Some people mistakenly think that the seasons are caused by Earth being farther from the Sun in winter and closer to the Sun in summer. This is incorrect; Earth’s elliptical orbit is close enough to a perfect circle that distance is not the reason. In fact, Earth is closest to the Sun in early January and farthest in early July, which is exactly the opposite of the summer and winter seasons in the northern hemisphere.

The reason for the seasons has to do with the angle at which sunlight strikes any particular place on Earth at any given time of year. The angle changes throughout the year because the tilt of Earth’s axis differs from the ecliptic. Put another way, the equatorial plane and the ecliptic plane are tilted with respect to one another by about 23.5 degrees. When one part of Earth is tilted toward the Sun, that part experiences summer; when it is tilted away from the Sun, it experiences winter; in between these phases Earth experiences spring and autumn.