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Equinox
Wed Sep 22, 2004
Listen in RealAudio 
Hi, I’m Bryan Yeaton, and this is The Weather Notebook. On the day of the Autumnal
Equinox, night and day lengths are equal. Right? Well, check the almanac tables; you’ll
be surprised.
For example, those tables show for Philadelphia, at 40 degrees North latitude, the day
length on the Equinox is 12 hours … and 8 minutes. Where do those extra minutes
come from?
The answer lies with the definitions of sunrise and sunset which occur the moment the
upper edge of the Sun’s disk visibly touches the unobstructed horizon. If Earth had no
atmosphere, the upper edge would lie on the horizon exactly six hours before and after
local noon on this day.
But, the atmosphere plays a trick on us, which works for both sunrise and sunset. Let’s
look at a sunrise — but only metaphorically! When we see the solar disk at the horizon,
we are seeing a mirage of the true sun which is actually still below the horizon.
The mirage appears when solar light rays are refracted upon entering our dense
atmosphere from empty outer space. The bending brings the solar light downward to
our eyes along a path we perceive as coming from above the actual sun. So, we see
the sun’s image several minutes before it actually rises.
The bending varies slightly with atmospheric conditions, and more so with latitude.
Calculations of sunrise and sunset at a given latitude are adjusted for average
refraction conditions: five minutes at 50 degrees N, four minutes at 40 degrees, and
three minutes in the Subtropics.
Although the Autumnal Equinox is today, September 22, the equal split between night
and day will occur on September 25th.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook
is funded by Subaru of America and the National Science Foundation.
Today's Links
US Naval Observatory
http://www.usno.navy.mil/
Sunrise/Sunset Tables
http://aa.usno.navy.mil/data/docs/RS_OneDay.html
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