Missing from the astronomical calendar I wrote about earlier this year was the possible appearance of two comets, one of which may represent a spectacular event. Several readers called my attention to that omission, which I address here.

First, let’s agree on what a comet is. It is a nucleus of ice and rock that ranges in diameter from a few hundred meters to several kilometers. When it comes within sight, its local atmosphere (called a coma) appears white and it often sports a white tail. It is this coma and tail that differentiate comets from asteroids.

I used to think that the tail followed the comet as it sped through space, but that is not true; the comet’s tail points directly away from the sun. It can stretch for hundreds of millions of miles. Although we usually notice only one tail, there are actually two: the most apparent is a dust tail, the other is an ion tail made up of gases.

Comets are attracted by the sun’s gravity and most, like earth, follow elliptical paths around the sun. The comet’s path, however, stretches far out into outer space and thus nears the sun rarely. The most famous, Halley’s Comet, does so only every 75 years. Its return in 1986 was a disappointment, however, because it was barely visible.

Comets are not rare. In fact, more than 4,000 have been identified and millions more are estimated to be in the outer reaches of our solar system. But only about one a year is dimly visible to the naked eye. The most recent widely observed comet was Hale-Bopp, which was seen for 18 months, coming closest to the sun in April 1997.

This year, two already identified comets could not only be visible but might be the brightest in many years. Notice that I said “could.” As David Levy tells us, “Comets are like cats; they have tails and they do precisely what they want.” Often as they approach the sun their visibility is markedly reduced as they are broken up by the force of solar gravity.

The first comet this year is variously known as Comet C/2011 L4 to astronomers, Comet Pan-STARRS for the discovering team (Panoramic Survey Telescope and Rapid Response System) or Comet Wainstock for Richard Wainstock, the Pan-STARRS member who first confirmed its existence. It was first seen in 2011 and has been followed with powerful telescopes since then. Comet Wainstock is already visible in the Southern Hemisphere. A report from observers there says it shines at about magnitude 4.5 and it might reach magnitude 1 or 2. (Magnitude 4.5 is the brightness of the dimmest visible stars; magnitude 1 or 2 is a little brighter than the Big Dipper stars.)

On March 10 this comet will pass the sun and, assuming it continues to be visible, it will appear in the evening sky near where the sun has set. It will be best seen then, but it will continue to be visible through mid-April.

The second is named Comet C/2012 S1, Comet ISON (International Scientific Optical Network) or Comet Nevski-Novichonok for its Russian discoverers, Vitali Nevski and Artyom Novichonok.

Comet Nevski-Novichonok could prove truly spectacular. It is scheduled to be first visible to the naked eye in early November in both the Northern and Southern Hemispheres and will pass very close to the sun on Nov. 28. If it survives this encounter, it could be brighter than the full moon and even be visible in the daytime sky. It could continue to be visible until January 2014. This comet will also be seen first near where the sun has set.