11/25/2018 – Ephemeris Extra – Comet 46P/Wirtanen may be naked eye in December

he path of Comet 46P/Wirtanen from November 21, 2018, to January 1, 2019. The labels are month, date, and expected magnitude. On November 22nd it was observed to be magnitude 5.5, about 5 magnitudes brighter than the predictions on the chart.  Click on image to enlarge. Created using Cartes du Ciel (Sky Charts).

Comet 46P/Wirtanen will be well placed in the evening sky for observation. Though a small comet, it has a history of being active, which is not disappointing us now. It will be closest to the Earth on December16^th at 7.1 million miles (11.4 million km). 

On December 16^th the comet will be closest to the Pleiades. On the 23^rd it will appear close to the bright star Capella. After that it will become circumpolar.

Comet Wirtanen is a small short period comet of 5.44 years.  It’s orbit doesn’t come as close to the Sun as the Earth.  It’s closest to the Sun, called perihelion it which it reaches December 12th is about 98 million miles (158 million km).  The orbit extends out to nearly Jupiter’s orbit.

Checkout photos and animations of this and other comets in http://www.spaceweather.com/’s Realtime Comet Gallery.

Also check out Seiichi Yoshida’s website and his weekly information about Bright Comets: http://www.aerith.net/comet/weekly/current.html.

Here is a black on white chart that I created for our society’s newsletter of the positions of the comet when it passes the Pleiades.  The positions are for 9 p.m. EST (01:00 UT on next date) on the displayed dates. Created with Cartes du Ciel (Sky Charts).

A note about comet magnitudes

Comet magnitudes are always devilishly hard to estimate. A comet always appears dimmer than its magnitude suggests because one is comparing the brightness of a diffuse object with the point source of a star. One either has to reduce the size of the comet to almost a point or defocus the star to the size of the comet to make the comparison if it doesn’t have a tail.

A point about magnitudes. They’re like golf scores. The lower the number, the brighter the object, and the better the golf score. Blame the ancient Greek astronomer Hipparchus, who ranked star brightness from first magnitude for the brightest stars to sixth magnitude for the dimmest stars visible to the naked eye. Modern astronomers put a mathematical basis for the system saying that a magnitude difference of 5 equals a brightness difference of 100. So each magnitude step equals the 5^th root of 100 or 2.512. So a 5^thmagnitude star is about two and a half times brighter than a 6^thmagnitude star, and so on.