ECLIPSING BINARY STARS Eclipsing Binaries are stars which vary in brightness periodi cally. The most famous of these is Algol or (3 Persei, and the explanation of the changes is that there are two stars revolving round one another in an orbit whose plane intersects the earth, or does so very nearly. Thus from time to time one star passes in front of or behind the other as seen from the earth and the light from one of them is temporarily lost to us. From the way in which the light from the double systems varies it is possible to calculate the shapes and relative sizes of the components of the binary system and to obtain certain other information (see STAR).