Reduction to observation

Abstract

The latitude of the Moon, multiplied by the equinoctial shadow and divided by 12, has to be subtracted from the true longitude of the Moon if the latitude is north at sunrise. At sunset it has to be added.

If the declination is south, then the application is reversed and there is no correction when the Moon is at the centre [zenith]. The product of latitude in minutes and the declination of the point whose longitude is 90 degrees plus that of the planet in degrees, [which is] in seconds, is added to or subtracted from the longitude of the planet depending upon whether the declination and the latitude are in different hemispheres or in the same hemisphere.