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Equinox means equal day and night, but not for a few days

The sun shines directly on the equator and, for that moment, the northern and southern hemispheres receive equal amounts of sunlight.

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Solstice Equinox Full Disks
By
Tony Rice
RALEIGH, N.C. — The September equinox arrived Sept. 22 at 4 p.m.

The equinox occurs at the same moment world wide, because it is determined by the Earth’s position in its orbit around the sun. The sun shines directly on the equator and, for that moment, the northern and southern hemispheres receive equal amounts of sunlight.

This equality is most visible in satellite images showing the terminator between day and night equally distributing sunlight to the northern and southern hemispheres.

From the Latin aequus (“equal”) and nox (“night”), daylight hours and hours of darkness, worldwide, are roughly equal on the equinox. They're considered roughly because sunrise and sunset times are a function of where you are on the Earth's surface -- mostly your latitude.

Equilux, the day where the length of day and night are closest, varies by latitude. In central North Carolina, our equinox occurs on Sept. 26 where day and night vary by just 31 seconds.

The equilux date for the March 2017 equinox was even closer to perfect, just a second off equal day and night on March 16.

The equinox is also known as the date when the sun rises and sets due east and west. But this is true only at the equator. This also varies by where you happen to be on the Earth’s surface.

Here in central North Carolina, the sun set just a fraction of a degree from due west Sept. 23 and rose a fraction of a degree from due east the following morning.

Each of these dates above was determined by calculating the sun's rise and set times along with azimuths for the year and finding those dates which most closely matched the definition of equilux. The source code is available online.
Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and a software engineer at Cisco Systems. You can follow him on Twitter @rtphokie.

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