If your family has school-age children, then the fall semester began anywhere from a few days to a few weeks ago. And if you’re in more northerly parts of the country, the weather may have felt like fall for a few days or weeks as well. But astronomical fall begins at 3:21 a.m. Central Time tomorrow, with the autumnal equinox. It’s the moment the Sun crosses the celestial equator from north to south. There are many ways to mark the changing seasons. Some of them — like the school calendar and the weather — are quite practical. But they’re also variable. School districts, colleges, and universities begin the fall semester at different times. And the arrival of autumn weather varies from region to region, and even year to year. The astronomical season is always fixed, though — it begins at the precise moment the Sun crosses the equator. That point can be predicted well in advance, so you can calculate the starting date of fall many centuries ahead. Even with this system, though, not ever
