
09/26/96 - 02:59 PM ET - Click reload often for latest version

Source: U.S. Naval Observatory


Day, night not equal on equinox

Each March at a particular instant, around the 20th, the sun is directly
above the equator. This is the spring equinox. In September, around the
22nd, the sun is again above the equator at the fall equinox. These times
are often called the "official" start of spring and fall. They are really
the astronomical beginnings of spring and fall. Meteorologists in the U.S.
consider winter to end the last day of February and spring to begin March
1. Summer begins June 1 and fall on September 1. Winter begins Dec. 1.
Often you hear that day and night are equal length on the spring and fall
equinox, but that's not the case. In fact, day and night are equal length
for a few days around the time of the equinoxes.

On the equinox, the sun's center rises and sets 12 hours apart. Sunrise and
sunset occur when the top of the sun, not its center, is on the horizon. As
you can see in the graphic above, the earth's atmosphere refracts, or
bends, light from the sun. As a result, the sun appears to be above the
horizon when it is actually still below the horizon. In calculating sunrise
and sunset times, the U.S. Naval Observatory uses 34 minutes for the angle
of refraction and 16 minutes for the semi diameter of the sun's disc. In
other words, the center of the sun is actually 50 minutes below a regular
and unobstructed horizon at the time of sun rise.


The following table shows sunrise and sunset times for Chicago around the
time of the spring equinox. They are typical.


Date     Sunrise   Sunset    Daylight

March 17 6:00 a.m. 5:59 p.m. 11 hours, 59 min.

March 18 5:58 a.m. 6:00 p.m. 12 hours, 2 min.

March 19 5:56 a.m. 6:01 p.m. 12 hours, 5 min.

March 20 5:54 a.m. 6:02 p.m. 12 hours, 8 min.



Twilight makes the day even longer. In much of the USA, morning civil
twilight and evening civil twilight are 25 to 30 minutes. long. Morning
civil twilight begins when the rising sun is six degrees below the horizon.
Evening civil twilight ends when the setting sun is six degrees below the
horizon. In practical terms, civil twilight is the time before sunrise and
after sunset when it's light enough for outdoor activities without
artificial light, especially when the sky is clear. On March 17, for
example, morning civil twilight begins at 5:31 a.m. and evening civil
twilight ends at 6:28 p.m.



All of the sunrise, sunset and twilight times here were obtained from a
U.S. Naval Observatory World Wide Web site that enables you to calculate
sunrise, sunset, twilight as well as moon rise and moon set times for any
city in the USA. Click here to go to a USA TODAY Online page with links to
that Naval Observatory Web page as well as links to information about the
sky, time and tides.
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