Page:Popular Science Monthly Volume 4.djvu/306

292 Merak, Phegda, Megrez, Alioth, Mizar, and Ackaïr. The last is the name given to the foremost horse. Persons possessed of good visual powers can discern above the second horse, Mizar, a small star which is called the Postilion. But these names are seldom employed in our times, the usual custom being to designate the seven principal stars of Ursa Major by the first seven letters of the Greek alphabet, as shown in the diagram. All these stars are of the second magnitude, except Delta (Fig. 1), which is of the third.



In the diagram, the arrows show the mean direction in which each of the seven stars moves. It will be seen that, of the seven, the first and the last, Alpha and Eta, are moving in a direction contrary to that of the other five. It must be added that they have not all the same velocity. Eta, for example, moves rapidly; Epsilon slowly; and so on with the others.

The quantity of their annual proper movements in right ascension, and in polar distance, is given for each of the seven stars in the following table:

In consequence of these proper movements, the relative distances between the stars of the constellation are ever changing. But, as this change only amounts to a few seconds in a century, many centuries must elapse before it is perceptible to the naked eye. Our human generations, our dynasties, nay, even our nations, are not sufficiently long-lived to measure this change.

We have here to deal with astronomical quantities, and, to