Parallax in Altitude

A term made use of in contradistinction to horizontal parallax.

Annular Parallax. The maximum value of the heliocentric parallax.

Binocular Parallax. The angular difference of an object as viewed by the eyes of the observer.

Diurnal or Geocentric Parallax. The difference between the place of a body as viewed at the same instant from the surface and from the centre of the earth. When viewed from the surface of the earth we have the apparent place, but could it be viewed from the centre of the earth we would have its true place. Thus the arc intercepted between the true and apparent places is the diurnal or geocentric parallax, which varies with the altitude of the body observed, being zero when the body is in the zenith, and having the greatest parallax when the body is O degrees, or without altitude.

Heliocentric Parallax. The difference in the place of a body as seen from the earth and from the sun; the angle drawn from the body to the centre of the earth and the centre of the sun.

Horizontal Parallax. The maximum value of the geocentric parallax--when the body observed is in the horizon, hence the term. Horizontal Parallax varies with the latitude of the observer, having its greatest value at the equator.