Why do the Northern and Southern Hemispheres have opposite seasons?

Here is a curious fact about our planet: when it is summer in countries like the United States or Spain, it is winter in countries like Australia or Argentina. The two halves of the Earth, the Northern Hemisphere and the Southern Hemisphere, always experience opposite seasons. The reason behind this is one of the most important ideas in understanding our planet.

A common guess is that seasons are caused by the Earth moving closer to or farther from the Sun. This sounds reasonable, but it is wrong. The Earth's distance from the Sun does change slightly during the year, but only by a small amount, not nearly enough to cause the seasons. In fact, the Earth is actually closest to the Sun during the Northern Hemisphere's winter. So distance is not the answer.

The real cause of the seasons is the tilt of the Earth's axis. The Earth spins on an imaginary axis, an invisible line running through its center from pole to pole. This axis is not straight up and down. It is tilted at an angle of about 23 and a half degrees. Crucially, the axis keeps pointing in the same direction in space all year long as the Earth travels around the Sun.

Because of this fixed tilt, the two hemispheres take turns leaning toward the Sun. For part of the year, the Northern Hemisphere is tilted toward the Sun, receiving more direct sunlight and longer days, which brings summer. At the very same time, the Southern Hemisphere is tilted away, getting less direct sunlight and shorter days, which brings winter. Six months later, when the Earth has traveled to the other side of its orbit, the situation reverses. This is exactly why the two hemispheres always have opposite seasons.

This article was written using information from Wikipedia.