progressive precession

My question started with astrology but the answer concerns astronomy. I
wanted to know the exact hour when the sun transits the degree and
minute of my birth. I didn't expect it to be the same time. I
understand the concept of precession, of the earth's axis and orbit.
What I didn't expect was to find that the return hour-and-minute is
later some years and earlier others. These are my data:
4/14/06-14:28hours
4/14/07-20:37hours
4/14/08-02:38hours
4/14/09-08:56hours
4/14/10-15:06hours
-and so on. The net precession over 4 years is about 1/2 hour. What I
don't understand is the irregular progression from year-to-year. Is it
a problem with my astrology program or a law of nature? The other
bodies also have a similar uneven progression. If it is too complicated
to answer in a newsgroup reply, how to articulate an Internet search
query would be a great help.

"wahid" wrote in message
oups.com...
My question started with astrology but the answer concerns astronomy. I
wanted to know the exact hour when the sun transits the degree and
minute of my birth. I didn't expect it to be the same time. I
understand the concept of precession, of the earth's axis and orbit.
What I didn't expect was to find that the return hour-and-minute is
later some years and earlier others. These are my data:
4/14/06-14:28hours
4/14/07-20:37hours
4/14/08-02:38hours
4/14/09-08:56hours
4/14/10-15:06hours
-and so on. The net precession over 4 years is about 1/2 hour. What I
don't understand is the irregular progression from year-to-year. Is it
a problem with my astrology program or a law of nature? The other
bodies also have a similar uneven progression. If it is too complicated
to answer in a newsgroup reply, how to articulate an Internet search
query would be a great help.


Can you define your terms more precisely? What exactly
do you mean by the degree and minute of your birth?
The position of the Sun (presumably in equatorial
coordinates) at that time?

The sidereal year is 365.256363 days in length.
The Julian year is 365.25 days.

After four years the difference in the two amounts
to 4*(0.006363day) = 36.6 minutes. Perhaps this is
what is turning up in your calculations.

Respectfully, check dates yourself; that would tell me if I have a
problem with my program. You know the y/m/d/h/m of your natal sun. What
is the y/m/d/h/m of the sun at the same degree/minute the following
year, the next, and so on. It's not a regular progression, according to
my data.

"wahid" wrote in message
oups.com...
Respectfully, check dates yourself; that would tell me if I have a
problem with my program. You know the y/m/d/h/m of your natal sun. What
is the y/m/d/h/m of the sun at the same degree/minute the following
year, the next, and so on. It's not a regular progression, according to
my data.


Sorry, I haven't time to lay out the calculations
(I would do them manually, not with a program).

But the difference between the sidereal year and
Julian year may be a clue to the reason for the
progression you've noticed, keeping in mind that
there will be period corrections every leap year.

wahid wrote:

My question started with astrology but the answer concerns astronomy. I
wanted to know the exact hour when the sun transits the degree and
minute of my birth. I didn't expect it to be the same time. I
understand the concept of precession, of the earth's axis and orbit.
What I didn't expect was to find that the return hour-and-minute is
later some years and earlier others. These are my data:
4/14/06-14:28hours
4/14/07-20:37hours
4/14/08-02:38hours
4/14/09-08:56hours
4/14/10-15:06hours
-and so on. The net precession over 4 years is about 1/2 hour. What I
don't understand is the irregular progression from year-to-year. Is it
a problem with my astrology program or a law of nature? The other
bodies also have a similar uneven progression. If it is too complicated
to answer in a newsgroup reply, how to articulate an Internet search
query would be a great help.

The effect you're seeing has very little to do with precession, but
is mostly due to the mismatch between the 365-day calendar year and
the 365.2422-day tropical year--most of which gets corrected for each
leap-year. Specifically, any four calendar years (that don't include
a century-year indivisible by 400) contain a total of 1461 days,
while there are about 1460.97 days in four tropical years: the
discrepancy is therefore about three-quarters of an hour every four
years. (Most of this error is corrected for by omitting a leap-year
in three of every four centuries, which was introduced with the
Gregorian reform to the Julian calendar.)

Over the short term, however, in three of every four years the time
the Sun reaches a given longitude advances by about 6 hours, falling
back by about 18 hours in those yearly intervals that contain a
February 29th. Note that the same is true of e.g. the vernal equinox,
which by definition is unaffected by precession. (That is, while of
course its position moves WRT the stars, its ecliptic longitude is
fixed at 0°, when the Sun appears to reach the point where the
ecliptic crosses the equator from south to north.)

--
Odysseus

Then it is the calendar. Thanks. I don't care what they say about
astrology. It's the best damn watch I ever owned.