THE CELESTIAL SPHERE.
In the second method, all measurements are referred to the centre of the earth, and thus the observer's position on the earth's surface does not interfere with subsequent comparison of the observa tions made. This system of measurement depends on the small magnitude of the earth's radius when compared with the distances of the bodies that arc being observed. As a first illustration, consider the directions in which a couple of men standing side by side must look to see the same object at a distance of five miles. Certainly there is an angle (T of a degree approximately) between their lines of sight, but the two directions are for all would take a year in travelling, the star's distance is termed a light-year. As far as position in the sky is concerned, the stars might all be at an equal linear distance away, apparently attached to the inside of a large imaginary sphere, rotating once every twenty-four hours (sidereal time) round the earth's axis. This imaginary
sphere, once believed to be a reality, is very useful for reference, and is called the Celestial Sphere.
practical purposes parallel. Suppose now that two observers, one at the north pole of the earth and the other at the south, point their telescopes at the same part of the sun's surface, the angle between the directions of the instruments, since the sun is 92 millions of miles away, would be less than of a degree. Now, multi tudes of the stars are so far away that, instead of taking the mile as a unit of measurement, it is more convenient to take half the diameter of the earth's orbit, i.e. the distance from the earth to the sun. This unit is too small for the more distant stars, and their distances are reckoned in light-years, If the light coming from a given star to the earth