On 2009 May 16, at 22:03 , Brij Bhushan Vij wrote:
" REVISING Average Mean Atomic Year Value (Y2007):
According to <http://en.wikipedia.org/wiki/Tropical_year> : The current values and their annual change of the time of return to the cardinal ecliptic points are: (1) vernal equinox: 365.24237404 + 0.00000010338×a days; (2) northern solstice: 365.24162603 + 0.00000000650×a days; (3) autumn equinox: 365.24201767 −0.00000023150×a days; and (4) southern solstice: 365.24274049 − 0.00000012446×a days, where a is number of years after Y2000. I consider the YEAR length can be fixed at the Average Mean Atomic Year i.e. [365.24237404 + 365.24162603 + 365.24201767 + 365.24274049] / 4 =365.2421895575 days. Astronomers, however, have fixed the Year value, Y2000 = 365.242189669781 days. This during the past century, Y1900 was 365d 5h 48m 45s.9747 i.e. 365.24219878125 days".
Irv replies: Like I wrote before, you can't calculate the average of those 4 "cardinal" points because they are not equally spaced, so when averaged the result is not the true average length of the solar cycle.
Also, if you look again at the atomic time chart that I cited previously:
... please note that the "Tropical Year" curve (in black) cyclically varies -- it is not a fixed constant length of 365+31/128 days, although that is its value today (near year 2000 on the x-axis).
The period of the cycle is 41000 years = the Earth axial tilt cycle. During that cycle Earth's axial tilt (=obliquity of the ecliptic) varies from a minimum of about 22.6° to a maximum of about 24.2°. This axial tilt cycle is both caused by and stabilized within these limits by Moon. The cycle is not perfectly predictable, because ice mass depression of Earth's poles and the resultant equatorial bulge interact with Moon to produce long-term variations.
The problem with using an average solar year length for any calendar is that there is no observable astronomical event available to evaluate or verify calendar drift.
