What are comets?
Marcus Chown and Govert Schilling continue to Tweet the universe
Comets were believed to be stars-with-tails, or hairy stars. Name comes from Latin for hair – coma. Can be visible for weeks in night sky.
Aristotle thought comets were vapours glowing in atmosphere. Tycho Brahe realised they were further away than the Moon and cosmic in nature.
Edmond Halley realised 1682 comet followed same orbit as comets of 1531 & 1607. All same object! Predicted return of Halley’s Comet in 1758.
Comets travel around Sun in highly elliptical orbits. Periods vary wildly, from a few yrs to thousands of yrs. Halley’s Comet takes 76 yrs.
Comets are porous chunks of ice/dirt few kms across. Primitive material left over from birth of Solar System, so scientifically priceless.
When close to Sun, ice evaporates, dust particles lost. Comet grows bluish gas tail and yellowish dust tail, pushed like windsocks from Sun.
Tweeting the Universe by Marcus Chown and Govert Schilling is published by Faber.