Samuel,
What is a solar year? The answer differs between measurement systems.
One is the measure between occurrences of the vernal equinox, the other
is a measure of the earth orbit according to the position of stars.
Civil calendaring systems usually have a solar basis, with a time
reconciled based on the vernal equinox (or northward equinox).
Astronomical (Sidereal) calendar systems have a basis on how long it
takes the earth to orbit the sun. The positions of the stars are used
as reference.
Sidereal day = 23 hours, 56 minutes, 4.09 seconds of mean solar time.
Sidereal time is based on a sidereal day consisting of 24 sidereal
hours.
Sidereal year = 365 days, 6 hours, 9 minutes, 9.54 seconds of mean
solar time.
Sidereal year = the period during which the earth makes a complete
revolution in its orbit according to the stars.
There is a precession of the earth's axis that causes a slight deviation
in the two measurements of what constitutes a solar year.
The Gregorian Calendar algorithm closer matches the vernal equinox year,
than it does the Sidereal year. The Gregorian Calendar closer matches
the seasons over time and will take many millennia to have significant
impact on human timekeeping.
Lunar calendars and timekeeping are a separate issue.
Sincerely,
Steven J. Hathaway
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