According to Newton's law of motion, all bodies traveling in elliptical orbits move faster at certain portions of their orbits, and slower in others. It is therefore important to observe whether the travel between two successive days is greater or less than their mean motion. In erecting a Figure for a specified hour it is necessary to reduce this to hourly motion, to determine the exact position occupied by the faster-moving bodies.
To Reduce Daily Motion to Hourly Motion Since 1° or 60 m., divided by 24 h. equals 2½; therefore: degrees per day X 2½ = minutes per hour; and minutes per day X 2½ = seconds per hour. Thus: Moon's travel of 14° p. d. X 2½ = 35' p. h.; and 24' less p. d. = 1' less p. h., or 34' p. h.; Mercury's travel of 2° 7' p. d. (computed as 2° X 2.5 = 5'; and 7' X 2.5 = 17.5") gives a rate of 5' 17.5" p. h.
