[Illustration: FIG. 10.—THE NORTH STAR AND BIG DIPPER IN SPRING.] You have, no doubt, noticed that a line joins the two end stars of the Big Dipper and the North Star in Figs. 3, 4, 10, and 11. These two end stars of the Big Dipper are called _pointer stars_, for they point directly to the North Star; that is if we draw a line with the eye through the pointer stars and produce, or continue the line, it will run into the North Star, nearly. By using these pointer stars it is easy for any one who knows the Big Dipper to be able to find the North Star on any clear night in the year, for the Big Dipper can be seen the year round. The seven stars which form the Big Dipper are not the brightest stars in the sky by any means, yet each one is a great white sun as large or larger than our own Sun. [Illustration: FIG. 11.—THE NORTH STAR AND BIG DIPPER IN SUMMER.] Now look sharply at the middle star in the handle of the Big Dipper, whose name is _Mizar_ (pronounced Me´-zar), and see if you can make out another little star whose name is _Alcor_ (pronounced Al´-cor) hugging up close to it. The Arabs who named them called these two stars the _Horse_ and its _Rider_. If you can see this little star Alcor you will have cause to shake hands with yourself, for if your eyes are good you can see it and if they are only fair to middling you cannot see it. This is one of the famous Arab tests for eyesight. [Illustration: FIG. 12.—TELLING TIME BY THE BIG DIPPER.] =How to Tell Time by the Big Dipper.=—We have seen how the Big Dipper seems to turn round the North Star and this being the case we can use the pointer stars for the hour hand of a big star clock. You must always bear in mind, though, that while the hands of a clock turn from right to left, the Big Dipper swings round from left to right; and there is another thing to be kept in mind and that is while the hour hand of a clock goes _twice_ round in 24 hours, the Big Dipper revolves only once in 24 hours, and for this reason the hand formed by the pointer stars of the Big Dipper moves only half as fast as the hour hand of a watch or clock. Each quarter of the circle, then, is equal to 6 hours and by dividing the quarter circles into 6 equal parts you can mark off the hours. The best way to do this at first is to make a large drawing of Fig. 12 on your starboard and compare it with the Big Dipper; then draw an imaginary circle round the North Star in the sky so that it will just clear the last star in the handle of the Big Dipper. With some practice you will be able to tell the time within half an hour or less. In telling the time by the Big Dipper you must remember that the stars in turning round the north pole run fast an hour every 15 days, and this makes them gain 6 hours in 3 months and so they gain a complete revolution in a year. But every time the Big Dipper makes one complete turn round the North Star, one complete day, as measured by star time, will have passed.