Transient Events That Can Give Us a Good Enough Reason
to Get Outside and Do a Little Observing
All month: the sunrise sky becomes a less lively place this month, as Venus ends its long morning apparition and joins Mars in the solar glare as they both head towards solar conjunction in January, abandoning the predawn gloaming to Mercury near the horizon and Jupiter higher up. Mercury begins the month as a fat crescent, 36% lit and 8 arcseconds across, glowing at mag 0.0 and rising an hour and forty minutes before the Sun. It remains visible before sunrise all month long: on New Year’s Eve, it shows us a nearly full phase, 94% lit and 5 arcseconds across, brightening to mag -0.5 and rising 49 minutes before the Sun. Its only planetary companion is Jupiter, which remains up after sunrise throughout December. There are no planets to be found near the sunset horizon this month: the first to set is Saturn, which will disappear below the edge of the Earth at 12:40 am on Dec 1 and two hours earlier by month’s end. The ringed planet shows us an 18 arcsecond disk at mag +1.1 as the month opens; by month’s end, it’s an arcsecond smaller and 0.1 magnitude less bright. The rings span 42 arcseconds and are nearly flat on Dec 1, at an angle of only 0.4° to our line of sight, but they now begin to open, reaching 1.5° by Dec 31. Neptune remains close to Saturn again this month – the pair’s separation begins at 4.3° and narrows to 3.5° by month’s end. Neptune’s deep blue disk shines at mag 7.9 and measures just over 2 arcseconds across. It sets just after 1:00 am on Dec 1 and two hours earlier on Dec 31. Uranus follows five hours behind Neptune, setting at 6:11 am at the beginning of the month and 4:07 am at its end. Its pale blue-green disk measures 4 arcseconds at mag 5.6. Bringing up the rear is mighty Jupiter. It rises at 7:30 pm on Dec 1 and 5:15 pm on Dec 31, while its disk grows from 44 to 46 arcseconds and it brightens from mag 2.5 to -2.7. It gets very high this month, reaching 70° elevation, on its way to opposition on Jan 9. Among the asteroids, minor planet 1 Ceres continues its trek through Cetus, and remains a relatively easy catch between mag 8.5 and 8.9. Although comets are rarely covered here, there seems to be a consensus that
24P/Schaumasse will brighten into the mag 9 range in December, and might reach mag 8.0, making it a smallscope or even binocular target for the wee hours. This month’s charts will show you where to find both the asteroid and the comet.
Dec 3: a nearly-full Moon marches through The Pleiades (M45) from 7:15 to 9:00 pm CST tonight, occulting several of them along the way, but the glaring moonshine will make it impossible to see the occultations without a telescope to keep the Moon (other than its edge) out of the field. Mag 3.7 Electra disappears at 7:16 pm, followed by mag 5.5 Celaeno at 7:18, mag 4.3 Taygeta at 7:40, and mag 3.9 Maia at 7:45. As always, start watching several minutes before the disappearance time so you don’t miss it, and remember that although the Moon moves westward across our sky, it appears to drift east compared to the stars, so the occultations will occur along the Moon’s left edge.
Dec 6: Mercury reaches its highest elevation for this morning apparition. Look for it 10° above the ESE horizon around 6:18 am CST (45 minutes before sunrise).
Dec 7: although its orbit usually takes it above or below the disk of Jupiter from our perspective, Jove’s outermost moon, Callisto, will execute a rare transit of the big planet tonight from 8:17 pm to 12:02 am CST. Callisto is easy to spot because it has a very low albedo and appears quite dark – expect to see something that looks a lot like a shadow transit. Catch it between 10 and 11 pm for a nice view, with Callisto roughly midway across the disk. Then step back for a broader view, as Jupiter makes a pretty 5° triangle with Pollux (Beta [] Gemini) and the gibbous Moon as they cross the sky together tonight.
Dec 9: the 66% lit gibbous Moon rises a half-degree from Regulus (Alpha [] Leonis) shortly before 10 pm CST, and the pair come even closer as the evening goes on, with just 12 arcminutes between them at 12:08 am.
Dec 13: the Geminids meteor shower peaks tonight, and the Moon cooperates for a change: it’s just 26% lit, and doesn’t rise until 2:11 am, when the Geminids’ radiant is near zenith. This is a prolific shower, with a nominal 120 ZHR: expect to see 50-75 meteors/hour from a dark site, or 25-50 from the suburbs. Geminids are fairly slow-movers (around 36 km/sec) with plenty of bright, long-lived meteors among them. Some observers report seeing red, yellow, or green trains. Unlike most showers, the Geminids provide good views in the evening as well as the overnight hours. It’s unclear whether their parent body, asteroid 3200 Phaethon, collided with another asteroid and produced a cloud of debris that was distributed along Phaethon’s orbit, or whether it’s the spent nucleus of an extinct comet whose debris stream persists and produces this shower.
Dec 15: Callisto has another interaction with the Jovian disk tonight as it disappears into the big planet’s shadow at 8:11 pm CST, reappears at 11:55 pm, then sneaks behind the disk at 1:52 am and re-emerges at 5:51. Don’t confuse it with Europa, which goes into the shadow at 7:46 and emerges at 11:49 pm.
Dec 21: the solstice arrives at 9:03 am CST, marking the year’s longest night. Once darkness arrives, the Ursids meteor shower peaks, but don’t expect a repeat of the Geminids: even with the Moon setting in the early evening, you’ll only see 5 meteors per hour from a dark site, or 1-2 per hour from the suburbs. The Ursids are debris from comet 8P/Tuttle, which last passed through the inner solar system in 2021. They’re slow movers, and include many bright meteors with yellowish trains. Some researchers have predicted that we’ll pass through a dense area in the stream between 11:30 and 11:45 pm tonight, but as usual, there are no guarantees.
Dec 24: while you’re waiting for Santa to arrive, you can watch Europa transit the face of Jupiter from 6:12 to 9:03 pm, chasing its shadow that made the trip from 5:22 to 8:13 pm.
Dec 25: if it won’t disrupt your Christmas dinner, take a minute (and a pair of binoculars) to watch mag 3.7 Lambda () Aquarii sneak behind the dark edge of the 31% illuminated Moon at 5:47 pm CST.
Dec 26: the not-quite-first-quarter Moon sits just 3° from Saturn tonight, and as a day-after-Christmas present to us all, the Lunar X will be visible in binoculars along the terminator, roughly midway between the Moon’s equator and its south pole, from around 7:15 pm until 9:15 pm CST. Once you spot the X, look farther north along the terminator for its companion, the Lunar V.
Dec 29: the shadow of Io transits Jupiter from 5:55 to 8:11 pm CST, followed by the hot little moon itself from 6:12 to 8:28 pm.
Dec 31: a nearly-full Moon (91% lit) poses in the eastern evening sky with The Pleiades (M45) and Aldebaran (Alpha [] Tauri), forming a pretty triangular tableau to end the year.
Rick Gering / December 2025 / Wishing everyone a new year filled with health, happiness, and clear skies!
