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Wednesday, January 23 , 2019, 5:19 pm | Fair 62º


Dennis Mammana: If You Know Where to Look, You May Catch a Comet This Week

Comet Click to view larger
Find a comet worth seeking after dark this week. (Creators.com illustration)

It’s been quite a while since we’ve seen a bright comet in our skies. Many stargazers remember the show put on by Comet Hale-Bopp 21 years ago, but there have been several other good ones since.

One is now approaching us, and while it won’t compare to any of those cosmic spectacles, it could become barely visible to the unaided eye.

Or not.

I’m referring, of course, to a visitor from space called 21P/Comet Giacobini-Zinner — affectionately known to astronomers as 21P.

21P, like all others, is one of billions of tiny icy remnants of the primordial solar system that tumble silently through space. Occasionally one of these cosmic nomads drifts inward toward the sun’s heat, and its ices disintegrate into a cloud of gas and dust around its nucleus (the “coma”). Sunlight and solar wind act as a fan and blow this material outward to create one or two tails that always point away from our star.

As compact as a comet’s tail may appear to us from Earth, its material is actually distributed over tens of millions of miles; in fact, to achieve the density of the air we breathe, a comet’s entire tail would need to be compressed to fit into the size of an average suitcase. In other words, a comet is the closest thing to nothing that’s still something!

Comet Giacobini-Zimmer is a type of comet known as a periodic comet; it passes through our cosmic neighborhood about every 6.6 years. On this passage, it will come closer to Earth than at any time during the past 72 years and any time until the year 2058.

Just how bright Giacobini-Zinner becomes this time around, however, is anyone’s guess. Comets are notoriously fickle; they can flare up at any time, or they can fade and go totally unnoticed by the average stargazer.

As noted comet hunter David Levy likes to say, “Comets are like cats. ... They both have tails, and they both do precisely what they want to do.”

Their unpredictable and ghostly nature has led people over the ages — even some today — to interpret them as cosmic harbingers of doom.

Nevertheless, it may be possible to spot this interplanetary nomad in the pre-dawn skies of early September ... if you’ve got very dark skies far from the lights of large cities. Over the next week, the comet will cross the constellation of Auriga, visible in the northeastern sky in the hours before dawn. Simply use the nearby illustration to find the star patterns in that area.

During the pre-dawn sky of Sept. 2, the comet will appear just slightly above the bright star Capella in the constellation Auriga. Unfortunately, the waning moon will also appear in the sky during this period, so binoculars or a small telescope will almost certainly be necessary.

Over the next few days, the comet will appear to drift toward the east. By Sept. 10, the comet will reach its nearest point to the Earth and the sun, and appear close to the open star cluster known as M37.

Yes, folks, the best sky show of this week will occur during the wee hours, so be sure to set your alarms!

Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at [email protected] and follow him on Twitter: @dennismammana. The opinions expressed are his own.

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