Equinox Autumnal
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Cruising Through the Equinox

+ Von Del Chamberlain +

Note: The 2006 autumnal equinox will occur at 10:04 p.m. MDT, 22 September 2006, when autumn begins in the Northern Hemisphere.

You can almost feel the planet cruising through the equinox.

We have reached the season of spectacular change! The hottest days have past and each is shorter than the last. Groves on mountains take on brilliant colors. The State Fair is over and school is in full swing. Everywhere, everyone seems more active and ready to get things done. Indeed, there is much to be done, for crops are ripe: it is the harvest season! The perfect time of year has come again, the time of change, for Earth has coasted through the equinox.

Here in the north it is autumn, while far to the south, across the equator, spring has arrived. It happens every year and we all feel it. Many enjoy it; others tolerate it; but few give much thought to what brings it.

Sometimes it is difficult to think about riding on a great spinning ball of rock, with pockets of water, all surrounded by a wisp of gas, zooming round a star. For most of human history no such concept crossed the minds of tribes that would grow into nations. Obviously, it was the sky that moved. Anyone could see that the Sun god leapt from sleep each morning to ride the sky and shower blessings down upon his children. Then stars moved about to beautify the night. We must be special, being at the center of all of this: nourished by warmth and light; refreshed by coolness and the beauty of a jeweled ceiling; given enough warmth, yet not too much.

It took no small effort to break age-old traditions, only a few hundred years ago: to dare to think new thoughts, formulate new theories that could be tested by observation and experiment, and to invent instruments that could expand our innate abilities to measure and perceive. The names of the scientific explorers loom large: Copernicus, Kepler, Galileo, Newton...the list goes on.

They made it easy to know the truth: Earth is a gyro whose spinning mass maintains the axis of its course. One pole points nearly toward a star, and so its name, "Pole Star." Each time it spins it carries Sun, Moon, planets and stars in review across our sky in the period called "one day." Each day begins at a slightly different place in space as Earth migrates along its orbit around a star, the Sun. The orbit is tilted to the plane Earth spins in, and so different parts of the planet receive continually different amounts of energy. For six months, more and more of the Northern Hemisphere is bathed in the Sun's rays until we reach the place where the Northern Hemisphere is tipped most toward the Sun. Just the opposite is happening in the Southern Hemisphere. Then, for the next six months, in the other half of the orbit, the northern climes of Earth receive less and less direct rays until the tilting axis of rotation brings shade to the north pole and less direct rays to those of us within the northern temperate zone. Then, the south receives the more direct illumination of the Sun. These ever recurring changes repeat with each yearly orbit.

Watching from the surface of the spinning ball, we pace our activities by two waves that move constantly around the planet: we awaken with the wave of light, go about our work and play, then relax as the wave of darkness moves over us. Day after day it is the same, but not quite. Each wave of daylight comes at a slightly different place upon our horizon.

In late June, the Northern Hemisphere of Earth is turned as much toward the Sun as the orbit permits: the solar-orb rises in the northeast, days are long and the Sun passes high at the middle of the day to set in the northwest. Nights are as short as they can get. Then, slowly, as Earth moves on, the Sun slips southward in its rising and setting points and lower across the sky each day. Days get shorter and nights longer. Faster and faster the change occurs until we reached the time of equality between light and dark.

The maximum speed of change, the point of balance between day and night, is reached every year in late September. So, on the day of the equinox the Sun rises directly east, crosses our sky in the middle of its range, and sets directly west. We feel its rays for 12 hours and live without them for the other 12. At the poles of Earth the Sun moves around at the base of the horizon, the north pole feeling its last solar rays until six months from now, and the south pole rejoicing in the returning Sun to light its sky for six months to come. Now, one day later, our portion of sunlight is a bit weaker, and so it will be until late December.

This is not cause for grief, for no matter what the season we do well to celebrate its gifts. Imagine, if you can, a planet that does not have rotation axis tilted to its orbit. Everyday would be the same: half light, half dark, no seasons, very little change. Perhaps it is change itself that most deserves our revelry, for we are creatures of change. The best things we do, from birth to death, result in changes for ourselves and others. The Roman poet, Ovid, wrote, "There is nothing constant in the universe. All ebb and flow, and every shape that's born bears in its womb the seeds of change."

Each day brings adventure: new stars coming into view; new sounds in moving air. If you really think about it, you can almost feel the planet moving: rolling upon its axis; flying upon wings of the constant tug of gravity that binds us to a star.

This article was modified from the original to serve as an information source for all autumnal equinox events.

For more equinox insights (and some pictures of balancing eggs) visit Phil Plait's Bad Astronomy site. Another good article is Phil's What Causes the Seasons?

Also available at the U.S. Naval Observatory's site is an article on the Length of Day and Night at the Equinoxes and UCAR has a nice explaination of the equinox.

And finally the Science Alliance has conducted egg balancing experiments with teachers.

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