After a longer and colder winter than normally experienced here in Kentucky, everyone is eagerly looking forward to the return of spring. But have you ever thought about what causes the change of seasons here in Kentucky?
A brief history lesson is important because before the 15th Century, it was believed that the sun orbited around the earth. Nicolai Copernicus (1473-1543) radically changed our understanding of astronomy when he proposed that the sun, not Earth, was the center of the solar system. This led to our modern understanding of the relationship between the sun and the Earth. We now know that Earth orbits the sun elliptically and so is closer to the sun during part of its orbit and farther away at other times. Earth has seasons because our planet’s axis of rotation is tilted at an angle of 23.5 degrees relative to our orbital plane. Over the course of a year, the angle of tilt does not vary and so Earth’s northern axis is always pointing the same direction in space.
But the orientation of Earth’s tilt with respect to the sun, our source of light and warmth, does change as we orbit. The northern hemisphere is oriented toward the sun for half of the year and away from the sun for the other half. The same is true of the southern hemisphere. When the northern hemisphere is oriented/tilted toward the sun, that region of Earth warms because of the corresponding increase in solar radiation. The sun’s rays are striking that part of Earth at a more direct angle; it’s summer. When the northern hemisphere is oriented/tilted away from the sun, the sun’s rays are less direct, and that part of Earth cools; it’s winter. Seasons in the southern hemisphere occur at opposite times of the year from those in the northern hemisphere. Northern summer = southern winter.
The seasons are marked by solstices and equinoxes — astronomical terms that relate to Earth’s tilt. The solstices occur each year on June 20 or 21 (summer) and Dec. 21 or 22 (winter), and represent the official start of the summer and winter seasons. In the Northern hemisphere, the summer solstice is the longest daylight day of the year, with the direct rays of the sun striking the Tropic of Cancer (23.5 degrees north of the equator) while the winter solstice is the shortest daylight day of the year, with the direct rays of the sun striking the Tropic of Capricorn (23.5 degrees south of the equator).
The vernal equinox and autumnal equinox herald the beginning of spring and fall, respectively. At these times of the year, the sun is directly over Earth’s equator, and the lengths of the day and the night are equal over most of the planet. On March 20 or 21 of each year, the Northern Hemisphere reaches the vernal equinox and enjoys the signs of spring. At the same time, the winds turn colder in the Southern Hemisphere as the autumnal equinox sets in. The year’s other equinox occurs on Sept. 22 or 23, when summer fades to fall in the north, and winter’s chill starts giving way to spring in the south.
So, today we welcome the first signs of spring, thanks to the vernal equinox!
This blog entry is written by Matt Lahm, Interpretive Ranger at The Parklands of Floyds Fork.