Comet Day (2061)
A great comet is a comet that becomes exceptionally bright. There is no official definition; often the term is attached to comets such as Halley's Comet, which are bright enough to be noticed by casual observers who are not looking for them, and become well known outside the astronomical community. Great comets are rare; on average, only one will appear in a decade. Comets are officially named after their discoverers.
Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 74–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and the only naked-eye comet that might appear twice in a human lifetime. Halley last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.
During its 1986 apparition, Halley's Comet became the first comet to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation.
These observations supported a number of longstanding hypotheses about comet construction, particularly the "dirty snowball" model, which correctly predicted that Halley would be composed of a mixture of volatile ices – such as water, carbon dioxide, and ammonia – and dust. The missions also provided data that substantially reformed and reconfigured these ideas; for instance, it is now understood that the surface of Halley is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.
The vast majority of comets are never bright enough to be seen by the naked eye, and generally pass through the inner Solar System unseen by anyone except astronomers. However, occasionally a comet may brighten to naked eye visibility, and even more rarely it may become as bright as or brighter than the brightest stars. The requirements for this to occur are: a large and active nucleus, a close approach to the Sun, and a close approach to the Earth.
The first confirmed sighting of Halley's comet was in 240 BC. The next predicted perihelion of Halley's Comet is the 28th of July, 2061.
Designed by Carol A.L. Martin, this tartan illustrations the blue and green glow of the ionized gases of a comet in the dark sky.
For more great comets in history, click one of the most recent Great Comets, Comet Hale-Bopp, 1997.