is now devoid of bright solar system objects, but early risers will notice the return of the constellations of spring, with Ursa Major and Leo at their highest positions just before twilight.

While one dwarf planet (Pluto) leaves our evening sky, the closest one, Ceres, is now easily visible in the morning sky. Rising just after midnight at the beginning of the month, Ceres is just a few degrees south-southeast of Denebola, the tail of Leo. It enters Virgo on the 5th, and will be worth keeping track of throughout winter as it makes an apparent loop around the Realm of the Galaxies. Currently shining at 8th magnitude, slightly dimmer than Neptune, Ceres can be observed with binoculars under dark sky conditions. 

Asteroid 2 Pallas reaches opposition in early January, but this is not a favorable apparition for observers in the northern hemisphere, as it is hovering quite low in the sky. If you have a good southern horizon, you can track Pallas moving among the hind legs of Canis Major.

Somewhat easier to observe, 3 Juno and 4 Vesta are visible in the early evening sky. Juno is 9th magnitude, and Vesta is hovering around magnitude 8. Both objects are moving eastward through Aquarius, between Jupiter and Saturn. They are highest in the sky at about the same time Fomalhaut (alpha Piscis Austrinus) is on the meridian.

The Geminids meteor shower is active during December, and peaks on the morning of the 14th. The 71% illuminated waning gibbous Moon rises after 9:00pm EST on the 13th, and 61% illuminated Moon rises after 10:00pm on the 14th, making this a somewhat favorable year for observing what is one of the more consistently productive meteor displays of the year. A patient observer may see several dozen meteors per hour during peak activity. This shower originates from the unusual asteroid 3200 Phaethon, which orbits the Sun in a more comet-like 17-month orbit that ranges from 0.14 to 2.40 AU.

While observing the meteor shower on the 13th, watch the Moon soon after it rises, as it will occult eta Leonis. The 3rd magnitude star disappears behind the bright limb of the Moon at 10:09pm, and reappears from the dark limb at 10:54pm.

Rounding out the lunar calendar, last quarter occurs on the 16th, new Moon is on the 23rd, and first quarter on the 29th.

It is with sadness that we’ve lost two notable figures from the world of astronomy recently. The first is Jay M. Pasachoff from Williams College. Pasachoff was known for being a prolific eclipse chaser, having observed 75 of them, including 36 total eclipses, from 1959-2022, as well as both transits of Venus, in 2004 and 2012.

It was from Pasachoff that I first learned of the transits of Venus. While I didn’t know Jay personally, much of my early knowledge of astronomy was obtained from my well-worn 1983 edition of The Peterson Field Guide to the Stars & Planets, which he co-authored with Donald Menzel. Among the knowledge I gleaned from this book was details on solar system observation, including the clouds and moons of Jupiter, the upcoming (far in future from my reading it in the mid-1980s) transits of Venus in 2004 and 2012, and the (even farther out) 2017 total solar eclipse.

I had always wanted to meet Jay, and have him autograph my copy of the Field Guide. I finally did get to see him in person, albeit briefly, at the 2022 Hartness House Workshop, held in Springfield, Vermont before the Stellafane Convention. He gave an engaging presentation about solar eclipses, including many of his personal experiences. He didn’t stick around long after the presentation, and my Field Guide remains unsigned. Jay passed in November at the age of 79.

The second big astronomer who has left us for the stars is Jim Kaler from the University of Illinois. I had never met Kaler, but he taught me much of what I know about stellar evolution and astrophysics through the many excellent books on the topic. While Carl Sagan informed us that “we are made of star stuff,” Kaler’s work tells us how that star stuff came to be.

Kaler maintained a database of about 1000 of the brightest and most significant stars that I reference often to produce content for this newsletter, as well as other personal knowledge and projects. After learning of his passing, I went into the database and picked a random star to learn about - a bright star, visible high in our sky this time of year, but one that is often overlooked.

Mirfak, alpha Persei, is a circumpolar star from our latitude, meaning that it never rises nor sets. At magnitude 1.75, it is as bright as the stars in the handle of the Big Dipper and in Orion’s Belt, yet it never gets its due attention. Perseus, to many, is a constellation associated with the annual August meteor shower, housing the most well-known eclipsing binary star, Algol, and for deep sky observers, the home territory of the beautiful Double Cluster (even though most star-hopping journeys start in Cassiopeia). To name another notable object in Perseus, many amateur astronomers would hesitate as they try to recall the Messier and Caldwell lists, perhaps recalling M34 from a past experience with the Messier Marathon. 

You may be surprised to know that Perseus contains an open cluster that is both larger and brighter than the nearby Pleiades. The Alpha Persei cluster, also known as Melotte 20, is a real treat to binocular observers, or for an extended gaze with a small, rich field telescope. The cluster consists of mostly hot, young stars, at a distance of 570 light years. Its brightest member, Mirfak, is a class F5 supergiant that lies 590 light years away, contains eight times the mass of the Sun, shines with 5,000 times its brilliance, and has a radius 62 times greater, which is 75% of the semimajor axis of the orbit of Mercury. More fascinating facts about Mirfak can be found on Jim Kaler’s Stars database.

The name Mirfak is derived from a longer Arabic name that translates to “elbow of the Pleiades,” a name which describes how I often associate the rising of Perseus to the forthcoming rising of the Pleiades, as the longer string of stars in Perseus points directly to the more well-known cluster of Taurus.

The next time you’re out under a clear, moonless sky, take a minute to become familiar with Mirfak and the Alpha Persei Cluster.