Comet 46P/ Wirtanen pushes north
BEST TIME TO SEE: 15-18 December
Comet 46P/Wirtanen passes relatively close to Earth during the middle of the month. Early predictions for Wirtanen had it peaking around 4th magnitude, making it an easy naked-eye object. More recent observations have tempered these expectations and Wirtanen may only reach mag. +5.5, barely scraping past the naked-eye threshold.
It’s important when describing comets to keep an open mind. They may obey predictions or they may not. The only way to be sure is to go outside and have a look.
Wirtanen will pass our planet by 0.078 AU mid-month. This will also impact on its brightness because the head of the comet is likely to appear quite large and consequently of low surface brightness.
If this all sounds a little negative, it’s not meant to. What’s important when going outside to view 46P/Wirtanen is to have the right expectations, because 46P should
still provide a number of interesting viewing and photographic opportunities. On the night of 9/10 December, for example, it’ll be located approximately 8° northwest of mag. +7.7 minor planet
3 Juno. Between 15 and 18 December it’ll pass between the Pleiades and Hyades open clusters. On the night of 23/24 December, it will be located very close to bright star Capella (Alpha (_) Aurigae), presenting an easy way to locate it.
The natural enemy of comets is the Moon, its light filtering out the diffuse detail in a comet’s head and tail. During December, the Moon will interfere more towards the end of the month. The period 1-18 December should be fine and, as long as you’re prepared to wait until the early hours for moonset, viewing through to 21 December should be possible. With careful planning, viewing early evening before moonrise from 24 December through to early January will mean you’ll only lose sight of the comet because of the Moon for a couple of days.
Wirtanen ends the month in the faint constellation of Lynx. By coincidence Lynx will also play host to another comet, 38P/ Stephan-Oterma. This is expected to be fainter than 46P/Wirtanen, starting December at mag. +9.2 and ending it at mag. +9.9, so telescope friendly.