8. Calendars: Solar vs. Lunar

The earth’s rotation, the apparent motion of the Sun and the apparent motion of the Moon, serve as a foundation for determining time. However, all attempts to combine these motions into a single system to determine time and to establish a calendar have always led to complications. The yearly cycle of the solar motion provides an unaided time keeping in years. However, as the earth does not make an even number of full rotations in one orbit around the Sun (it makes about 365 ‘A rotations), it is periodically necessary to adjust the calendar to make up for the accumulated extra quarter days. This is why there are leap years. Nevertheless, the division of the Solar year into months and weeks is artificial. The primary difficulty of a purely lunar calendar is that a year of 12 lunar months falls short by about 11 days with respect to a seasonal (solar) year.

Thus, Lunar months do not keep a fixed relationship with the seasons with which so much human activity is tied very closely. It takes 235 Lunar months, which is equal to 19 solar years of 365 1/4 days for the Moon to come back to its original position again. Thus, any correction to keep the lunar months tied to the seasons is extremely difficult unless an adjustment of one month to every three years is made. Overlooking this difficulty a lunar calendar provides a much simpler system of time keeping without external aids, i.e. watches, diaries, calendars etc. The main stumbling block with Lunar months is when to begin the month, for advance preparations of calendars that will be consistent with actual visibility of the Moon. There are two obvious solutions:

  1. A purely calculated method for the start of the Lunar month based on the exact times when the Moon and the Sun have the same right ascension, i.e. the conjunction. This time is extremely accurate as it is independent of any terrestrial locations.
  2. To start the Lunar month one day after the day on which the Sun and the Moon have the same right ascension. This will fix the age of the new Moon to 24 hours with a high degree of probability as to its visibility. There is no possibility of predicting the exact time of its appearance, as the measurements are not independent of terrestrial locations. It is also important to keep in mind weather conditions which might affect visibility and which are beyond human control.