Prominently document that UT1, not UTC, is most appropriate for ancient dates

I am getting very strange results for sunset, the older the date, it seems to be out by a few hours, (Expected value from Starry Night software)

Got 2020-06-01T16:05:01Z, expected 2020-06-01T16:05:00Z, diff mins 0, alt -0.83
Got 1800-06-01T16:03:42Z, expected 1800-06-01T16:04:00Z, diff mins 0, alt -0.83
Got 1000-06-01T16:31:04Z, expected 1000-06-01T16:05:00Z, diff mins 26, alt -0.83
Got 0200-06-01T18:23:57Z, expected 0200-06-01T16:00:00Z, diff mins 143, alt -0.83
Got -400-06-01T20:14:49Z, expected -400-06-01T15:56:00Z, diff mins 258, alt -0.83
Got -800-06-01T21:59:03Z, expected -800-06-01T15:53:00Z, diff mins 366, alt -0.83

To reproduce:

load = Loader(path.as_posix())
ts = load.timescale()
ts.julian_calendar_cutoff = GREGORIAN_START
eph = load('de431t.bsp')
babylon = Topos("32.55 N", "44.42 E")

def altitude_of_sun(t0):
    b = eph['earth'] + babylon
    apparent = b.at(t0).observe(eph['sun']).apparent()
    alt, az, distance = apparent.altaz()
    return alt

def sunset_for_day(t0, expected):
    t1 = ts.tt_jd(t0.tt + 1)
    t, y = almanac.find_discrete(t0, t1, almanac.sunrise_sunset(eph, babylon))
    for ti, yi in zip(t, y):
        if yi == 0:
            diff_mins = int(abs(ti.tt - expected.tt) * 60 * 24)
            print("Got {}, expected {}, diff mins {}, alt {:.2f}"
                  .format(ti.utc_iso(), expected.utc_iso(), diff_mins, altitude_of_sun(ti).degrees))

if __name__ == "__main__":
    sunset_for_day(ts.utc(2020, 6, 1), ts.utc(2020, 6, 1, 16, 5))
    sunset_for_day(ts.utc(1800, 6, 1), ts.utc(1800, 6, 1, 16, 4))
    sunset_for_day(ts.utc(1000, 6, 1), ts.utc(1000, 6, 1, 16, 5))
    sunset_for_day(ts.utc(200, 6, 1), ts.utc(200, 6, 1, 16, 0))
    sunset_for_day(ts.utc(-400, 6, 1), ts.utc(-400, 6, 1, 15, 56))
    sunset_for_day(ts.utc(-800, 6, 1), ts.utc(-800, 6, 1, 15, 53))

Am I doing something wrong? I’m guessing its something to do with the timescale? I’m on version 1.34

[brandon-rhodes]

Great, I’m glad it helped! If you don’t mind, I’m going to re-open this with a different title to remind myself to highlight this difference more in the documentation.

[brandon-rhodes]

(Merry Christmas!) The UTC timescale uses leap seconds to keep up with the Earth’s actual rotation, but leap seconds are only specified from 1972 forwards — there are no leap seconds going backwards that would keep it in sync in past centuries with the length of the day, or at least with our retrospective guesses about how long the day was in past centuries. We currently use the sparse ancient records of eclipses to try to guess where the Earth was pointed during the day in ancient times, and those guesses guide the UT1 timescale that follows the Sun rather than an artificial scheme of leap seconds. I’d first try UT1 and see whether that gives you closer values to the ones you expect.