SEASONS - THE LIFE
SUSTAINERS
By Paige Williams
MSNBC
THE ABILITY to predict the seasons — by tracking the rising and setting points of the sun throughout the year — was key to survival in ancient times. Babylonians, Mayans and other cultures developed complex systems for monitoring seasonal shifts. But it took centuries more to unravel the science behind the seasons.
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 Earth. We now know that Earth orbits the sun elliptically and, at the same time, spins on an axis that is tilted relative to its plane of orbit. This means that different hemispheres are exposed to different amounts of sunlight throughout the year. Because the sun is our source of light, energy and heat, the changing intensity and concentration of its rays give rise to the seasons of winter, spring, summer and fall.
The seasons are marked by solstices and equinoxes — astronomical terms that relate to Earth’s tilt.
The vernal equinox and autumnal equinox herald the beginning of spring and fall. At these times of the year, the sun appears to be 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 is reaching the vernal equinox and enjoying the signs of spring. At the same time, the winds are turning cold in the Southern Hemisphere as the autumnal equinox sets in.
On Sept. 22 or 23 of each year, it’s the other way around: Summer fades to fall in the north, and winter’s chill starts giving way to spring in the south.
The solstices mark the points at which the poles are tilted at their maximum toward or away from the sun. This is when the difference between the daylight hours and the nighttime hours is most acute. The solstices occur each year on June 20-21 and Dec. 21-22. This December’s solstice comes at 8:14 p.m. ET Saturday.
Here’s how the seasonal change affects the weather: Around the time of the June solstice, the North Pole is tilted toward the sun and the Northern Hemisphere is starting to enjoy summer. The sun’s rays are strong because they are directly overhead. They are concentrated over a smaller surface area and traveling through a relatively small amount of energy-absorbing atmosphere before striking the earth.
At the same time that the Northern Hemisphere is entering summer, the South Pole is tilted away from the sun, and the Southern Hemisphere is starting to feel the cold of winter. The sun’s rays are weak because they are spread over a greater surface area and must travel through more energy-absorbing atmosphere to before reaching the earth.
The situations are reversed in December, when it’s the Southern Hemisphere that basks in the most direct rays of the sun, while the Northern Hemisphere receives the weakest rays.
Although the solstices represent the pinnacles of summer and winter with respect to the intensity of the sun’s rays, they do not represent the warmest or coldest days. This is because temperature depends not only on the amount of heat the atmosphere receives from the sun, but also on the amount of heat it loses due to the absorption of this heat by the ground and ocean. It is not until the ground and oceans absorb enough heat to reach equilibrium with the temperature of the atmosphere that we feel the coldest days of winter or hottest days of summer.
| 
