The Earth's axis is an imaginary line that runs from the north and south poles through the center of the planet. The Earth spins on its axis, thus giving rise to day and night. But the Earth's axis is not perpendicular to its orbit around the Sun. The Earth's axis tilts about 23.4 degrees from the vertical. This axial tilt and its precession (cyclical movement of the orientation of the tilt relative to the Sun) give rise to the different seasons on Earth's hemispheres at different times of the year. The exact of value of Earth's axial tilt appears to have been relatively stable over time.