Autumn Equinox 2025

 

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.

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

But this is not necessarily true.

The definition of the equinox as being equal day and night is actually NOT TRUE! If the sun was nothing more than a point of light in the sky and if the Earth lacked an atmosphere, then, at the time of an equinox, the sun would spend one half of its day above the horizon and one half below. But in reality, the refraction from the Earth's atmosphere raises the appearance of the sun by more than its diameter while it is rising or setting.

 What we see is an optical illusion of the sun as a red-orange ball, sitting on the horizon. The sun is actually completely below the horizon. Refraction!

In addition to refraction giving us an earlier sunrise and a later sunset, there is another factor that makes daylight longer than night at an equinox! Sunrise and sunset are defined as the times when the first or last speck of the sun is visible above the horizon — not the center of the disk. That is why, when you look at the sunrise sunset on the tables online for the Equinox, the Day is LONGER than the night (so they are not equal as the term Equinox suggests. 

Now you know the facts!