The axial tilt is expressed as the angle made by one half of the planet's axis and one half of a line drawn through the planet's center and perpendicular to the orbital plane, such that its rotation is in the same rotary direction relative to the former as the planet's orbital motion is to the latter.
The axial tilt may equivalently be expressed in terms of the planet's orbital plane and a plane perpendicular to its axis. In our solar system, the Earth's orbital plane is known as the ecliptic, and so the Earth's axial tilt is also called the obliquity of the ecliptic.
The Earth has an axial tilt of 23.45°. The axis is tilted in the same direction throughout a year; however, as the Earth orbits the Sun, the hemisphere tilted away from the Sun will gradually come to be tilted towards the Sun, and vice versa. This effect is the main cause of the seasons. Whichever hemisphere is currently tilted toward the Sun experiences more hours of sunlight each day, and the sunlight at midday also strikes the ground at an angle nearer the vertical and thus delivers more heat.
The Earth's axial tilt varies between 21.5° and 24.5° with a 41,000 year periodicity (currently decreasing: 24.049 in 3300 BC, 23.443 in 1973, 23.439 in 2000), while the direction of the tilt gradually undergoes precession, moving in a slow circle over a period of about 25,800 years. However, other factors may change the axial tilt of Earth (and of other planets).
Through time, axial precession changes the position of the Earth in its orbit at which the seasons occur (precession of the equinoxes). This has little effect on the amount of solar influx (insolation) during times when the orbit is circular, but can have large effects on the strength of the seasons when the Earth's orbit is highly elliptical (see Milankovitch cycles). The effects of axial precession on seasons can be seen in the following chart.