An Evening Moon, it’s the Week to Peek at Comet Wirtanen, Mercury at Maximum Visibility, and Geminid Meteors Peak, too!

Star Walk
7 min readDec 10, 2018

Bright Comet Update

(Above: Damian Peach of Australia captured this image of Comet 46P/Wirtanen on December 7, 2018. Note the distinctive green halo and the the very faint tail extending to the lower right. This image is inverted from a binocular view. His galleries are accessed at

Comet 46P/Wirtanen is predicted to brighten until December 16, when its orbit will carry it closest to both Earth and the sun. It’s now bright enough to see without binoculars if you are under a dark sky away from artificial lights. It’s quite easy in binoculars, if you know where to look. Don’t try searching for the comet with a telescope — the patch of sky seen in the eyepiece is so small that you’ll likely miss the comet. But once you know where it is, use the telescope to look closely at it! For the early part of this week, the moon will not affect the night sky, but the moon will become quite bright as we head toward the comet’s peak time this weekend. So you should try to see the comet on the first clear night this week.

(Above: The orbit of comet 40P/Wirtanen crosses the solar system between Jupiter’s and Earth’s orbital distances on a 5.4 year loop. This year, when that comet is near perihelion and at maximum brightness, Earth will be nearby, setting up the potential for a naked-eye comet in our skies.)

Look for a faint, green, fuzzy blob surrounding a bright point of light. Reports are that the halo around the comet is as wide as a full moon — that’s half a finger’s width held at arm’s length. If Comet 46P/Wirtanen grows a tail, the tail will extend east (to the left), away from the sun. You can begin to look for the comet as soon as the sky is dark. The comet will then climb to its highest point, halfway up the southern sky, at around 10:15 pm local time. Then it will set in the west at about 4:20 am local time.

The orbit of this comet is carrying it up through the plane of the solar system from below. This week, the comet will continue to drift north, moving it higher in the sky for observers in mid-northern latitudes. Tonight (Sunday) Comet Wirtanen will be located in eastern Cetus (the Whale) and just less than a palm’s width (or 5°) below that constellation’s brightest star Menkar. Because the comet is rapidly moving to the left and upwards, on Monday and Tuesday night it will pass a palm’s width to the lower left, and then directly to the left, of Menkar, respectively.

On Wednesday evening, the comet will pass two finger widths to the left of a medium-bright star named Omicron Tauri, which represents the Taurus the bull’s foot. On Thursday the comet will move to the upper left of that star. On Saturday night, the comet will land four finger widths (or 4.5°) below the very easy-to-identify Pleiades star cluster. That’s the bright little cluster of bluish stars above the very bright orange star Aldebaran in Taurus. On Sunday, the night when comet 46P/Wirtanen should be at peak brightness, the comet will shift to the lower left of the Pleiades.

The Geminids Meteor Shower Peaks

(Above: To record the splendor of the Geminids Meteor Shower, Yin Hao captured this composite of 37 frames spanning 8.5 hours on December 12–13, 2017 at a location in Inner Mongolia. The pair of bright stars centred within the meteors are Castor and Pollux in Gemini. Orion is to their lower right, with the Milky Way rising through centre-frame. NASA APOD for December 15, 2017.)

The Geminids meteor shower, one of the most spectacular of the year, runs from December 4 to 16 annually. In 2018, it will peak before dawn on Thursday, December 14, when up to 120 meteors per hour might be seen under dark sky conditions. Geminids meteors are often bright, intensely coloured, and slower moving than average because they are produced by particles dropped by an asteroid designated 3200 Phaethon.

The best time to watch for Geminids will be from full darkness on Wednesday until dawn on Thursday morning. At about 2 am local time, the sky directly overhead, which will be positioned near the bright star Castor in Gemini (the Twins), will be plowing into the densest part of the debris field. True Geminids will travel away from that part of the sky, but don’t just watch that location — the meteors will be shortest there, and they can appear anywhere in the sky. The early-setting crescent moon on the peak night will provide a dark sky for meteor-watchers.

(Above: The radiant location for the Geminids meteor shower. For best results, wait until the radiant is overhead around 2 am local time. But don’t watch the radiant — the meteors will be shortest there.)

To see the most meteors, find a wide-open dark location, preferably away from light polluted skies, and just look up with your unaided eyes. Binoculars and telescopes are not useful for meteors — their field of view are too narrow. Try not to look at your phone’s bright screen — it’ll ruin your night vision. And keep your eyes heavenward, even while you are chatting with companions. If the peak night is cloudy, a night or two on either side of that date will be almost as good. Happy hunting!

The Moon and Planets

This is the best week of the moon’s monthly circuit around Earth to look at our nearest celestial companion. Tonight (Sunday), a thin crescent moon will shine prettily over the southwestern horizon after sunset. The planet Saturn will be situated a generous palm’s width to the moon’s lower right. Saturn, which is embedded in the western twilight, will set at about 6 pm local time all week long, even as the moon shifts eastward away from it.

During the remaining weekdays, the moon will wax fuller and shift eastward across the southern evening sky, first moving through the dim stars of Capricornus (the Sea-Goat) and then traversing Aquarius (the Water-Bearer) on Thursday and Friday. On Thursday evening, the nearly half-illuminated moon will occupy a spot 4 finger widths below Mars.

The moon will officially reach its First quarter phase on Saturday morning, so it will be slightly more than half-illuminated by the time we see it on Saturday evening. (Lunar phases are only controlled by the angle between the sun, Earth, and moon, not the time of day or night on Earth.) On the evenings around first quarter, sunlight strikes the moon at a shallow angle — arriving from near the moon’s eastern horizon. This light casts dramatic and spectacular shadows from even slightly elevated terrain on the moon’s surface, allowing the fine details of the moon’s geography to be seen in binoculars and small telescopes.

The inner planets are both in the eastern pre-dawn sky this week. Mercury is currently making an excellent appearance for anyone living in the Northern Hemisphere. It will be low, in a fairly dark southeastern sky, at around 6:30 am local time, and remain in view until about 7:15 am local time while it is carried higher. When viewed in a telescope, Mercury will exhibit a crescent phase. On Saturday, Mercury will reach its widest separation (21°) from the sun and also will become its brightest for the current appearance. From Sunday onward, Mercury will descend towards the sun again.

Venus is much higher in the eastern sky than Mercury, because it is now rising hours earlier — at about 4 am local time. Viewed through a telescope, Venus will also show a crescent phase. It’s much brighter than Mercury, too. Venus recently reached its maximum brightness for the year — a stunning magnitude of -4.87!

(Above: Jupiter will join the inner planets Mercury and Venus in the eastern pre-dawn sky this week. Mercury will achieve peak visibility on December 15, as shown here at 7 am local time.)

Jupiter recently passed solar conjunction (when it was in the same part of the sky as the sun) and is returning to visibility in the eastern pre-dawn sky this week. It will be a while before it climbs high enough to catch your eye, but you might glimpse Jupiter low over the southeastern horizon after it rises at 6:30 am local time — just as the sky is beginning to get lighter. Jupiter will return to the evening sky from May onward.

Mars continues to dominate the southern evening sky, even as it is slowly shrinking in size and brightness. This week, the reddish planet will shine in the lower part of the southern evening sky among the stars of Aquarius (the Water-Bearer). Mars will set in the west just before midnight local time.

Blue-green Uranus (“YOU-ran-us”) is in the southeastern evening sky. It’s still close to its peak brightness (magnitude 5.7) and size for this year. You can see Uranus without optical aid under very dark skies, but binoculars and telescopes work better. Look for Uranus about 1.5 finger widths to the upper left (east) of the modestly bright star Torcular (or Omega Piscium). That star sits a generous palm’s width above the “V” where the two starry cords of Pisces (the Fishes) meet. This week, Uranus will be at its highest point, over the southern horizon, at about 9 pm local time — the best position to to see it clearly.

Neptune met Mars last week, but the red planet will now be rapidly drawing away from the distant blue planet. Tonight (Sunday) Mars will be located 1.5 finger widths to the upper left (east) of Neptune. Next Sunday night, Mars will be a palms’ width away Neptune. This week, Neptune will become visible in strong binoculars or a telescope once the sky becomes fully dark. The planet will set at about 11 pm local time. With Mars now unavailable to help us find Neptune, look for the magnitude 7.9 planet sitting about two finger widths to the upper left of the modestly bright star Hydor (Lambda Aquarii), where it’s been for quite some time.

Treats in Taurus

Every year in early December, the distinctive constellation of Taurus (the Bull) returns to our evening skies, rising as darkness falls and crossing the sky through the night.

(Above: Taurus, the Bull leads the bright winter constellations up the eastern sky in December)

Astronomy Skylights for the week of December 9th, 2018 by Chris Vaughan.

Keep looking up, and enjoy the sky when you do. I love questions and requests — so, send me some!



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