The graphic to the right depicts the relationship of the Earth’s orbit around the sun, called its orbital plane, versus its day-and night-producing axis of rotation, tilted 23.5 degrees in relation to the plane of orbit.

Earth is slightly closer to the sun in winter, but proximity to the sun is not a significant causal factor in seasonality, as some might think. Instead, it’s that tilted axis that causes Earth’s seasons.

During our winter, the northern hemisphere is tilted away from the sun. Sunlight falls at more of an oblique angle and the sun is over the horizon a comparatively short time. Less warming occurs.

In spring, the Earth is neither tilted toward or away relative to the sun. During the equinox, day and night are of equal length. Temperatures moderate toward summer.

During summer, the northern hemisphere is tilted toward the sun. Sunlight falls more directly overhead, and the sun is over the horizon longer. Greater heating occurs.

In the fall, the Earth is neither tilted toward or away relative to the sun. During the equinox, day and night are of equal length. Temperatures cool toward winter.

In the southern hemisphere, these seasons are exactly reversed.

The more time the sun is over the local horizon, the more solar heat is applied. The higher the sun reaches overhead, the more direct are the sun’s rays and, again, the more heat is applied. These are the two important reasons for Earth’s seasonality resulting from the axial tilt. In regard to the angle of the sun’s rays, it may be helpful to think of a flashlight. Held perpendicular to a level surface, a flashlight will create a circle of comparatively bright light. Held at a lesser angle, say 45 degrees, the light will be spread out over a greater area. Because the flashlight produces a finite amount of light, the greater the area covered, the less intense the light is per square inch. Sunlight behaves in the same way — less intensity means less heating per square mile.

Often overlooked in discussions such as this is the length of the night, shorter in summer and longer in winter. A longer night provides more time for Earth’s warmth to radiate into space.

— Kent Fairfield is a volunteer with Pine Mountain Observatory and a lifelong amateur astronomer. He can be reached at kent.fairfield@gmail.com. Other PMO volunteers also contributed to this article.