|
|
|
|
|
|
|
Sidereal The interval between two successive passages of the Sun through a particular point among the fixed stars. The value is 365.2564 MSD. |
|
|
|
|
|
|
|
|
Tropical The interval between two successive passages of the Sun through the first point of Aries. The value is 365.242191 MSD. |
|
|
|
|
|
|
|
|
The term "year," without adjectives, generally is taken to mean the tropical year. The length of the civil year depends upon the accuracy of a given calendar in representing the length of the tropical year. The Gregorian calendar postulates an average length of 365.2425 MSD, which is an accuracy adequate for virtually all practical applications. |
|
|
|
|
|
|
|
|
The month is the least well defined and most troublesome unit incorporated into a calendar. There are, of course, several kinds of month: |
|
|
|
|
|
|
|
|
Draconic or nodal The period between two successive passages of the Moon through the ascending node. The value is 27.2122 MSD. |
|
|
|
|
|
|
|
|
Sidereal The period between two successive passages of the Moon through a point among the fixed stars. The value is 27.3217 MSD. |
|
|
|
|
|
|
|
|
Synodic Like sidereal, but using the Sun as a reference point. The value is 29.5306 MSD. |
|
|
|
|
|
|
|
|
Anomalistic Like sidereal, but uses the perigee as the reference point. The value is 27.5546 MSD. |
|
|
|
|
|
|
|
|
The definition of month relevant for calendar time is the synodic month. The average synodic lunar period is 29.530588 MSD. The length of a given lunation (the period between two successive new moons) varies from this value. Meeus, for example, (Astronomical Algorithms, p. 324) gives lunation durations varying from 29 days 6 hours 35 minutes to 29 days 19 hours 55 minutes for the period A.D. 19002100. |
|
|
|
|
|
|
|
|
These values, and their variations, have consequences. First, the value of roughly 29.5 days means that many calendars eventually settle on some form of alternation between 29- and 30-day months to produce an average of 29.5 days per month. |
|
|
|
|
|
|
|
|
Second, the variability in this period means that, lacking a sophisticated astronomical and mathematical understanding of lunar motion, early societies were not able accurately to predict the occurrence of new moons or other lunar phases. |
|
|
|
|
|
|
|
|
As a consequence, many early calendars depended upon observation of the actual new moon (actually, the first visible crescent) and effectively adopted a wait-and-see strategy for months, which then called for a count-forward of the waxing moon and a |
|
|
|
|
|