Autumn arrives this evening as the Sun crosses the equator from north to south — the moment of the September equinox.

Over the last few decades, the most common date for the equinox here in the United States has been the 22nd. But it also can occur a day earlier or a day later.

That’s a result of the difference between the calendar year and the true year — the time it takes Earth to make one full turn around the Sun — a gap of 5 hours, 48 minutes, and 45 seconds.

On average, the equinox advances by that extra bit each year. But Leap Year basically pushes the equinox back to its starting point. Today, the equinox occurs on September 22nd for most of the Lower 48 states for three years in a row. It then jumps to the 23rd the year before Leap Year for the eastern part of the country.

This arrangement doesn’t completely balance the books, though. If uncorrected, it would cause the equinox to move about one day earlier every 128 years. But the calendar drops three Leap Years every four centuries. For the most part, these adjustments keep the September equinox on the 22nd or 23rd — for now.

As you might expect, though, it won’t stay that way. The equinox will begin to shift toward September 21st later in the century.

Tomorrow: a possible rain of fire across the American Midwest from an exploding comet.
 

Script by Damond Benningfield

Support McDonald Observatory