NavList:

   

Reply

> *From: *Robert H. van Gent
> *Date: *2024 Sep 21, 21:32 -0700
>
> The autumnal equinox, defined as the moment when the apparent geocentric
> longitude of the Sun equals 180 degrees, will occur nearly 32s earlier,
> at 12h 43m 40.82s.

My solstice and equinox page says the 2024 September equinox occurred at
12:43:40 UTC. The JPL Horizons web calculator agrees. I have not tried
for precision beyond 1 second of time.

http://sofajpl.com/solstice

https://ssd.jpl.nasa.gov/horizons/app.html#/

Ecliptic latitude of the Sun is +0.60″, so the discrepancy between
declination zero and ecliptic longitude 180 is greater than average.

IERS Bulletin A predicts UT1-UTC will increase from +0.06 to +0.10
during the next 365 days. In other words, Earth will rotate a little
faster than the atomic second. If correct, we will go another year
without a leap second.

Note my long term table says the 2025 March equinox is 09:01:29 UTC
(after you subtract 37 s to convert from TAI to UTC). If you round to
the nearest minute, the correct direction to round may be too close to
call. The precise time depends on your ephemeris and precession /
nutation model. I think plus or minus two seconds of time is a
reasonable discrepancy.

The 2026 June solstice is even worse. Unless we get a leap second,
TAI-UTC will remain 37 s, and the solstice will fall exactly in the
middle of a UTC minute.

A few years ago Capitaine and Soffel ("On the definition and use of the
ecliptic in modern astronomy") noted, "It should be clear that the use
of an ecliptic will not be required for providing a future
semi-analytical precession-nutation solution... While the equinox (and
the tropical year) will always have some value for the seasons, the
organisation of everyday life and the calendar, the geometric,
kinematical and dynamical uses of ecliptic in modern astronomy are now
limited to uses for continuity with historical references and parameters."

http://arxiv.org/abs/1501.05534

So, future precession / nutation models may not provide a route to
ecliptical coordinates. But in that case, legacy models will enable us
to calculate solstices and equinoxes with all necessary accuracy.

--
Paul Hirose
sofajpl.com

   

Reply