vi The Solar System and Universe 89 



path is changed by attraction either into a closed curve an 

 ellipse or into a hyperbola. Comets are thus viewed as 

 the carriers of new stores of matter and energy into the 

 solar system from remoter realms of space. Halley's comet 

 is believed to have been captured by the attraction of 

 Neptune when it was sweeping through the solar system, 

 and the other periodic comets are similarly the slaves of the 

 great planets. The planes of the orbits of comets show no 

 relation to that of the ecliptic, sometimes indeed being 

 perpendicular to it. To revert to a former simile ( 112), 

 if the Sun be compared to a large ship, and the ecliptic to 

 the surface of the ocean, steam-launches manoeuvring round 

 the ship represent the planets, all nearly in the same plane, 

 though the swell of the ocean causes them to be above the 

 mean level at one part of their evolutions and beneath it at 

 another. A comet would be represented by a diving bird 

 going round the ship by diving under the keel and flying 

 above the deck.S 



133. Nature of Comets. The tail of a comet, some- 

 times several million miles long, is greatest when near the 

 Sun, away from which it points whether the comet is 

 approaching or receding. Comets shine, according to the 

 spectroscope, partly with reflected sunlight and partly with 

 the light of glowing vapour. The density of their substance 

 is very slight, and they were long supposed to consist of 

 masses of glowing gas. Recent observations, however, make 

 it almost certain that they are swarms of very small solid 

 bodies far enough apart to let starlight pass between them, 

 and these when heated by approach to the Sun give off 

 vapour at first composed of a compound of carbon and 

 hydrogen, latterly, as the temperature is higher, of metals 

 such as sodium and iron. The particles which make up 

 comets may be only a few inches, or possibly only the 

 fraction of an inch in diameter, and they are known as 

 meteorites. 



134. Meteors. Attentive observers may see a few 

 meteors or " falling stars " on any clear night. A star 

 apparently detaches itself from its neighbours on the star- 

 dome and silently glides downward, sometimes leaving an 



