While we had the autumnal equinox on Sunday and while the name equinox means ‘equal night’ in Latin, it wasn’t equal day and night on that day. The earth’s axis is not tilted away or towards the sun on the two equinoxes during the year but length of day wasn’t equal.
Since ‘sunrise’ is defined as the instant when the upper edge of the sun’s disk becomes visible above the horizon – not when the center of the sun is visible. In the same sense, ‘sunset’ refers to the moment the upper edge disappears below the horizon. At both instances, the center of the sun is below the horizon, and therefore the equinox day lasts a little longer than 12 hours.
And if the ‘center’ of the sunrise or sunset doesn’t add enough of a wrinkle, refraction and latitude play a factor also. The Earth’s atmosphere refracts sunlight and can appear to be six minutes longer in a day. And one’s latitude can vary the ‘equal day’ by days or weeks.
For me, today is the approximate date of ‘equal’ whereas the most southern cities in the United States have an extra four days before their ‘equal’ day and it doesn’t occur until October 17 if living five degrees north of the equator.
Equinox Sunset
