Solar Time is a 'Planetary hours' clock based on the true astronomical position of the Sun as seen by an observer on earth. Sunrise, sunset, and the maximum Sun's altitude are used to calculate the beginning of the hours and to separate day hours from night hours. The observer's latitude and longitude are needed to perform the calculations. The result is a clock synchronized with nature but the duration of the hours will vary with the season. These variable-length hours are known as temporal, unequal, or seasonal hours. A planetary hours clock like Solar Time, goes further, taking into account the ancient tradition of the 'Planetary Rulers'.