The Vernal Equinox
Nature Bulletin No. 559-A March 22, 1975
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation
THE VERNAL EQUINOX
The 1975 calendar has hung on the wall for eighty-one days. We nailed
it there back in the middle of one of the mildest winters we have had in
recent years. Yesterday, March 21, we welcomed the official end of
winter and hope it means the end of snow, sleet, ice and zero
March 21st is the first day of spring -- the Vernal Equinox. This is the
day when the sun crosses the equator and spends the next half-year
lighting and warming us dwellers of the northern hemisphere. The Latin
word equinox means "equal night" because on this day, and again on
September 23 when the sun goes back across the equator, days and
nights are the same length. However, if we look up the times of sunrise
and sunset on these dates, we find that the day is about 12 hours and 10
minutes long. This happens because the earth's atmosphere bends the
sun's rays enough so that we see the sun for a few minutes each morning
and evening while it is actually below the horizon.
At this time, our days are lengthening at their fastest -- some three
minutes each day -- about two minutes earlier in the morning and one
minute later in the evening. At different times during the year, the
increase or decrease at the two ends of the day are not equal, partly
because the earth's path around the sun is not quite circular, and partly
because the earth spins on an axis which is tilted at an angle of 23-1/2
degrees. If the earth's axis were not tilted, the sun would stand right
above the equator year in and year out -- a continual equinox. There
would be no spring, summer, autumn or winter and very little variety in
the weather. There would be torrid, temperate and frigid zones but each
might be as uniform and monotonous throughout the year as the tropical
rain forests of equatorial Africa and South America now are.
In a country such as ours, spring has a more dramatic meaning than the
mere mechanics of the vernal equinox. It is not an abrupt break from
winter into spring on March 21. Instead, it is regular orderly pageant
with a multitude of actors dancing ahead of the sun on its northward
march, a parade that stretches from subtropical Florida to the bleak
north coast of Alaska. Snow and ice disappear. Streams run full. Wave
after wave of migrating birds sweep northward to nest. Spring flowers
push up and bloom in the warming woodlands, seeds sprout, and
gardeners bend their backs to the spade.
Illinois is a long state -- 385 miles. Down near the Ohio River, flowers
come into bloom over a month earlier than the same kinds growing in
our northern tier of counties. In the fifty mile length of Cook County
there is fully ten days difference in flowering dates between its northern
and southern borders.
Each spring, for thirty years, the naturalists of the Forest Preserve
District have carefully recorded scores of signs of spring. Some of them
at about the time of the vernal equinox are as follows: Red-winged
blackbirds, bronzed grackles, killdeers, meadow larks and flocks of
robins have been back for two weeks or more. Bluebirds are arriving.
Duck migration is about at its peak. Crows are nesting. Chipmunks are
awake. Cricket frogs and spring peepers are singing. Spotted
salamanders are laying their jelly-like masses of eggs in still cold
ponds. Painted turtles are out of hibernation and basking on sunny days.
The flowers of the American elms and silver maples are wide open. The
bloodroots are pushing up. Tender wild onion sprouts are large enough
to flavor a salad or sandwich. Spring is the sunrise of the year.
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Update: June 2012