Said of the method of calculating the time of an event from an Arc of Direction. The Ptolemy unit is 1° of R.A. for each year of life. Later authorities divided the arc by the diurnal increment of R.A. made by the Sun on the day of birth. Others employ a mean arc of 1° of RA., as the equivalent of the mean motion of the Sun, thus adjusting the circle of 360 degrees to a year of 365¼ days. A method employed by Worsdale, Simmonite, Morrison and others, consists of adding the Arc of Direction to the Sun's R.A. at birth, taking as years the number of days after birth at which the Sun reaches this Advanced R.A. The method employed by Naibod (q.v.) is to determine the arc in R.A., and consider each degree equal to 1 year, 5 days, 8 hours - a minute equal to 6 days, 4 hours. v. Directions.
ASTROLABE ENCYCLOPEDIA
