First understood by astronomer Edmond Halley in 1705, this comet has a 76 year period, though that number varies from century to century, depending on the gravitational effects of other objects in the solar system. Its orbit, like that of all comets, is highly elliptical, and follows a retrograde path inclined eighteen degrees to the ecliptic. It's last pass by the earth came in 1986. The tail of the comet is made up of illuminated dust and ionized plasma, the former pointing back along the comet's path, the latter pointing away from the sun.