361. _Parallax of the Stars._--Such is the distance of the stars, that only in a comparatively few instances has any displacement of these bodies been detected when viewed from opposite parts of the earth's orbit, that is, from points a hundred and eighty-five million miles apart; and in no case can this displacement be detected except by the most careful and delicate measurement. Half of the above displacement, or the displacement of the star as seen from the earth instead of the sun, is called the _parallax_ of the star. In no case has a parallax of one second as yet been detected. 362. _The Distance of the Stars._--The distance of a star whose parallax is one second would be 206,265 times the distance of the earth from the sun, or about nineteen million million miles. It is quite certain that no star is nearer than this to the earth. Light has a velocity which would carry it seven times and a half around the earth in a second; but it would take it more than three years to reach us from that distance. Were all the stars blotted out of existence to-night, it would be at least three years before we should miss a single one. _Alpha Centauri_, the brightest star in the constellation of the _Centaur_, is, so far as we know, the nearest of the fixed stars. It is estimated that it would take its light about three years and a half to reach us. It has also been estimated that it would take light over sixteen years to reach us from _Sirius_, about eighteen years to reach us from _Vega_, about twenty-five years from _Arcturus_, and over forty years from the _Pole-Star_. In many instances it is believed that it would take the light of stars hundreds of years to make the journey to our earth, and in some instances even thousands of years. Proper Motion of the Stars. 363. _Why the Stars appear Fixed._--The stars seem to retain their relative positions in the heavens from year to year, and from age to age; and hence they have come universally to be denominated as _fixed_. It is, however, now well known that the stars, instead of being really stationary, are moving at the rate of many miles a second; but their distance is so enormous, that, in the majority of cases, it would be thousands of years before this rate of motion would produce a sufficient displacement to be noticeable to the unaided eye. [Illustration: Fig. 420.] 364. _Secular Displacement of the Stars._--Though the proper motion of the stars is apparently slight, it will, in the course of many ages, produce a marked change in the configuration of the stars. Thus, in Fig. 420, the left-hand portion shows the present configuration of the stars of the Great Dipper. The small arrows attached to the stars show the direction and comparative magnitudes of their motion. The right-hand portion of the figure shows these stars as they will appear thirty-six thousand years from the present time. [Illustration: Fig. 421.]