FactSentinel

Earth's seasons are caused by the roughly 23.5-degree tilt of its rotational axis relative to its orbital plane. Because the axis keeps pointing toward nearly the same spot in space (close to Polaris) as the planet circles the Sun, each hemisphere alternately leans toward or away from the Sun over the course of a year. When the Northern Hemisphere is tilted toward the Sun, sunlight strikes it at a steeper angle and days are longer, concentrating more energy on the ground and producing summer. At the same time, the Southern Hemisphere receives more oblique, spread-out sunlight and shorter days, bringing winter. Six months later the situation reverses.

The evidence is consistent and easy to verify. Seasons in the two hemispheres are opposite, which would not happen if distance from the Sun were the controlling factor. Day length also varies with latitude and time of year in exactly the pattern a tilted axis predicts, including the midnight sun near the poles in summer and polar night in winter. The equinoxes and solstices, when the Sun's position over the equator or the tropics changes, follow directly from the tilt. Planets with different tilts, such as Mars and Uranus, show seasonal patterns that match their own axial angles.

A common misconception is that seasons result from Earth being closer to the Sun in summer. In fact, Earth is closest to the Sun, called perihelion, in early January, during Northern Hemisphere winter. The variation in distance is only about 3 percent and has a minor effect compared with tilt.