The autumnal equinox, when daylight hours nearly equal the hours of darkness, occurs Thursday at 1:29 p.m. MDT, the earliest appearance so far in this century.

The moment that the sun appears to cross the Equator on its annual journey south occurs at 19:29 universal time, which is 1:29 p.m. MDT. Not since 1897 has the equinox come this early, reports LeRoy Doggett, head of the Nautical Almanac Office at the U.S. Naval Observatory.The equinox will occur even earlier in four years, when it comes at 18:44 universal time on Sept. 22, 1992, and also at 18:01 on Sept. 22, 1996.

And in 2000, the equinox will edge all the way back to 17:29 universal time on Sept. 22 of that year.

The tendency of the equinox to occur earlier will continue, Doggett said, at least until the year 2100, when the absence of a leap year will shift it to later dates again.

The equinoxes are days of approximately equal amounts of daylight and darkness. Many people arbitrarily consider the autumnal equinox the start of fall, and the spring equinox the start of spring.