Fall Arrives With Autumnal Equinox, But Sunrise And Sunset Don't Equal Perfect Half-Days

(CBS SF) -- Fall is officially here, as Monday, September 22nd marks the Autumnal Equinox, even if the Starbuck's Pumpkin Spice Lattes aka #PSL began arriving way back in August.

The equinox marks the time when days and nights are of equal length, but anyone with a smartphone or a Google search will quickly notice that the sunrise at 6:57 a.m. and the sunset at 7:06 p.m. do not provide a perfect 12 hour day and 12 hour night.

The reason for the discrepancy is not some bizarre astronomical twist, nor the tilt of the earth or our latitudes.  It's our definitions.

Sunrise is determined by the moment the top edge of the sun's disc appears above the horizon, while equinox is measured from the center of the sun.  Sunset is determined by the moment the sun's disc disappears below the horizon.

Add into that the atmospheric effect of bending and magnifying light, and you get a 7- to 9-minute discrepancy.  Have no fear though, the actual periods of light and dark are equal. Some people have referred to the day as "equilux" to avoid unnecessary vagueness.

The next big seasonal date is the official start of winter on the longest night of the year, December 21st.