(COLORADO) — Fall can be defined in two different ways, and the start date depends on whether you look at it from a meteorological or an astronomical perspective.

Meteorological fall began earlier this week, on Sept. 1, and will continue through Nov. 30. This version of fall is based purely on the calendar. Meteorologists and climatologists use this system because it keeps record-keeping, climate statistics, and seasonal comparisons consistent from year to year. Each season is divided neatly into three-month blocks: September through November for fall, December through February for winter, March through May for spring, and June through August for summer.

Astronomical fall, on the other hand, is tied to the tilt of the Earth and the position of the sun. It begins with the autumnal equinox, typically around Sept. 23, though the exact date can shift slightly depending on the year. The equinox marks the point when the sun’s direct rays cross the equator, moving into the Southern Hemisphere. This transition reduces direct sunlight for the Northern Hemisphere and signals the official start of fall from an astronomical perspective.

In short, meteorological fall is calendar-based and useful for climate data, while astronomical fall is science-based and tied to Earth’s position in space.

No matter which definition you follow, the arrival of fall brings cooler air, changing leaves, and one of the most enjoyable times of the year.