AUTUMNAL EQUINOX

i love the sciences -- biology, ecology, physics, chemistry, genetics, and of course, astronomy. today is the autumnal equinox, when the tilt of the earth's axis is inclined neither toward nor away from the sun. the moment of equinox this year happens at 2118 hours, UTC (greenwich mean time, or zulu time). the vernal and autumnal equinoxes are the two times of year when, around the globe, we experience an equal timespan of daylight and night.

it is the tilt of earth's axis with regard to the plane of its orbit around the sun, that is responsible for northerly and southerly latitudes having seasons. currently that tilt measure roughly 23.44 degrees. as you can see from the illustration above (click to enlarge), in the northern hemisphere when the north pole is angled toward the sun, we experience summer, centered on the summer solstice. when the north pole is angled away from the sun, we experience winter, centered on the winter solstice.

the earth's orbit is elliptical, meaning that at two phases of its orbit the planet revolves closer to the sun, and at the intervening phases it is more distant. some think mistakenly that it is this difference in distance from the sun that generates our seasons. but the distance effect is minimal, compared to the effect of our axial tilt. the changes in the angle of incidence of the sun's light and heat on the earth's surface, is the source of seasonal variations in weather, plant growth, and animal activity (including our own).

over geologic time, complicating matters is the fact that earth's axis undergoes precession, or wobbling, resulting in less or more severe seasonal variations, with consequences (in combination with other factors) as monumental as ice ages. not to worry, it takes about 26,000 years for the axis to describe a single cycle. any dizziness you may feel is psychological.