The seasons of the Earth occur because the rotational axis of the Earth is not perpendicular to its orbital plane. The axis of the Earth is tilted at an angle of about 23.44 degrees from its orbital plane. This tilt is called the axial tilt or "obliquity of the ecliptic." As a consequence, for approximately half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum amount occurring on about June 21. For the other half of the year, the same happens, but in the southern hemisphere instead of the northern, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North Pole and the South Pole of the Earth are just on the terminator, and hence day and night are equally divided between the northern and southern hemispheres.

The seasons are not caused by the varying distance of Earth from the Sun due to the orbital eccentricity of the Earth's orbit. This variation does make a contribution, but is small compared with the effects of exposure because of Earth's tilt. Currently the Earth reaches perihelion at the beginning of January - during the northern winter and the southern summer. Although the Earth is at its closest to the Sun and therefore receiving more heat, the whole planet is not in summer. Although it is true that the northern winter is somewhat warmer than the southern winter, the placement of the continents may also play an important factor. In the same way, during aphelion at the beginning of July, the Sun is farther away, but that still leaves the northern summer and southern winter as they are with only minor effects.