Twice a year our valley joins the rest of the world.

In the spring and in the fall, the length of day is nearly equal throughout the planet, with twelve hours of daylight and twelve hours of darkness. This event is called equinox, literally, from Latin, "equal night". It happened again last Saturday.

In the spring it is called vernal equinox, again from the Latin, for "equal night of spring", and in the fall this event is called autumnal equinox. We enter fall at this time and for those in the southern hemisphere, spring is just beginning.

What is happening is actually rather curious. The earth itself is not shifting or leaning to make the seasons change. It is doing what it always does, moving in an orbit around our sun. The earth's poles are tilted as this happens though--they form an angle as compared to the orbit of the earth around the sun. What is also significant is that the earth is spinning as it orbits, like a gyroscope, and it continues to stay with each pole pointed toward some imaginary spot far off in space even though it is circling the sun. Because of this, when the earth orbits the sun, our north and south poles slowly change their relationship to the sun.

At this time of year, at equinox, the north and south pole are equal in this relationship. As the earth moves on in its orbit, our pole will stay pointed at that imaginary spot in space, and will thereby point further away from the sun as we go, and the south pole will point more toward the sun. As this happens, less sun will reach us, and more will reach the southern hemisphere. Eventually, about December 21, the earth will reach the place in its orbit when the north pole is pointed as far away from the sun as the tilt will allow. Winter will arrive for us and summer will reach the south. This event is called solstice. The earth then continues on its orbit into the next cycle, with the days growing longer in the north and shorter in the south until June 21, our summer.

This cycle of the seasons affects much of life on earth, as it does in our valley. Most plants in the temperate or middle latitudes of the earth where we live have adapted to take advantage of these seasonal changes. Most animals have evolved or learned to meet the adaptations of these plants or the animals that depend on them. Even near the equator, life adjusts according to seasonal changes, with wet and dry cycles that are driven by these changes.

As we arrive here at the momentary balance of light and darkness, the equinox reminds us that we too are creatures of these forcesÐwho among us doesn't begin to think of firewood or shoring up our homes for the coming darker, colder days as the daylight lessens. There is a sense of preparation that you can feel in the plants and animals around us, and from most of our human neighbors as well. This is no doubt an instinctive response to an ancient pattern of behavior that understands our fragile place on earth, linked in our very lives to the seasons.