Classical absolute/differential programs.

In astronomy we try to determine a non-rotating frame from analyses of the observed motions of three mechanical systems - the solar system, the galaxy and the extragalactic nebulae. The rotation of the extragalactic frame is of the order 10^-10arcsec per century, so, for all practical purposes, this frame may be regarded as having no rotation. The other two frames are model-dependent and, as such, cannot be regarded ab initio as constituting non-rotation frames of reference. These reference frames are linked by various techniques. The author discusses the so-called classical (optical) techniques, using absolute and differential methods. The instruments involved are transit instruments (meridian circles, vertical circles), astrolabes and photographic zenith tubes (PZTs).