A variable star is one whose brightness changes from time to time. A great many variable stars are known, but very few of them can be seen with the naked eye. There are different reasons given for a star varying in brightness. Our Sun is a variable star, and we are told that this is due to his spots. Another type of variation is perhaps produced by pulsations--the star periodically expanding and contracting. Again a double star formed of two bright stars which revolve round each other, as many double stars do, may eclipse one another, and this would cause a change in brightness. Here, then, are three good reasons for a star being variable. The following are a few of the _variables_ which can be seen with the naked eye: _Betelgeux_, in Orion. _Alpha Cassiopeia_, that is, the brightest star in Cassiopeia. _Beta Lyra_, that is, the second brightest star in Lyra, and _Beta Pegasus_, that is, the second brightest star in Pegasus. _Mira_, in Cetus. _Algol_, in Perseus. APPENDIX K INVISIBLE OR DARK STARS Stars are born, live and die, just like human beings. All the stars, including the Sun, are either in the process of making, are at their brightest brilliancy, are dying out, or are cold and dead or have not yet become bright. Procyon and Sirius is each attended by a comparatively dark companion. Procyon and Sirius both move slowly back and forth a short distance in the sky and this motion was attributed to the pull of satellites long before these bodies were discovered with the telescope. APPENDIX L THE EQUATION OF TIME TABLE OF THE EQUATION OF TIME --------+---------+---------+---------+---------+---------+-------- Day of | January |February | March | April | May | June Month | | | | | | --------+---------+---------+---------+---------+---------+-------- |_m_ _s_ |_m_ _s_ |_m_ _s_ |_m_ _s_ |_m_ _s_ |_m_ _s_ 1 |S 3 31 |S 13 44 |S 12 37 |S 4 6 |F 2 56 |F 2 30 6 |S 5 49 |S 14 15 |S 11 33 |S 2 37 |F 3 27 |F 1 41 11 |S 7 59 |S 14 27 |S 10 19 |S 1 14 |F 3 45 |F 0 44 16 |S 9 49 |S 14 19 |S 8 57 |F 0 4 |F 3 48 |S 0 18 21 |S 11 24 |S 13 52 |S 7 28 |F 1 12 |F 3 38 |S 1 23 26 |S 12 39 |S 13 10 |S 5 56 |F 2 10 |F 3 15 |S 2 27 31 |S 13 35 |S 12 13 |S 4 24 |F 2 56 |F 2 39 |S 3 38 --------+---------+---------+---------+---------+---------+--------