Halley’s works fill several shelves in the library of the Royal Society. His fame is kept green by the periodical return of the wandering star known by his name.
WHAT ARE COMETS?
The modern answer to the question “What are Comets made of?” is this:
Probably the heads are a mixture of solid and gaseous matter. The tails are gaseous—the result of the volatilisation of the solid matter of the heads.
The spectroscope shows that gases appear to be a constituent of all Comets. The spectra of Comets are very similar to those of a Bunsen flame. Recent spectroscopic photographs have revealed the presence of hydrocarbons, nitro-carbons, of cyanogen and of the vapours of sodium, iron and other metals.
The connection between Comets and Meteors implies the presence in Comets of solid matter. A modern theory, voiced by Schiaparelli, is that meteor showers are broken up Comets.
The tails of Comets appear to be composed of luminous gases ejected from the head of the Comet through a solar force held to be “Light Pressure,” which causes these tails to shoot off and disperse into space at the rate of 865,000 miles an hour.
The length of some Comets’ tails has been estimated at 125,000,000 miles, while the Comets’ heads themselves are generally much larger in size than our Earth. Halley’s Comet is more than ten-fold the size of our Earth.
E. W. Maunder, of the Royal Observatory of Greenwich, a modern astronomer, has thus summarized the latest theories of the substance of Comets:
“Though the bulk of comets is huge, they contain extraordinarily little substance. Their heads must contain some solid matter, but it is probably in the form of a loose aggregation of stones enveloped in vaporous material. There is some reason to suppose that Comets are apt to shed some of these stones as they travel along their paths, for the orbits of the meteors that cause some of our greatest ‘star showers’ are coincident with the paths of Comets that have been observed. But it is not only by shedding its loose stones that a Comet diminishes its bulk; it loses also through its tail. As the Comet gets close to the Sun its head becomes heated, and throws off concentric envelopes, much of which consists of matter in an extremely fine station of division.”