Comets are members of our Solar System, usually with highly elongated orbits, which become visible near the Sun. They have bright heads and diffuse tails of variable length which always point away from the Sun. Comets can be divided into two categories: those which are gravitationally bound to the Sun like the planets and travel in elliptical orbits, the periodiccomets, and those which do not seem to be bound to the Sun but appear once and then shoot off into space never to be seen again, the paraboliccomets. Routine PCOMET deals with the latter sort; you can use routine ELOSC (7500) for periodic comets.

The approximate position of a parabolic comet can be found from its parabolic orbital elements. As the name suggests, the orbit is a parabola and needs one less element to define it than does an ellipse. Subroutine PCOMET (7900) performs this task, taking the five parabolic orbital elements and calculating the ecliptic coordinates of the comet for a given instant. This is input in the usual way via DY, MN, YR, returning with the ecliptic longitude, EP</b., the latitude, BP, and the distance of the comet from the Earth, RH. These quantities have all been corrected for light-travel time. The routine also returns the instantaneous values (i.e. not corrected for light-travel time) of the heliocentric ecliptic longitude, L0, latitude, SO, radius vector, P0, and distance from Earth, V0.