A Christmas Star, a Pre-dawn Planet Parade, Comet Wirtanen Wends North, and Algol Amps Up!

Star Walk
7 min readDec 24, 2018

A Christmas Star

Merry Christmas, Stargazers!

(Above: Sirius, also known as Alpha Canis Majoris, is the brightest star in the winter evening sky. This pretty image is by Akira Fuji.)

Sirius (also known as Alpha Canis Majoris) is the brightest star in Canis Major (the Big Dog) and the brightest star in the sky (after the sun). Canis Major is the constellation to the lower left of Orion (the Hunter). After this brilliant stellar jewel rises in the southeast at just before 8 pm local time, it becomes hard to miss. This week, Sirius will climb to its highest point, about a third of the way up the southern sky, at 12:45 am local time. If you are walking through your darkened house in the middle of the night, it might catch your eye out a window because it never climbs very high.

Sirius is a hot, blue-white, A-class star located only 8.6 light-years from the sun. It is about twice the size of our sun. Its extreme brightness and low position in the sky combine to produce spectacular flashes of colour as it twinkles. J.K. Rowling used the name Sirius for a character in her books because the star is nick-named “the Dog star”.

(Above: The hunting dogs and Orion, at right. Sirius is at bottom centre, while Procyon is at the upper left. Imaged by Akira Fuji. A gallery of his images is here.)

A very large telescope may allow you to see Sirius B, a faint white dwarf companion located 10 arc-seconds from Sirius. Astronomers coined the name “the Pup” for that little star, but I prefer to call it “the Flea”. A white dwarf is the core of a star that has stopped fusing hydrogen in to helium and blown off its outer layers to leave the extremely hot core exposed.

Bright Comet Update

Here’s an update on how to see Comet 46P/Wirtanen this week and what to expect. The comet will now slowly fade in brightness. And, unfortunately, the full-ish moon’s brightness will continue to overwhelming the dim comet for the first part of this week.

Comet 46P/Wirtanen reached peak brightness around December 16, when its orbit carried it closest to both Earth and the sun. It’s still bright enough to barely 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 have located it in binoculars, switch to the telescope to see it magnified. I observed the comet last week in my 12” telescope and it showed no tail, just a dim, greenish, fuzzy glow around a tiny point of light.

The comet will be nearly overhead in late evening during this week, perfect for seeing it through the least amount of intervening air. You can begin to look for the comet as soon as the sky is fully dark. At that time, it will be about halfway up the northeastern sky. The comet will then climb to its highest point, nearly overhead, just before midnight local time. The orbit of this comet is carrying it up through the plane of the solar system from below. The comet is now close enough to the North Celestial Pole, where Polaris resides, to be circumpolar — meaning that it will never drop below the horizon for mid-Northerly latitude observers.

This week, the comet will continue to drift north, carrying it higher in the sky, and towards the left for observers in mid-northern latitudes. After dusk tonight (Sunday), the comet will pass a thumb’s width (or 1.25°) to the lower left of the bright star Capella in Auriga (the Charioteer). Between 11 pm and midnight EST, the comet will pass closely to the right of a star cluster called NGC 1883, putting the pair of object together in the field of view of your telescope. (Remember that the telescope might flip or invert the view.)

From there, Comet Wirtanen will move farther north every night, pulling away from Capella and heading towards the star Muscida, which marks the nose of Ursa Major (the Big Bear). Next Sunday night it will end up about halfway between those two stars.

The Moon and Planets

The moon will spend this week transitioning from the Full Moon Before Yule to this Saturday morning’s Last Quarter phase. During this period, the moon will rise later and wane in phase. In fact, from Monday on, the moon will make a pretty sight in the pre-dawn sky and also linger to appear in the morning daytime sky. In late evening on Monday, the orbital motion of the moon will carry it just below the center of the large open star cluster known as the Beehive, and Messier 44, in Cancer (the Crab). Both objects will fit within the field of view of binoculars or a telescope at low magnification, although the bright moonlight will obscure the cluster’s dimmer stars.

We’ve entered a period when Mars is the only bright planet available for viewing in the evening. Mars continues to dominate the southern evening sky, although it is slowly shrinking in size and brightness. This week, the Reddish Planet will shine a little less than halfway up the southern evening sky. Right now, it’s a few finger widths below the westerly circlet of stars that form one of the fish of Pisces (the Fishes). The ring of stars is about 4 finger widths across. Mars will set in the west at about 11:40 pm local time.

The distant ice giant planets are also in the evening sky. Tonight (Sunday) dim, blue Neptune will be a fist’s width to the lower right of Mars. This week, it will be available for viewing after the sky becomes fully dark. The planet will set a little before 10:30 pm local time, but take your look earlier, while it’s higher. In binoculars, Neptune appears about two finger widths to the upper left of the modestly bright star Hydor (Lambda Aquarii), where it’s been for quite some time. Hydor, and a pair of stars to its left, form a sideways narrow triangle. Neptune is inside of that triangle.

Blue-green Uranus (“YOU-ran-us”) is in the southern 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 make it relatively easy. Uranus is about 1.5 finger widths above, and slightly to the left 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 7:45 pm local time — the best position for seeing it clearly.

All of the remaining bright planets, except Saturn, are gathered in the eastern pre-dawn sky. This week, Saturn will be buried in the eastern twilight after sunset. Next week, it will slide past the sun to join the morning pre-dawn planets. Speaking of the pre-dawn…

Mercury is currently continuing 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 almost 7:30 am while it is carried higher. When viewed in a telescope, Mercury will exhibit a nearly full phase. As the week wears on, Mercury will drop lower.

Very bright Jupiter is now moving to a more prominent position in the eastern pre-dawn sky. This week, it will sit to the upper right of Mercury, but that swift planet will rapidly drop away from the king of Planets. You can look for Jupiter between 6 am and 7:30 am local time.

Venus is much higher in the eastern sky than Mercury and Jupiter, because it rises hours earlier — at about 4 am local time. Viewed through a telescope, Venus will appear less than half illuminated, and it’s stunningly bright now!

Watch the “Demon Star” Brighten

The star Algol in Perseus (the Hero) is among the most accessible variable stars for beginner skywatchers. Its visual brightness dims noticeably for about 10 hours once every 2 days, 20 hours, and 49 minutes because a dim companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star. On Friday, December 28 at 6:06 pm Eastern Time, Algol will reach its minimum brightness of magnitude 3.4. At that time, for observers in the GTA, it will sit more than halfway up the eastern sky. By 11:06 pm, Algol will be high overhead and will have brightened to its usual magnitude of 2.1.

In the constellation, Algol marks the glowing eye of Medusa the Gorgon, whose severed head Perseus is carrying. That’s why this star has the nickname the “Demon Star”. The constellation sits to the upper left of the bright Pleiades star cluster.

Astronomy Skylights for the week of December 23rd, 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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