Practical Astronomy, which comprises a description of the instruments used in astronomical observation, and of the principles and methods underlying their application.
Spherical or Geometrical Astronomy.
In astronomy, as in analytical geometry, the position of a point is defined by stating its distance and its direction from a point of reference taken as known. The numerical quantities by which the distance and direction, and therefore the position, are defined, are termed co-ordinates of the point. The latter are measured or defined with regard to a fixed system of lines and planes, which form the basis of the system.
The following are the fundamental concepts of such a system.
(a) An origin or point of reference. The points most generally taken for this purpose in astronomical practice are the following:—
(1) The position of a point of observation on the earth’s surface. We conceive its position to be that occupied by an observer. The position of a heavenly body is then defined by its direction and distance from the supposed observer.
(2) The centre of the earth. This point, though it can never be occupied by an observer, is used because the positions of the heavenly bodies in relation to it are more readily computed than they can be from a point on the earth’s surface.
(3) The centre of the sun.
(4) In addition to these three most usual points, we may, of course, take the centre of a planet or that of a star in order to define the position of bodies in their respective neighbourhoods.
Co-ordinates referred to a point of observation as the origin are termed “apparent,” those referred to the centre of the earth are “geocentric,” those referred to the centre of the sun, “heliocentric.”