Fading Wirtanen
BEST TIME TO SEE: Early January
At the time of writing, the performance of comet 46P/ Wirtanen is still uncertain. Hopefully, it will have brightened enough to be seen with the naked eye while moving north through the sky during December. Its path ended 2018 in the constellation of Lynx and this is where it can be found at the start of January, 1.2° to the south-southeast of 15 Lynxis. Predicted to be a tricky mag. +5.0 object at this time, 46P arcs across the Lynx/ Ursa Major border, ending up in Ursa Major from the middle of the month.
The reason why we describe it as a tricky mag. +5.0 is that the head of the comet may have a reasonable apparent size and this will result in a low surface brightness, making it harder to see than its magnitude would suggest. At the end of the month, 46P/Wirtanen is predicted to have faded to mag. +7.8 which means
that, in theory at least, it should be possible to see with binoculars.
The Moon is full on 21 January and at this time of year the fuller phases ride high across the UK’s sky, making hunting diffuse comets somewhat
tricky. Your best chance of seeing it will be at the start of the month, as the new Moon is on 6 January. The best date for star-hopping to it will be 11 January, when 46P lies just over a degree from mag. +3.4 Omicron (k) Ursae Majoris.