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My solstice / equinox page says the 2025 March equinox occurred on the
20th at 09:01:29 UTC. This is one of those cases where rounding to the
nearest minute could go one way or the other. It would depend on the
solar system ephemeris and precession / nutation model.

I used the JPL DE441, IAU 2006 precession and 2000A nutation. A
prediction 25 years ago might have used the DE406 and IAU 1976/80. With
those tools I get 09:01:27, only 2 seconds different. Of course a
prediction 25 years in the future should use a time scale such as TT or
TAI, not UTC or UT1, to avoid error due to Earth's irregular rotation.
That's why my long term table gives times in TAI. Some web sites don't
allow for that problem. They use GMT (UT1) for predictions at remote epochs:

Solstices and Equinoxes: 2001 to 2100
http://astropixels.com/ephemeris/soleq2001.html

Table of Gregorian New Moons, Vernal Equinoxes and Nisan1 year start
dates from 2400 BC to 3100 AD
https://www.truebiblecode.com/BLCTable.html

The first is OK for this recent equinox, but the second is in error by
10 minutes.

Sun ecliptical latitude at the equinox was +0.74″, which is about its
maximum deviation from the ecliptic. Thus, an "equinox" time calculated
from right ascension or declination would likewise have near maximum
discrepancy vs. a strictly correct computation based on ecliptical
longitude. I get:

09:01:21 RA = 0
09:01:29 longitude = 0
09:02:10 declination = 0

My page mentions yet another possible definition of the equinox: the
instant when Sun apparent geodetic latitude is zero as seen from the
center of a transparent Earth. This differs from declination = 0 due to
polar motion. As explained on the page, this definition, and the one
based on declination, are not practical at high precision since the
instant of a solstice is indeterminate.

http://sofajpl.com/solstice/index.html

In the long term table, note the 2026 June solstice at 08:25:07 TAI.
It's almost certain there will be no leap second at the end of this
year, so UTC will remain 37 seconds behind TAI. Therefore, the solstice
will occur at 08:24:30 UTC. Right on the tipping point if you round to
the nearest minute!

--
Paul Hirose
sofajpl.com

   

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