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Equinoxes and solstices: tilted spinning
![[Steele Hill, SOHO] The Sun, radiating energy Earthward](images/2003-05-30-solar-wind.jpg) Q:
Please explain an equinox and solstice. (I love your site.) —Jakob
Chiasson, Moncton, New Brunswick, Canada
[Steele Hill, SOHO] The Sun, radiating energy Earthward
A: We Earth folk know good and well what an equinox and a solstice are.
An equinox is when the day length roughly equals night length (occurring about March 21
and September 23). The two solstices (on or near June 21 and December 22) give
us, respectively, the longest daylight and the longest night in the year.
(Related:
Why day,
night not equal on equinox).
The tricky stuff is imagining a spaceman’s view as shown in the figure.
(Related: figure of
Earth orbiting the Sun.)
But it’s basically simple. A spaceman can see that Earth’s rotation axis
tilts. As Earth orbits Sol, the direction of the tilt remains fixed. So,
sometimes one hemisphere tilts mostly toward the Sun and gets more sunshine than
the other one. The first hemisphere knows summer and the other experiences
winter. In between, neither hemisphere tilts much sunward and that’s spring or
fall.
We get almost equal length days and nights on the equinoxes because then neither
hemisphere tilts toward the Sun. The tilt effect is gone. The summer’s solstice
gives the longest day because then our hemisphere tilts directly toward the Sun
and we see the Sun the longest. At the pole, we see the Sun all day (and a day
lasts the summer). Likewise, the winter’s solstice harbors the longest night
because then our hemisphere tilts directly away.
Further Surfing:
USA
Today: Earth’s seasons graphic
USA Today:
Earth’s seasons
(Answered May 30, 2003)
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