Equal and Unequal Hours

There were two types of hour systems used in the past: equal and unequal hours. The unequal hours divided the day-duration by 12 and the night-duration by 12. The hours would change length at different times of the year, sometimes stretching to 90 minutes and shrinking to 30 minutes depending on the duration of the day and night, but the day and night would always be divided into twelve portions each.

The equal hours of constant 60 minutes (900 prāṇa) are based on the division of the day and night into 12 portions each on the equinox, wherein the day and night durations are exactly twelve hours each. With equal hours the day will have a different amount of hours throughout the year as the days and nights change proportion. Unequal hours gives the night and day equal hours but itself became unequal duration, while equal hours gives the day and night unequal hours but stayed uniform. The gnomen on a traditional Indian sundial made ritually before building a temple was 12 aṅgula above ground and 12 aṅgula below ground symbolic of 12 day and 12 night hours.

In modern times, we are so used to clock time that unequal hours is very foreign. But on a solar clock it is a simple adjustment of lines. Below is the difference between unequal and equal hours at 38 degrees of latitude.

Unequal hours above and equal hours below:

The ancients were able to accurately tell hours in the day with a sundial and at night were able to use an astrolabe to tell the hours by stellar movement. Star maps to keep the hours of the night are found in Egyptian tombs, and the instruments for this are found throughout the ancient world. The keeping of time was a common and valued science in the ancient world.   

A proper sundial can be accurate up to two minutes. It can also indicate the day of the month, solstices and equinox. Below is the type of sundial used by the author (created in Shadows software). It is computer generated, but could also be made with patience by marking the Sun throughout the year.

The placement of the shadow (prabhā) of the gnomen (śanku) will vary in length throughout the year. In the summer, the Sun is higher in the sky, the shadow is shorter. In the winter, as the Sun is lower in the sky, the shadow is longer. Notice below, the solstice and equinox markings indicated by the length of the shadow.