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telescopic stars do not have the slightest change in their relative positions, but remain as if fixed on the celestial sphere from year to year. Now and then, however, an exception is found. A very bright star is probably nearer to us than the fainter ones, and if a star shows any change in its position, the astronomer may proceed to measure and determine its parallax.

So far as has yet been determined, the nearest star to us is Alpha Centauri, a star of nearly the first magnitude, in the southern hemisphere. The parallax of this star is 0.75". By the rule we have given, its distance will be nearly 275,000 times that of the sun. Such a distance transcends all our power of conception over and over again. A crude idea of it may be obtained by reflecting that light itself, the speed of which we have already described, would require more than four years to reach us from this star. We see the latter, not as it is now, but as it was more than four years ago. At such a distance not only does the earth's orbit itself vanish away to a point, but a ball as large as the whole body of Neptune would be barely visible to the naked eye as the minutest possible point.

The next star in the order of distance is supposed to be about one half as far again as Alpha Centauri, and there are some half dozen others, within three or four times its distance. In all, the parallaxes of about one hundred stars have been determined with more or less exactness; but even in these cases the parallax is sometimes so small that we cannot be sure it is real. It seems likely that only about fifty stars are within seven times the distance of