Parallaxes and Eclipses
The theories of the Sun and Moon developed in the preceding chapters have the feature in common that they provide exactly geocentric co-ordinates. In other words they presuppose an ideal observer situated at the centre T of the Earth and observing the centre of the planet in the direction TP (see Figure 7.1). A real observer placed at the point A on the surface of the Earth will see P in the direction AP. Unless P is at the zenith of A, the lines of vision TP and AP will include an angle called the parallax of P (in modern terms the daily parallax). It is the difference between the observed and the geocentric zenith distances of P. If we want to know the actual position of P (the direction of AP) then it is necessary to adjust the geocentric position derived from theory with a certain correction for parallax.