The Moon Moves into Late Night, Mars Remains, and we Welcome Comet Wirtanen!

Bright Comet Alert

For some time now, astronomers have been eagerly awaiting the return of a periodic comet named 46P/Wirtanen. This comet traverses the solar system like a commuter train — shuttling between the orbits of Jupiter and Earth on a 5.4 year-long cycle. But this December’s return is a special one for star-gazers throughout the world.

(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.)

When any comet reaches perihelion, its closest approach to the sun, it becomes the most active — growing a halo (or coma) around its “dirty snowball” core and sporting one (or two) tails composed of released gases and dropped particles. Comet Wirtanen’s perihelion is only 6% farther from the sun than Earth’s orbit. Most of the time, Earth is somewhere else when the comet passes perihelion. But this year, Earth will be sitting nearby when that event occurs on December 12. And we’ll be even closer to the comet, only 11.5 million km from it, a few days later on December 16!

What all this means is that we are hopeful that Comet 46P/Wirtanen will appear larger and brighter this year. It might even become bright enough to see without binoculars at locations away from artificial lights, especially when the moon is out of the night sky. We can start enjoying the comet now, even while it is still brightening. By the end of this week, the moon will be waning to a crescent and rising after midnight, leaving the sky darker for comet hunting.

Comets move faster across the sky when they are near perihelion. This week, the comet will be gradually shifting higher, passing through the southeastern section of the dim constellation of Cetus (the Whale). By 10 pm local time, the comet will be directly over the southern horizon. At the latitude of Toronto, it will be about two fist diameters high. Unfortunately, there aren’t any bright objects nearby to help you find it right now, but that will change in the coming weeks when the comet will be MUCH higher in the sky and passing through Taurus and then Auriga.

The Moon and Planets

This week finds the moon waning and rising later in the night after the recent Full Frost moon. Tonight (Sunday), the moon will be among the legs of Gemini (the Twins). When the waning gibbous moon rises at about 9:15 pm local time on Tuesday, it will be located less than a palm’s width below the large open star cluster known as the Beehive and Messier 44 in Cancer (the Crab). The moon and the cluster will both fit within the field of view of binoculars, or a telescope at low magnification — but the bright moon will overwhelm most of the stars.

On Thursday, the moon will reach its Last Quarter phase, when it will be half-illuminated on its western side — toward the pre-dawn sun. At its last quarter phase, the moon rises around midnight and remains visible in the southern sky half of the sky all morning. After this phase, the waning moon will traverse the last quarter of its orbit around the earth, on the way to next week’s new moon.

Next Sunday morning in the southeastern sky, the elderly waning crescent moon will sit above the very bright planet Venus, after that planet rises at about 4 am local time. Independent of the moon’s monthly visits near her, Venus will be situated in that part of the sky for quite a while. It’s located only about four finger widths to the lower left of the bright white star Spica in Virgo (the Maiden) and drawing farther away from that star every morning. Viewed through a telescope, Venus will exhibit a slim crescent phase similar to the moon’s.

We are quickly running out of evening planets to look at. Mercury and Jupiter are crossing close to the sun now and will soon become pre-dawn objects. Yellowish Saturn is getting closer to the sunset, too. This week, the ringed planet will already be less than a fist’s diameter above the southwestern horizon by the time the sky gets dark at 6 pm local time. The increased amount of Earth’s atmosphere we have to look through when objects are that low will make Saturn “swim” in your telescope. (The Earth’s blanket of air is thinnest directly overhead.)

Thank goodness for Mars! Mars will remain visible, even while it shrinks in size and brightness, until next May. This week, the reddish planet will shine as the brightest object in the lower part of the southern evening sky among the stars of Aquarius. It will set in the west at about midnight local time.

Mars’ orbital motion has been carrying it eastward, directly towards distant Neptune. The two planets will get very cosy next week. Tonight, Mars will be located a generous palm’s width to the lower right (west) of Neptune. Next Sunday night, Mars will be only half that distance away from the blue, ice giant planet. All week long, Neptune will be visible after dusk. It will reach its highest point over the southern sky (and at the best viewing conditions) at about 6:30 pm local time, and then set in the west at about midnight.

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 will make seeing it easier. Look for Uranus about 1.5 finger widths to the upper left (east) of the modestly bright star Torcular (Omega Piscium). That star sits a generous palm’s width above the “V” where the two starry cords of Pisces (the Fishes) meet. By late evening, Uranus will be high enough in the southeastern sky to see it clearly.

Don’t forget to look for the major main belt asteroid Juno. It recently reached opposition, causing it to appear at its brightest and largest for this year. This week, the magnitude 7.45 object will be visible in binoculars and small telescopes all night long after it rises in the east at 6 pm local time. Juno will be positioned about two fist diameters from the bright stars Aldebaran in Taurus (the Bull) and Rigel in Orion (the Hunter). It will reach its highest position, about halfway up the southern sky, just before midnight local time.

Astronomy Skylights for the week of November 25th, 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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