It is now known that some comets require hundreds and even thousands of years to perform a revolution around the sun; Donati's comet, one of the most famous comets, which appeared in 1858 "like a plume of fire, shaped like a bird of paradise," travels on such a long circuit that it returns only every 2000 years. The brilliant comet of 1811, which remained visible for nearly ten months, has a period estimated as being around 3000 years. But the orbits of some comets are even longer than this, some traveling so far into space that they not only over-leap the boundary of Neptune but fly to a turning-post so distant that we may never see them return. More than 30 short period comets have been discovered and these have their orbits entirely within the solar system. Encke's comet, with a period of 3½ years, has the shortest period known. [Figure 143] Although it was discovered to the relief of mankind that comets have no influence upon the earth, it was also discovered that some of the planets with great mass and strong gravitational power, can exert considerable influence on a comet, in fact, enough influence to attract many of these celestial bodies out of their courses and force them to make their outer turn around a point near that planet's orbit. Jupiter has a larger family of comets than any other planet. If a planet once captures a comet it is his captive forever, unless the near approach of some planet of greater mass pulls the comet away and forces it to enclose its new master in the outer end of its orbit. [Figure 144] A large comet first appears among the stars as a dim spot of light. It may appear from any direction, for, although comets are considered members of the solar system, their orbits do not lie in the same plane as the planets. As it sweeps toward the sun with terrible velocity, which increases hourly, a long streamer of luminous gas gradually develops and floats backward from its head. A small comet often has no tail, but a large comet usually possesses a long and beautiful one, and has been known to possess two, and even half a dozen. This tail, or tails, are at their best when close to the sun, although at all times swung carefully away from the terrific flames of that luminary. Scientists almost hold their breath sometimes at the close call of comets, for the Great Comet of 1843 almost grazed the surface of the sun and many another has passed right through the outstretched banners of the corona. After darting swiftly around this brilliant center, the comet again heads into fathomless space, its long, bright tail gradually disappearing and its fierce speed relaxing as it escapes farther and farther from the sun's control.