Why We Have Seasons

Why do we have summer and winter? It is not because Earth moves closer to the Sun. The real reason is that Earth's axis is tilted. Imagine a line going through Earth from the North Pole to the South Pole. That line is not straight up and down compared to Earth's path around the Sun. It leans by about 23.5 degrees.

As Earth travels around the Sun during the year, the tilt stays pointing the same way. So for part of the year, the Northern Hemisphere leans toward the Sun. Sunlight hits it more directly, and the days are longer. More direct light and longer days mean more warming, so it is summer there. At the same time, the Southern Hemisphere leans away from the Sun. Its sunlight comes in at a slant, days are shorter, and it is winter.

Half a year later, Earth has moved to the other side of the Sun. Now the Southern Hemisphere leans toward the Sun and gets summer, while the Northern Hemisphere gets winter. That is why the seasons are opposite in the two hemispheres.

Earth is actually closest to the Sun in January, right in the middle of winter for the Northern Hemisphere. The tilt, not the distance, is what makes the seasons.

Earth's axis is tilted. As Earth orbits the Sun, different parts of Earth point toward or away from the Sun. This is what causes the seasons.

When the Northern or Southern Hemisphere points toward the Sun, it is summer in that hemisphere. During this time, the Sun's rays hit that half of Earth more directly, and direct rays bring more warming.

When a hemisphere points away from the Sun, it is winter there. The Sun's rays arrive at more of a slant, so they spread out and bring less warming.

The Northern and Southern Hemispheres always have opposite seasons. When it is summer in one, it is winter in the other. This only makes sense because of the tilt. If distance from the Sun caused seasons, both hemispheres would have the same season at the same time, because the whole planet is about the same distance from the Sun.

In June, the Northern Hemisphere is tilted toward the Sun. Some people say summer happens then because the Northern Hemisphere is physically closer to the Sun, so it must be warmer. Can you find the problem with that idea?

First, if being closer to the Sun caused summer, then the whole Earth would have summer at the same time. But when it is summer in the north, it is winter in the south. The two hemispheres are on the same planet, nearly the same distance from the Sun, yet they have opposite seasons.

Second, Earth's whole orbit barely changes its distance from the Sun. In fact, Earth is closest to the Sun in early January, which is winter in the Northern Hemisphere. If closeness made things hot, January would be the warmest time of year there.

The better explanation is the tilt of Earth's axis. Tilting toward the Sun gives a hemisphere more direct sunlight and longer days, and that is what really warms it up.