| According to NOAA, the first day of Fall this year is Monday, September 22. The official time of the Equinox (the moment when the Sun is positioned directly over the Earth's equator) is 10:44 am (1544z) Monday morning. During the day of an Equinox, nearly every point on the Earth's surface has the same sunrise and sunset, (exception is when there is an obstruction to the horizon like a mountain). |
During an equinox, the center of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of nearly the same length.
The word equinox derives from the Latin words aequus (equal) and nox (night). In reality, the day is longer than the night at an equinox.
According the the US Navy,
Day and night are not exactly of equal length at the time of the March and September equinoxes. The dates on which day and night are each 12 hours occur a few days before and after the equinoxes. The specific dates of this occurrence are different for different latitudes.
On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth.
Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon.
Daytime continues to be longer than nighttime until after the September equinox. In the southern hemisphere, the dates of equal day and night occur before the September equinox and after the March equinox.
In the northern hemisphere, at latitude 5 degrees the dates of equal day and night occur about February 25 and October 15; at latitude 40 degrees they occur about March 17 and September 26. On the dates of the equinoxes, the day is about 7 minutes longer than the night at latitudes up to about 25 degrees, increasing to 10 minutes or more at latitude 50 degrees.
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