Comet Wirtanen Brightens Before the Winter Solstice, Telescope Shopping Tips, the Moon Covers a Star, and the Full Moon Before Yule Spoils the Ursids!

Star Walk
11 min readDec 17, 2018


(Above: This image of Comet 46P/Wirtanen was captured by talented Canadian astrophotographer Alan Dyer on December 15, 2018. More of his amazing sky work is here.)

It’s the Solstice!

Happy Holidays, everyone! Or, as we astronomers say, “Have a Happy Solstice and a Merry Perihelion!”

For the Northern Hemisphere, the first day of winter, also called the Winter Solstice, occurs on Friday, December 21 at 5:23 pm Eastern Time. At that precise moment, the north pole of Earth’s axis of rotation will be tilting directly away from the sun. Every day, at local noon, the sun reaches its highest position in the sky for that day. But at the Winter Solstice, that highest position is the lowest (i.e., farthest south, celestially) for the entire year, and we receive the shortest amount of daylight. The sunlight that we do get this time of year is diluted because it’s spread over a larger area, the same way a flashlight beam looks dimmer when you shine it obliquely at a wall (try it!).

(Above: At the Winter Solstice, the Earth’s axis of rotation tips away from the sun, putting the noonday sun lowest in the southern sky for the year.)

Fewer hours and weaker sunlight both translate into less received solar energy (insolation) and therefore colder temperatures! Good news for us, though — after Friday, our days start growing longer again! For our friends in the Southern Hemisphere, the Sun will attain its highest noon-time height for the year on the solstice, and it marks the start of their summer season.

It is NOT the case, as some people think, that we are colder in winter because we are farther from the Sun (a position called aphelion). That event happens every year in early July! On the contrary — we’re approaching Earth’s nearest position from the Sun (perihelion), which occurs every January 4, or thereabouts.

Some people think that Christmas was deliberately placed close to the solstice, and Easter placed close to the Vernal Equinox, because early non-Christian “pagans” were already holding celebrations to mark the astronomical changing of the seasons.

Telescope Buying Tips

A few people have asked me about buying a first telescope. Two of the most important characteristics in telescope performance are its aperture (the diameter of the main “tube”) and the sturdiness of the mount/tripod. The aperture is the diameter of the main mirror or lens in the telescope — and the larger it is, the more light it can gather, allowing fainter objects to be seen more easily. A larger aperture also increases the telescope’s angular resolution, or sharpness, which is especially helpful for planets.

In a refractor telescope (the type with the lens at the front end of the main tube), look for a minimum diameter of 60 mm. This type of telescope will offer nice views of the brighter planets, the Moon, double stars, and brighter nebulae and clusters. The performance of a telescope improves with the square of the aperture, so an 80 mm telescopes is (80*80)/(60*60) = 1.8 times more powerful than a 60 mm telescope! That’s nearly twice as good!

(Above: A Dobsonian-style telescope offers the ideal mix of ease of use, stability, and light-gathering for seeing the moon, planets, and deep sky objects.)

In reflector telescopes (the type with the large mirror inside the bottom of the main tube), diameters of 6 or 8 inches are common and affordable. Because of that aperture scaling rule, they outperform the refractor telescope by showing dimmer galaxies and clusters, especially under a dark sky, while also showing the same brighter objects. (An 8-inch reflector gathers seven times as much light as an 80 mm refractor!)

The second important factor when shopping is the mount, including the tripod. A high quality telescope on a rickety mount is practically useless. Every time you touch it, for focusing or centering an object, you set off a wild shaking of the image. This is one of the main reasons why cheap department store telescopes are poor investments — people get frustrated when they can’t put objects in the field of view and keep them there.

Skywatcher makes a good affordable series of starter telescopes. Here’s a 70mm diameter model that ticks all the boxes. They have larger aperture versions, too. Look for the “AZ3” part of the model number. It’s a good, lightweight, and sturdy tripod that includes slow motion controls for following the object as the Earth turns. Any other brand with the same specifications should be okay, too.

The type of telescope I recommend most for beginners is the Dobsonian style. While larger and heavier, they are simple and easy to set up and use. They are less portable, but are quite rugged and can be transported in the trunk of your car if padded decently. The beauty of the Dobsonian design is the extremely sturdy mounting system — not a tripod, but a swivelling and tilting box that holds the big tube. Once aimed at a target, a simple nudge with your finger will re-centre the object without any undue shaking. These types of telescopes typically show a generous amount of sky in their eyepiece, making finding targets much easier.

The best brands of consumer telescopes include Meade Instruments, Celestron, Skywatcher, Orion, and iOptron. Some of the GTA telescope vendors include New Eyes Old Skies, Ontario Telescope, KW Telescope, and Khan Scope. Many of these vendors offer both new and used equipment, which can be a very cost effective solution. Dobsonians in particular can be found used on sites like Kijiji and the Canadian astronomy selling site Astrobuysell, mainly because people find them harder to store. Prices for good, used telescopes range from $250 to $400. If you are buying used, make sure that you try the telescope on a star — to ensure that you can achieve a sharp pinpoint focus.

One last tip — the eyepiece does the job of magnifying. So look for telescopes that accept eyepieces with 1.25” diameter barrels — the standard for quality equipment. That way you can add extra eyepieces, or upgrade to better quality ones later. Here’s an online beginner telescope buying guide from the Backyard Astronomer’s Guide folks.

Bright Comet Update

Here’s an update on how to see Comet 46P/Wirtanen this week and what to expect.

(Above: The path of Comet 46P/Wirtanen through the sky for the next week, starting below the bright Pleiades Cluster at top right. Yellow dots represent 6-hour intervals.)

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! The moon will increasingly affect the night sky this week, so you should try to see the comet on the first clear night this week.

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 10:45 pm local time. Then it will set in the west at dawn.

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, and to the left, for observers in mid-northern latitudes. Tonight (Sunday), the comet will land four finger widths (or 4°) below, and a little to the left of, 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. For the rest of this week, the comet will make a bee-line towards the bright star Capella in Auriga (the Charioteer). On Wednesday night, the comet will pass a thumb’s width (1.3° below the Northern Trifid Nebula (also designated NGC1579), setting up a potential photo op for astrophotographers. Wirtanen will pass a pinky finger’s width (25 arc-minutes) above the medium-bright star Saclateni (Zeta Aur) on December 21, and then slide a finger’s width below (i.e., 1° to the east of) the very bright, yellow Capella on Sunday, December 23. In late evening the comet will be nearly overhead, perfect for seeing it through the least amount of intervening air.

The Ursids Meteor Shower Peaks

The annual Ursids meteor shower, produced by debris dropped by periodic comet 8P/Tuttle, will peak during the early hours of Saturday, December 22, when seeing up to 20 meteors per hour is possible, under dark skies. The best time to watch will be from midnight to dawn that morning. Unfortunately, a full moon on the peak night will spoil the show for Ursids meteor watchers in 2018. True Ursids will appear to radiate from a position in the sky above Ursa Minor (the Little Dipper) near the star Polaris, but the meteors can appear anywhere during the time between dusk on Friday and dawn on Saturday.

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. Try to put the full moon behind a tree or a building. 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!

(Above: The radiant for the Ursids meteor shower is in the northern sky near the star Kochab in Ursa Minor, which is also known as the Little Dipper.)

The Moon and Planets

This first half of this week will still be a good opportunity to pull out your telescope or binoculars and view the moon after dinner time. (Let your telescope pre-cool outside in a secure location for an hour or two before you look through it. Keep the lens caps on until you begin your observing, and wrap the cold telescope in a plastic bag or telescope case before you bring it inside. That will minimize any frost or dew forming on the cold parts.)

The moon was at its First Quarter phase yesterday (Saturday), so it will wax and be illuminated by slanting sunlight until Friday evening. In the meantime, the moon will move through Cetus (the Sea-Monster), then Pisces (the Fishes), and then return to Cetus’ head on Tuesday night. That night will also bring a chance to easily see the moon pass in front of, or occult, a star.

As the southeastern sky is darkening on Tuesday evening, December 18, the dark leading limb of the moon will move over the medium-bright (magnitude +4.30) star Xi Ceti (also known as Al Kaff al Hidhmah II) at approximately 5:35 pm EST. The star will reappear from behind the moon’s opposite, illuminated east limb at 6:47 pm. Sharp eyes can watch the event without aid, but binoculars or a small telescope will make it very easy. Start looking a few minutes beforehand. It’s fun to watch the star disappear suddenly as the dark moon covers it!

(Above: Between 5:35 pm and 6:47 pm Eastern time in the southeastern sky on Tuesday evening, December 18, the moon will occult the medium-bright star Xi Ceti (also known as Al Kaff al Hidhmah II). Binoculars or a telescope will aid in seeing the event, which is shown here towards the end, at 6:47 pm EST.)

Starting in mid-evening on Thursday, December 20, in the eastern sky, the waxing gibbous moon’s orbital motion will carry it directly through the Hyades star cluster, the stars that form the triangular face of Taurus (the Bull). The moon will enter the cluster at about 8 pm EST. It will be in the middle of the “V” of Taurus at about 11:30 pm. By 4 am EST, the moon will exit the cluster after passing within a finger’s width above the bright, orange, foreground star Aldebaran. By that time, the moon and Taurus will have moved over to sit above the western horizon.

The December full moon, traditionally known as the Oak Moon, Cold Moon, Long Nights Moon, and the Moon before Yule, will occur at 12:49 pm EST on Saturday. This Full moon always shines in or near the stars of Taurus, but by the time the moon rises at 5 pm local time, it will be among the stars forming the feet of Gemini (the Twins). Since the moon is opposite the sun on this day of the lunar month, the moon is always fully illuminated and rises at sunset and sets at sunrise. Interestingly, full moons during the winter months in North America climb as high in the sky as the summer noonday sun, and cast similar shadows.

Wow! The eastern pre-dawn sky is loaded with planets 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. As the week wears on, Mercury will drop lower.

By around 7 am local time, look for very bright Jupiter sitting about four finger widths below, and slightly to the left of, Mercury. On the coming weekend, Mercury will descend to pass less than a finger’s width to the upper left of Jupiter.

(Above: The eastern pre-dawn sky hosts the planets Jupiter, Mercury, and Saturn, as shown here at 7 am local time on December 17. On the weekend of December 22–23, Jupiter and Venus will be very close together.)

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 also show a crescent phase and it’s stunningly bright now!

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 on the border between Aquarius (the Water-Bearer) and Pisces (the Fishes). Mars will set in the west just before midnight local time.

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 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 8 pm local time — the best position for seeing it clearly.

(Above: The position of distant blue Neptune, with respect to Mars this week.)

Neptune met Mars last week, but the red planet is rapidly pulling away from the distant blue planet. Tonight (Sunday) Mars will be a palms’ width to the upper left of 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 Neptune about two finger widths to the upper left of the modestly bright star Hydor (Lambda Aquarii), where it’s been for quite some time.

Astronomy Skylights for the week of December 16th, 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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