Big, bright moons are comet killers. Those of you seeking comet 46P/Wirtanen, now a naked-eye sight from rural areas, have probably noticed the waxing moon creeping up in the comet’s direction the past few nights. Some comets are compact and bright, but 46P is large and diffuse and best seen in a moonless sky. Almost everything other than the moon and planets is best seen in a moonless sky!
I’ve prepared a fresh map to help you follow our fuzzy friend through Christmas and a table of sorts showing when the comet will be visible in a dark, moonless sky. These times will vary somewhat depending on your location, so use them as guidelines then check the moonrise and moonset calculator to find exactly when the moon sets where you live. Because the moon has been gaining on the comet (as it moves east in its orbit), you’re going to have to stay up later and later for moonset.
Best comet viewing times. If you go out after the moonset times, the sky will still be dark and ideal:
- Tonight (Dec. 12) — Anytime. Moon’s not a problem.
- Thurs., Dec. 13 — Moon starting to brighten the sky. Sets 10:30 – 11 p.m. local time.
- Fri., Dec. 14 — Moon at first quarter phase (half-lit). Sets 11:30 to midnight. Some light but still not a comet-quencher.
- Sat., Dec. 15 — Moon sets 12:30 – 1 a.m. Sunday morning.
- Sun., Dec. 16 — Moon sets 1:30-2 a.m. Monday morning. Getting bright now.
- Mon., Dec. 17 — Moon sets 2:30-3 a.m Tues. morning.
- Tues., Dec. 18 — Moon sets 4 a.m. Weds. morning.
- Weds., Dec. 19 — Probably the final moonless night for diehard observers. Moon sets around 5 a.m., and the comet is low in the northwestern sky.
Dark skies return during early evening hours starting Dec. 24. While it’s always exciting for long-time comet observers to see one with the naked eye, the average person won’t be visually impressed by this fuzzball. It doesn’t show a tail (except a short stub in a telescope) and it’s basically little more than soft, diffuse glow. You could compare it to a painting done in minimalist style vs. a Van Gogh. As so often happens in astronomy, it’s what you bring to the object that helps you to appreciate it. Knowing you’re looking at a kilometer-wide spinning ball of dusty ice as ancient as anything in the solar system is just cool. And it will soon be less than 30 times the moon’s distance as it swoops by Earth.