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Latin aequus, meaning equal, and nox, meaning night. This is derived from the oldest understanding of an equinox, which suggests it is the day when daytime and night are of equal duration. Were you an ancient astronomer marking sunrise and sunset points along your horizon with large rocks you would have noted that the sun has extreme north and south points along the rising and setting horizons. The equinox would occur at the midpoint of the extremes. One effect is that the amount of daylight compared to darkness will decrease from now until late December (the winter solstice -Monday, December 21, 2015 at 8:49 PM PST this year or Tuesday, December 22, 2015 at 04:49 UTC) and then start increasing until Spring Equinox (Saturday, March 19, 2016 at 9:31 PM PDT or Sunday March 20 4:31 UTC) when the daylight and darkness will again be equal.

But there are problems. We live on a complicated planet. If you are trying to measure the length of a day

First, the observer needs a clear view of a lot of  the horizon. That can get tricky in winter or bad weather. Or if you live in or near mountains.

Second, it matters where (longitude and latitude, and perhaps altitude) the observer is.

Third, the Earth’s atmosphere influences light so one can see the sky lightening before the sun actually rises. A common definition of sunrise is when the sun is first visible above the horizon. Technically, it probably should be when the center of the sun is above the horizon.

The extent of refraction mentioned above depends on atmospheric pressure and temperature. Assuming the standard atmospheric pressure of 101.325 kilopascal and temperature of 15°C or 59°F.

When day and night are equal by location

60° North Mar 18 Sep 25
55° North Mar 17 Sep 25
50° North Mar 17 Sep 25
45° North Mar 17 Sep 25
40° North Mar 17 Sep 26
35° North Mar 16 Sep 26
30° North Mar 16 Sep 27
25° North Mar 15 Sep 27
20° North Mar 14 Sep 28
15° North Mar 12 Sep 30
10° North Mar 8 Oct 4
5° North Feb 24 Oct 17
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