Our Calendar and the Vernal Equinox
Equinox literally means "equal
night." �On the Vernal (spring) and
Autumnal (fall) Equinoxes, daylight and night are roughly the same
length of time [more on this later!] whether you live north or south
of the equator. Each hemisphere gets the "same amount of sunlight"
during the equinoxes, however, if it is spring at say 40�N latitude,
then it will be autumn at 40�S; the term, autumnal being from the perspective of
those in the northern hemisphere.
Most people used to consider March 21st and September 23rd to be
the Vernal and Autumnal Equinoxes in the northern hemisphere. However, you should
know that most media sources have, for many years now, often announced the equinox as
having occurred on the previous day. Why? Because they finally started asking astronomers,
"just exactly when is the equinox?" Well folks, " the times they are a
changin'." Not only is an Astronomical equinox actually a precise moment in
time, but due to the small differences between our calendar and what could be termed, astronomical
reality, the equinoxes are on the average coming sooner in the year every spring and fall! Some time before 2100 AD, the Vernal Equinox will almost always be on the 19th of March, and only then will it begin a gradual shift back to the 21st of the month. �Why?
�� In order to make our calendar (the Gregorian calendar) conform to the actual seasons of the earth, it must include a number of corrections (or, "leap days"). These corrections produce a cycle in which about 400 years must pass until we return to approximately the same day and time for one of the equinoxes.
At present, we already had the latest date for the Vernal Equinox on the 21st of March, 1903 [at about 11:17am PST] and it will not be until the 19th of March, 2096 [at about 5:59am PST] that we arrive at the earliest date in this cycle. So, we are already about half way from the latest to the earliest date at this time; the span of time between the two being about 54 hours for each cycle.
An astronomical definition of an equinox:
"The precise moment in
time when the plane that cuts through the earth's equator is exactly
in line with the plane of the earth's orbit around the sun -- which is
called the ecliptic."
Thus, the Vernal Equinox is the instant when the sun is directly
above the earth's equator while the 'track' (or apparent motion) of
its daily dance across the skies is slowly heading from the
south to the north.
�� This year, on the west coast of the United States, that occurred at
approximately 11:54am (PST) on the 20th of March.
The main reason, of course, for all the changes in how the sun appears
to move in the heavens, and why we experience the yearly seasons, is that old 23��
tilt in the earth's axis of rotation !
A Table of Past and
Future Vernal Equinoxes,
(rounded off to the nearest minute),
From 1992 (a leap year) through 2005 AD
All of
the following times occur on the 20th of March (PST)
| YEAR |
The Vernal Equinox |
Time
Differences |
| 1992 |
00:48 - close to 1 AM ! |
Leap Years = +24h |
| 1993 |
06:41 - after 6 am |
-(5h 53m) |
| 1994 |
12:28 - AFTER Noon |
-(5h 47m) |
| 1995 |
18:14 - after 6 pm |
-(5h 46m) |
| 1994 |
00:03 - Midnight ! |
-(5h 49m) + 24h |
| 1997 |
05:55 - about 6 am |
-(5h 52m) |
| 1998 |
11:54 - about Noon ! |
-(5h 59m) |
| 1999 |
17:46 - close to 6 pm |
-(5h 52m) |
| 2000 |
23:35 - near Midnight ! |
-(5h 49m) + 24h |
| 2001 |
05:31 - near 6 am ! |
-(5h 56m) |
| 2002 |
11:16 - no longer Noon ! |
-(5h 45m) |
| 2003 |
17:00 - actually 5 pm ! |
-(5h 44m) |
| 2004 |
22:49 - close to 11 pm! |
-(5h 49m) + 24h |
| 2005 |
04:34 - close to�5 am ! |
-(5h 45m) |
�(Note: the year 2000 -- sometimes abbreviated as "Y2K," will
�� be the first century year in 400 years that is also a leap year!)
�� As can be seen in the table above, there is a definite advance of almost � of a day (6 hours) per year in the time of the Vernal Equinox. This
is clearly�due to the difference between our 365-day calendar year and the actual
length of a tropical year (the "astronomical reality"). This is why we have leap years!�We add a whole day every 4 years to make up for the almost 6 hours�that was lost each preceding year.
�� A tropical year, however, is actually equal to about 365.2422 days!� So, for an arbitrary series of years, assuming a starting point at which the years were synchronized, we are usually introducing an�
overcompensation of 0.0312 day every four years:
1 day - (4 x 0.2422 day) = 1 day - 0.9688 day = +0.0312 day.
This equals +44.928 minutes, or an average of +11.232 minutes per year for every leap day that is added to our calendar.� If we consider 1600AD to be our arbitrary starting point, then by 1696AD, we would have had 24 leap years; and would already have almost 3/4ths of a day greater than the tropical year (+0.7488 day).� But, according to the rules of the Gregorian calendar, a century year that is not evenly divisible by 400 does not have a leap day added.� So, the seven non-leap years between 1696AD and 1704AD would lose 0.2422 day for each of those years.� Therefore, the difference between the calendar year and the tropical year in March of 1700AD would fall back to a loss of:
+0.7488 day - (4 x 0.2422 day) = +0.7488 - 0.9688 = - 0.22 day.
And by March of 1703AD, the difference would be:
- 0.22 day - (3 x 0.2422 day) = - 0.22 day - 0.7266 = - 0.9466 day.
�Finally, in 1704AD, after adding another leap day, the difference becomes:
- 0.9466 day - 0.2422 day + 1 day = - 0.1888 day; still a negative number.
As a matter of fact, the calendar year would remain behind the tropical year until the leap year of 1732AD (the difference being +0.0296 day).
Computations for the next two centuries would be similar:
The difference through 1704AD (- 0.1888 day) plus the corrections of 23 more leap years for 1708AD through 1796AD (+0.7176 day) minus 4 more years without a leap day being added (- 0.9688 day) gives us:
- 0.44 day for March of 1800AD.
And by March of 1803AD, the difference would be:
- 0.44 day - (3 x 0.2422 day) = - 0.44 day - 0.7266 = - 1.1666 day.
Finally, in 1804AD, after adding another leap day, the difference becomes:
- 1.1666 day - 0.2422 day + 1 day = - 0.4088 day, and the calendar year remains behind the tropical year until the leap year of 1860AD; with a difference of +0.0289 day.
The difference through 1804AD (- 0.4088 day) plus the corrections of 23 more leap years for 1808AD through 1896AD (+0.7176 day) minus 4 more years without a leap day being added (- 0.9688 day) gives us:
- 0.66 day for March of 1900AD.
And by March of 1903AD, the difference would be:
- 0.66 day - (3 x 0.2422 day) = - 0.66 day - 0.7266 = - 1.3866 day.
Finally, in 1904AD, after adding another leap day, the difference becomes:
- 1.3866 day - 0.2422 day + 1 day = - 0.6288 day, and the calendar year remains behind the tropical year until the leap year of 1988AD; with a difference of +0.0264 day.
The difference through 1904AD (- 0.6288 day) plus the corrections of 23 more leap years for 1908AD through 1996AD (+0.7176 day) minus 3 more years without a leap day being added (- 0.7266 day) gives us:
- 0.6378 day for March of 1999AD.
By the Vernal Equinox of 2000AD, however, there would be another leap year; the first century leap year, giving us a total of 97 leap years for the complete 400-year cycle; the difference between the calendrical and tropical years being computed as follows:
NOTE: The following is still UNDER CONSTRUCTION !
I can NOT vouch for any data below this line !!!
- 0.66 day + 0.7488 day - 0.9688 day + 1 day
= +0.12 day
and just a slight overcompensation� now of + min.;
or� .
We would see a series of differences similar to this:
-5h 48.768m, -11h 37.536m, -17h 26.304m, (-23.2512hrs. + 24.0000hrs.)or
+0.7488 hr. = +44.928 mins., then
-5h 3.84m, -10h 52.608m, -16h 41.376m, (-22.5024hrs. +
24.0000hrs.) or +1.4976hr. = +89.856 mins., then
. . . . . . .
However, you can also see that the difference between the equinoxes for
each year is still decreasing; by about 9min. every year on average.� This is due to
a phenomenon called� Nutation.
I can't help but add a note here about how this affects history:
In Catholic countries, the adoption of the new (Gregorian) calendar was
made in 1582, with the elimination of 10 days, October the 4th being followed by October
15th. It was not until 1752 that Britain and her colonies made the change from the Julian
calendar, at which time New Year's Day was finally changed from March 25th (Spring
equinox) to January 1st! In 1752, the change was made by having September 14th follow the
day after September 2nd. It's quite important to know when/if the calendar was changed in
a particular location when trying to fix certain historical events!
�� A scrap of paper which states that something happened on say New Year's Day,
could be months from the reality (according to our own calendar) until we know exactly
when and where the event was recorded!
The Gregorian Calendar
The Details of Astronomical Times
LINKS to other sites about the Equinoxes:
Back to The Starman's HomePage
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