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The astronomical definition for equinox is "The instant at which the
apparent longitude of the Sun is 0deg (or 180deg)". This may or may not be
the moment at which the declination of the Sun is 0 deg. The reason being
that the latitude of the sun need not be 0 (this year it will be 0.06" at
time of equinox) and that the Earth is wobbling around a little.

According to MICA, the next equinox will be on Mar 20 at 19:17:12 ET.
Assuming 64 seconds for delta T this gives 19:16:08 UT. However, the Sun
will have crossed the equator 4 seconds earlier than this.

This subtle difference is of no significance to the navigator, but it is of
utmost importance to Rob Gendreau, who would miss the proper moment at which
to balance his eggs, if he followed an official ephemeris.

Rob Gendreau wrote:

> I always assumed the equinox was when the sun crossed the equator, but
> since taking a Cel Nav class I realize my naivete, and now assume that it
> must be more complicated than that ;) For instance, is it when the center
> of the sun crosses, or the lower limb, or what?

> Tradition has it that at the moment of the equinox you can stand an egg on
> end. So an accurate time for the equinox is essential.

   

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