When is the first day of fall in 2025? A carefully worded answer is that on Monday, Sept. 22, at 2:19 p.m. Eastern daylight time (11:19 a.m. Pacific daylight time) autumn begins astronomically in the Northern Hemisphere, and spring in the Southern. At that moment, the sun would be shining directly overhead as seen from a point in the equatorial Pacific Ocean, 1,320 miles (2,124 km) south-southeast of Acapulco, Mexico.

Twice each year — around March 20 and Sept. 22 — the sun is said to "cross the equinoctical, or cross the line," that line, of course, being the equator. These two dates are called an equinox, from the Latin for "equal night," alluding to the fact that day and night are then of equal length worldwide.

But this is not necessarily so.

Not so equal

The definition of the equi

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