Page:Astronomy for Everybody.djvu/353

As a general rule the starry heavens may be taken as a symbol of eternal unchangeability. The proverb-makers have told us in all time how everything on the earth is subject to alternation and decay, while the stars of heaven remain as we see them, age after age. But it is now known that, although this is true of the great majority of the stars, there are some exceptions. These are so little striking that they were never noticed by the ancient astronomers.

The first person in history to observe a change in a star was one Daniel Fabritius, a diligent watcher of the heavens, who lived three centuries ago.

In August, 1596, he noticed a star of the third magnitude before unknown in the constellation Cetus, which soon faded away again, and disappeared from view in October. In subsequent years it was found to show itself at regular intervals of about eleven months.

Two centuries elapsed before another case of the kind was known. Then it was found that the star Algol, in Perseus, faded away from the second to the fourth magnitude for a few hours at intervals of a little less than three days.

Early in the nineteenth century other stars were found to be subject to a more or less regular variation of their