Dear Jos,
You raise an interesting point...
> According to the United States Naval Observatory
> ... the 2009 equinox falls on sept 22 at 21:18:00 UT.
> This is 61 minutes different from your value.
> Who is right?
I would use the USNO time myself but your question
isn't as easy as it seems...
There is more than one definition of "equinox". For
example there is "the mean equinox of the date" and
so on.
Here is a simple definition which turns out to need
further interpretation:
The instant of an equinox is the moment the
sun reaches one of the two points where the
ecliptic and the celestial equator intersect.
One snag is that both the ecliptic and the celestial
equator are slightly wobbly...
The earth's orbit is disturbed by the moon and by
Jupiter and is therefore not quite a plane. Hence
the perceived orbit of the sun round the earth, the
ecliptic, is not quite a plane either.
Another effect is nutation which means that the
orientation of the earth's axis wobbles about, so
the celestial poles wobble in sync, and hence the
celestial equator wobbles too.
You therefore have a wobbly ecliptic intersecting
a wobbly celestial equator. You have a choice:
1. Do you average out the wobbles and take
only long-term effects into account?
2. Do you take everything into account and
give yourself a stiff calculation?
Most published results are the result of option 2
and this means that the intervals between successive
vernal equinoxes (say) can vary by several minutes
year on year.
By several minutes I mean about 5 to 7 minutes,
definitely not 61 minutes!
You will have many long winter evenings ahead of
you to ponder all this.
Best wishes
Frank King
Cambridge, U.K.
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