What planetary events are coming up in the night sky? Read about Venus, Mercury, Mars, Jupiter, Saturn, Uranus and Neptune and what’s going on in our Solar System in Planets at a Glance.
Mercury
Mercury will begin the month of December hidden in the sky glow above the southwestern horizon after sunset. Its continuous slide eastward will allow it to become visible from mid-northern latitudes after the middle of December, but observers in the tropics will glimpse it several days sooner.
Mercury will be outpacing 20 times brighter Venus, which will shine about a palm’s width to its lower right, or five degrees to the celestial southeast. Mercury will reach a maximum angle of 20 degrees east of the Sun on December 21. That night the speedy, magnitude -0.56 planet will be most easily visible for about half an hour starting at 5 p.m. local time.
When viewed in a telescope, Mercury will display a 60 per cent illuminated waning phase, and an apparent disk diameter of 6.8 arc-seconds. Venus will continue to shine to Mercury’s lower right until December 27, with the young crescent Moon posing to their left on December 24. Mercury will descend sunward and fade in brightness, passing a mere 1.6 degrees to the right (celestial north) of Venus on December 29 — a scene best viewed in binoculars, but only after the Sun has completely set.
Venus
Like Mercury, Venus will spend the early part of December lurking just above the west-southwestern horizon after sunset. Its brilliant -3.38 magnitude might allow observers with cloud-free, unobstructed horizons to glimpse the planet. Around mid-month, Venus will have slid far enough east of the Sun for sky-watchers at mid-northern latitudes to more easily see it.
Mercury, which is 20 times fainter, will shine about a palm’s width to Venus’ upper left (or celestial east) for most of December. After Mercury’s greatest elongation, it will drop sunward, passing only 1.6 degrees to the right of Venus in a tight conjunction on December 29. The very young crescent Moon will shine a palm’s width to the left of the two planets on December 24.
Viewed in a telescope during the latter half of December, Venus will exhibit a nearly fully-illuminated disk that spans 10 arc-seconds — but only observers at southerly latitudes, where Venus will be higher, will see a clear image.
Mars
During the early hours of December 1, Mars will be at its closest proximity to Earth — a distance of 50.61 million miles, 81.446 million kilometres, or 4.5 light-minutes.
That will be the best night to view the markings on Mars’ 17.2 arcseconds wide globe, including the dark triangle of Syrtis Major and the bright disk of Hellas. As well, the Martian moons Phobos and Deimos are more readily seen around closest approach — especially when they venture farthest from Mars’ glare.
In the sky during December, bright, reddish Mars will rise at sunset and culminate high in the southern sky after midnight. The planet will be a fine sight in backyard telescopes, as it travels in retrograde westward towards the Pleiades star cluster — beginning the month between the horns of Taurus, and ending December nearly a fist’s width above the bright, warm-hued star Aldebaran.
From December 1-5, Mars will pass close enough to the celestial north of the large open star cluster NGC 1746 for them to be viewed your telescope’s eyepiece. Because Mars’ elliptical orbit is veering wider from Earth’s during December, Mars’ opposition will occur a week after its closest approach — specifically after midnight on December 7 in the Americas.
Since the Moon will also be opposite the Sun that night, observers in northwestern Mexico, the Continental United States (except for southern and eastern states), all of Canada (except southwestern Nova Scotia), Greenland, Svalbard, western Europe, and the northern coast of Africa can watch the Moon occult Mars. This event can be observed with unaided eyes, binoculars, and telescopes. Exact times vary by location, so use an app like Starry Night to determine your own circumstances.
In Toronto, Ontario, the leading edge of the Moon will cover Mars at 10:29 p.m. EST on Wednesday evening. Mars will reappear from behind the Moon’s opposite, southern limb at 11:17 p.m. EST. In Europe, the occultation will occur before dawn on Thursday morning.
Jupiter
Jupiter will be well placed for observing during early evening in December. Although the bright planet will be setting shortly after 11 p.m. local time by the end of the month, the early sunsets at mid-northern latitudes will deliver plenty of hours to view the planet starting at dusk. Skywatchers can view it with unaided eyes, through binoculars, and in backyard telescopes.
Visually, Jupiter will decrease slightly in apparent brightness over the month, from magnitude -2.6 to -2.4. Because it ended its retrograde loop in late November, Jupiter will spend December ramping up its easterly prograde motion through southwestern Pisces, pursued by slow-moving Neptune and the speedy asteroids (3) Juno and (4) Vesta.
In a backyard telescope, Jupiter will exhibit dark equatorial bands across a disk that diminishes in size from 44.4 to 39.4 arc-seconds. The Great Red Spot will appear every second or third night, and Jupiter’s four large Galilean satellites will eclipse and occult one another, and cast their round, black shadows on the planet. The waxing Moon will shine near Jupiter twice in December — as a gibbous Moon three degrees to the south on December 1, and as a crescent moving past Jupiter on December 28-29.
Saturn
The creamy-yellow dot of Saturn will be visible in the southwestern evening sky in December, but its elongation from the Sun will decrease from 70 degrees to 42 degrees over the month, giving us less and less time to view it in telescopes.
On December 1, magnitude 0.77 Saturn will become visible in the lower part of the southern sky after dusk, and then set towards 10 p.m. local time. At month’s end, the slightly less bright ringed planet will already be sinking in the southwestern sky at dusk. During December Saturn will travel eastward above the medium-bright stars Deneb Algedi and Nashira, which form the tail of Capricornus, the Sea-Goat.
Viewed in a telescope, Saturn will show an apparent disk diameter that diminishes slightly from 16.4 to 15.8 arc-seconds over the month. Its rings will stretch across 2.33 times that span. Saturn’s rings will tilt more edge-on to Earth every year until the spring of 2025. This year they are already closed enough for Saturn’s southern polar region to extend well beyond them. The pretty waxing Moon will shine five degrees to the south of Saturn on December 26.
Uranus
Uranus reached opposition on November 9, so the blue-green, magnitude 5.67 planet will still be an all-night target during December — especially after mid-evening, when it will climb higher and appear more clearly in a backyard telescope or binoculars.
All month long, Uranus’ small, blue-green dot will be moving slowly retrograde westwards through southern Aries, a generous fist’s width to the lower left (or 13 degrees southeast of) that constellation’s brightest stars, Hamal and Sheratan. It will also be only a palm’s width north of the star Mu Ceti.
On the evening of December 5 in the Americas, the very bright, nearly-full Moon will shine several degrees to the left (or celestial east) of Uranus. Hours earlier, observers with telescopes in Europe, the Middle East, and northern Asia can see the Moon occult Uranus around 17:00 GMT — the 12th in a series of consecutive lunar occultations of that planet.
Neptune
During December, the distant, blue planet Neptune will be observable during the evening hours as a magnitude 7.9 speck in northeastern Aquarius — near that constellation’s border with Pisces. On December 3, Neptune will complete a retrograde loop that has been slowly carrying it westward since late June.
Like nearby Jupiter, Neptune will ramp up its regular eastward motion during the rest of the month. The planet will be easiest to see when it is highest in the sky after dusk. To aid your search, far brighter Jupiter will shine about six degrees to Neptune’s east-northeast, and the medium-bright star Phi Aquarii will shine almost as far toward its west-southwest.
In a telescope, Neptune’s tiny apparent disk size will span 2.3 arcseconds. Larger telescopes can show Neptune’s large moon Triton.
Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the Universe together.
