Rainbow-ga has answered your question, but you might also be
interested in the effect of the difference between a calendar year and
a solar year. The solar year is about 365 1/4 days while the calendar
year is 365 days (but with an occasional leap day/year thown in to
keep the calendar year somewhat in sync with the solar year).
If I take the dates of the winter solstice provided in the noaa
reference provided by rainbow-ga and convert them to date differences
in excel, I get year-to-year differences of
365.2423611 days
365.24375
365.2423611
365.2423611
365.2423611
365.24375
365.2423611
365.2423611
365.2423611
365.24375
365.2423611
365.2423611
365.2430556
365.2430556
365.2423611
365.2430556
365.2423611
365.2430556
365.2430556
These intervals are close to the solar year (approximately the time it
takes the earth to revolve about the sun once, I believe); it appears
that the winter solstice occurs near the same time each solar year.
Side note: the solar year is defined in terms in the difference
between vernal equinoxes, so the above reasoning is somewhat circular.
Nevertheless, I believe that it takes about 365 1/4 days to revolve
about the sun, which should account for most of the year-to-year
change in the winter solstice expressed in terms of the shorter
365-day calendar year. Each year, the winter solstice occurs about 6
hours later according to the calendar; this eventually changes the
calendar day of the solstice from 12/21 to 12/22 until a leap year
brings the date back to 12/21.
Defintions of the solar and sideral years:
"Year: 1) The period during which the Earth completes one revolution
around the sun. This has several interpretations. a) Sidereal Year:
Time of true revolution around the sun--that is, the time it takes the
Earth (as seen from the sun) to reappear at the same fixed star, equal
to 365.2564 mean solar days or 365 days, 6 hours, 9 minutes, 10
seconds. b) Tropical Year: The time measured from one vernal equinox
to the next--that is, the apparent revolution of the sun through the
zodiac, equal to 365.2422 mean solar days or 365 days, 5 hours, 48
minutes, 46 seconds. This is not a constant, but only decreases by 5
seconds in every one thousand years. This is also called mean solar
year or ordinary year. c) Calendar Year: Fixed by the Gregorian
calendar of 365 days in an ordinary year and 366 days in leap year. "
The above is from
http://www.crh.noaa.gov/dtx/glossary/y.php
----
"A solar year (365.2422 days) consists of the period of time (365
days, 5 hours, 48 minutes, and 46 seconds) spent by the sun in making
its apparent passage from vernal equinox to vernal equinox. In fact, a
solar year is the time it takes the earth to make one orbit about the
sun using the instant in the spring when the sun?s center crosses the
equator as a reference point."
The above is from
http://www.bibarch.com/concepts/Calendrics/MarkingTime.htm
----
"Solar year
The period of time required for the earth to make one complete
revolution around the sun, measured from one vernal equinox to the
next. It is equal to 365 days, 5 hours, 48 minutes, and 45.51 seconds.
It is also called an Astronomical year or a Tropical year."
The above is from
http://en.wikipedia.org/wiki/Solar_year
with more information if you click on the "Tropical year" link. |
