<news@[EMAIL PROTECTED]
> wrote:
>"Judson McClendon" <ju...@[EMAIL PROTECTED]
> wrote:
>> Actually, the fact that they're adding *any* leap seconds shows the
year
>> is longer. ;-)
>
>Longer than in 1820 (on which approximate year the definition of the
>second is based), yes, but not necessarily still changing in length.
>The long-term trend is expected to be a slowing down, as you said, but
>in the short term that has not been happening. In fact we came fairly
>close to needing a *negative* leap second, which would have caused
>great concern because (having never happened before) so many systems
>might have malfunctioned.
>
>> All this is due to gravitational interaction between the Earth and
other
>> bodies. Even we humans give the earth a wee ****ge now and again. :-)
>
>Isn't the effective increase in the earth's radius, as sea-level rises
>(due to climate change), also likely to be a factor?
That might affect the earth's rotation because of conservation of
angular momentum, but it wouldn't affect the length of a year. For
orbital calculations the mass is considered to be a point at the
center of mass of the body. Do you remember when they first orbited
the Gemini command module around the Moon at 60 miles altitude? They
noticed that its speed tended to increase and decrease slightly over
certain parts of the Moon. They realized that concentrations of mass
(or MASCONS) like mountain ranges were causing it. But except for
orbits *very* close to the surface of a large body like a planet,
this effect is negligible and ignored.
When the sea lavel rises, it's because ice previously on a continent
melted. Ice floating in the water doesn't change the sea level when
it melts (Archimedes principle). So, the sea level rising is actually
the mass getting closer to the center of the earth. That should cause
the rotational speed of the earth to increase, much like a spinning
skater pulling in her arms.
Another factor that affects the equinoxes and solstices is precession.
A spinning object that is also moving in another direction tends to
wobble, and this is called precession. This causes the North pole to
wander around a bit, and affects the seasonal position of the sun to
a minute degree. If you check an accurate table for the equinoxes and
solstices (http://aa.usno.navy.mil/data/docs/EarthSeasons.php)
you
will see they are not absolutely uniform, because of the orbital
perturbations and precession. In fact, they can't be predicted with
absolute precision far into the future.
The biggest reason the seasons do not always start on the same date is
because of the pattern of leap years causes the calendar to ****ft a
bit relative to the solar year.
--
Judson McClendon judmc@[EMAIL PROTECTED]
(remove zero)
Sun Valley Systems http://sunvaley.com
"For God so loved the world that He gave His only begotten Son, that
whoever believes in Him should not perish but have everlasting life."
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