The Commonest Comet

Encke’s Comet is swooping in, on the latest of its many returns.

Orbits of Earth and Encke.  Grid lines on the ecliptic plane are 1 astronomical unit apart; the thick one points to the vernal equinox direction.

This is the 63rd of its known visits to the inner solar system – far more than for any other comet.  It is designated 2P, second in our list of recognized periodic comets after the only one exceeding it in fame, 1P Halley.  It was discovered in 1786 by Pierre Méchain, during the surge of comet-enthusiasm caused by the successful prediction of Halley’s, but, like Halley’s, is known by the name not of its discoverer but of the thinker who interpreted its nature: Johann Franz Encke, a young mathematician whose years of work showed that several comets were incarnations of this same one.  Its very short period, 3.3 years, and the relative closeness of its orbit to Earth’s result in its having been observed so many times.  The 3.3-year period also means that each third return is close to 10 years later and roughly similar; thus, it was at perihelion on 2007 April 19 and is now heading for a perihelion on 2017 March 10.

It crossed inward over our orbit, behind us and therefore in the evening sky, on Jan. 26.  So now, getting rapidly nearer to the Sun, it is getting rapidly brighter, but is also about to become inconveniently nearer in angle to the setting Sun.

The geometry is helpful in that the angle of the ecliptic at this time puts the comet vertically up from the Sun, and it is 19° above the horizonh at the time of the picture.  It may have reached a magnitude of about 6.5, at or below the threshold for the naked eye in ideal conditions, but a target for binoculars.  The Moon fortunately is deep below the evening horizon.

At perihelion it will be just about directly between us and the Sun.  When, on its way out, it is at opposition in August it will be more distant and farther south.