When we think December, we tend to think winter solstice, long nights, Christmas and end-of-year parties. But December is also Geminid season.
Although not as well known or as popular as the Perseids (which mainly owe their fame to the warm August weather!), the Geminids are considered the most consistent and reliable meteor shower year after year. They’re also one of the most prolific, alongside the Quadrantids in January.
Remember that a meteor is the brief streak of light produced when a small interplanetary particle (typically smaller than a grain of sand) enters Earth’s atmosphere at very high speeds. The intense friction generates extreme heat, causing the air around the dust particle to glow. Meteor showers occur when Earth runs into trails of dusty debris along its orbit. Most meteor showers are associated with periodic comets that shed material as they repeatedly swing by the Sun, constantly replenishing the stream with dust. The Geminids are unique in that their source is an unusual asteroid—3200 Phaeton—believed to be the dead nucleus of an ancient comet that has lost virtually all its volatiles, leaving only a rocky core.
In 2023, the Geminids are expected to peak around 2 p.m. (EST) on December 14—that’s broad daylight in our time zone. Since the Geminid radiant is visible all night long and culminates around 2 a.m., the nights of December 13-14 and 14-15 will provide equally good displays for Quebec astronomers this year. Around midnight on both nights, when the radiant (near Castor and Pollux, the twin stars in the constellation Gemini) is very high in the sky, you can see a good 30 or so Geminids per hour under dark, crystal-clear skies. Experts consider Geminid meteors to be slow-moving because they enter Earth’s atmosphere at about 36 kilometres per second (or 130,000 km/h); however, they are often very bright due to the relatively large-sized dust particles.
Since the new Moon falls on December 12, the interfering light from our natural satellite will not be a limiting factor this year. So, given these particularly favourable conditions, will you be heading out to catch a glimpse of the Geminids? If possible, find a location away from light pollution, bundle up warmly and have your wish list ready!
The giant planets
It’s not too late to check out Saturn and admire its beautiful rings through a telescope, before the planet sinks too low. Saturn is at its highest altitude this month, appearing 32 degrees above the southern horizon as soon as the sky darkens, shortly after sunset. But don’t wait too long, because it will continue sinking toward the west-southwestern horizon as the evening advances. Saturn is the most luminous object in this area of the sky at nightfall, making it easy to spot with the naked eye. On the evening of December 17, the crescent Moon hangs 3 degrees below the ringed planet.
The situation is far more favourable for Jupiter. The largest planet in the Solar System was at opposition in early November and is now extremely well positioned for early evening viewing. In fact, Jupiter already shines high in the sky at nightfall, about 50 degrees above the southeastern horizon. When observation conditions are good, this elevation ensures greater image stability when looking at the planet through a telescope: The fine details of Jupiter’s cloud bands will appear sharper than when the gas giant is lower on the horizon.
On December 30, once night sets in, owners of 150-mm and larger telescopes will be treated to an interesting phenomenon as the Galilean moons Europa and Ganymede simultaneously cast their shadows onto Jupiter’s clouds. The show begins shortly before 5 p.m. (EST): After spending several hours transiting Jupiter’s disc, Europa leaves the planet’s face and, for a few minutes, can be seen as a tiny bump on the western limb. At that moment, the tiny black dot of its shadow begins to cross the Jovian clouds, appearing as a small dark notch on the opposite limb. After a few minutes, Europa completely moves off Jupiter’s disc and stands clearly against the sky, with its shadow now appearing as a single spot on the planet’s clouds.
The double shadow transit actually begins around 5:17 p.m., when Ganymede’s shadow enters the Jovian clouds. This shadow is much larger than Europa’s and has a somewhat elongated appearance, since it falls onto the high latitudes of Jupiter’s southern hemisphere.
At around 6 p.m., the two shadows are clearly visible one above the other, nearly on the Jovian central meridian, with both moons shining just to the west of the planet. Ganymede’s shadow is the first to move off Jupiter, at around 6:55 p.m., followed by Europa’s some 20 minutes later, at around 7:19 p.m.
Venus still shines at dawn
The final hours of the night are still dominated by Venus, which shines like a spotlight above the southeastern horizon from 4:30 a.m. until dawn. However, the beautiful Morning Star has dropped a few degrees since its peak last October and will lose another 10 or so degrees over the course of December alone: On the 1st, Venus can be found at about 30 degrees in altitude just as civil dawn begins; on the 31st, it stands only 20 degrees high at the same time of day. It will continue to drift downward over the coming months.
Through a telescope, Venus currently appears as a gibbous disc, 75% illuminated and only 15 arc seconds in diameter by mid-December. On the morning of December 9, note the thin waning Moon hovering less than 4 degrees to the right of Venus, slightly lower than the planet.
The December solstice will occur on the 20th at 10:28 p.m., EST. For those of us in the Northern Hemisphere, it’s the official start of astronomical winter, which will last “only” 88 days, 23 hours and 38 minutes. Although the length of each season slowly varies as millennia pass, our boreal winter is currently the shortest, and will remain so for several more centuries.