Autumn Begins at 7:43 am CDT Sunday

The fall equinox this year is on Sunday, September 22nd at 7:43 am CDT.

The word "equinox" comes from the Latin term meaning "equal night". Despite this, the length of day and night is not quite the same on this date. Why not? While the geometric center of the Sun is above the horizon for 12 hours, sunrise is defined as when the edge of the Sun's disk becomes visible, not when the center of the Sun is visible. The atmosphere also causes the light to bend ("refraction"), affecting when the Sun starts to be seen versus when it actually crosses the horizon. The combination of these factors will cause the equal day-night (known as "equilux") to vary by latitude.

Typically, the equinox occurs around September 22-23 each year. Around our latitude, the date of equal day and night is September 25th this year.

Why do we have seasons? Image courtesy of NASA

(Image courtesy of NASA)