angles made by the intersecting lines are each said to be ninety
degrees, marked thus °. The arc of a circle included between any two
of the lines is also 90°. Every circle, great or small, is divided
into these 360°. If the sun rose in the east and came to the zenith
at noon, it would have passed 90°. When it set in the west it would
have traversed half the circle, or 180°. In Fig. 20 the angle of the
lines measured on the graduated arc is 10°. The mountain is 10°
high, the world 10° in diameter, the comet moves 10° a day, the
stars are 10° apart. The height of the mountain, the diameter of the
world, the velocity of the comet, and the distance between the
stars, depend on the distance of each from the point of sight. Every
degree is divided into 60 minutes (marked '), and every minute into
60 seconds (marked ").

[Illustration: Fig. 20.--Illustration of Angles.]

Imagine yourself inside a perfect sphere one hundred feet in diameter,
with the interior surface above, around, and below studded with
fixed bright points like stars. The familiar constellations of
night might be blazoned there in due proportion.

If this star-sprent sphere were made to revolve once in twenty-four
hours, all the stars would successively [Page 60] pass in review.
How easily we could measure distances between stars, from a certain
fixed meridian, or the equator! How easily we could tell when any
particular star would culminate! It is as easy to take all these
measurements when our earthly observatory is steadily revolved
within the sphere of circumambient stars. Stars can be mapped as
readily as the streets of a great city. Looking down on it in the
night, one could trace the lines of lighted streets, and judge