Definition of prime vertical in English:
A great circle in the celestial sphere passing through the zenith and the east and west points of the horizon.
- Regiomontanus, however, is based upon an equal division of the equator rather than the prime vertical - it is the same method as that suggested by Morinus, but bows to convention by commencing from the ascendant.
- For the observations closer to the meridian than the prime vertical, make the calculation of latitude with the estimated longitude.
- For star observations a star should be selected which will be about three hours from the meridian and near the prime vertical at the times of observation.