Page:The Music of the Spheres.djvu/177

 No wonder that the appearance of Sirius was hailed with reverence when its rising just before the sun meant also the rise of the life-giving river and the prosperity of the inhabitants of Egypt! However, as mentioned before, with the precession of the equinoxes, the heliacal rising of Sirius has changed and its brilliant light no longer announces the rising of the Nile.

Sirius is one of our nearest stars for its light requires only 8½ years to reach the earth. Yet its nearness does not account altogether for its quite exceptional brightness, for our sun at the same distance would appear as a star of the 6th magnitude and be invisible to the unaided eye. Only two stars, as far as has yet been discovered, lie closer to the earth than Sirius. The second brightest star in the sky is Canopus, visible from the southern hemisphere, but so far away that its distance can scarcely be estimated.

Halley, a celebrated English astronomer born 1656, made the first discovery of the relative motion of the stars when he noted that Sirius had moved from the position assigned to it on Ptolemy's map of 150 We have now reliable data for discussing the proper motion of about 10,000 stars. The proper motion of stars consists of a displacement in various directions of the individual stars. Thus the configuration of a constellation may slowly change, and, although many groups of stars all travel in the same direction, there are also conspicuous instances where they move in different directions. The Big Dipper and the Southern Cross are two star groups whose stars are traveling along different courses and after a period of time extending over thousands of years, the 'dipper' and the 'cross' effect of these two constellations will have altogether disappeared.

For a while Sirius was believed to be traveling in a straight line, but soon irregularities were discovered in the great star's motion, an "undulatory progressive motion" on each side of a middle line. From a knowledge of these oscillating movements,