Celestial objects in orbit such as the planet Earth move in two ways: revolution and rotation. The former refers to the elliptical motion of a satellite around its parent body, in this case the sun; the latter refers to the circular motion of the body around a central axis as it revolves. The axis of rotation frequently makes a tilted, non-perpendicular angle from the orbital plane. This phenomenon, known as axial tilt or obliquity, has profound effects on the conditions on the planet.
Earth's axial tilt is currently 23.5 degrees and varies between 22 and 24.5 degrees over a period of 41,000 years. In relation to the orbital plane, also known as the ecliptic, the north and south celestial poles move slowly, which causes the planet to almost always tilt in the same direction with respect to the solar system as a whole. As the Earth revolves around the sun, the distribution of sunlight and across the planet varies; the Northern and Southern Hemispheres only get equal amounts of light during the spring and fall equinoxes. When Northern Hemisphere is tilted in the direction of the Sun, people in that part of the world go through the summer season, while the Southern Hemisphere has winter. On the opposite side of the orbit, the Southern Hemisphere tilts toward the sun and experiences the season of summer; the Northern Hemisphere tilts away and has winter
One of the indicators of seasonal shift is the variable ratio of daytime to nighttime throughout the year, controlled by the Earth's axial tilt. The side of Earth facing the sun at any given moment experiences daytime. When the Northern Hemisphere is facing the sun, the latitudes north of the equator will have more area in the day side of the planet, causing longer days, and the Southern Hemisphere experiences shorter days. Going farther away from the equator increases this distance; the Arctic Circle located at 66 degrees north marks the point at which the entire rotation is on the daytime or nighttime side of the sun, referred to as the summer and winter solstice, when the day or night lasts a full 24 hours. At the poles, day and night last a full six months.
Scientists have not discovered the cause of Earth's axial tilt, but have put forth a few main theories. The prevailing theory is that a large celestial body collided with Earth early in its lifetime and caused a tilt in the axis. Another theory is the chaotic distribution of dust at the formation of the solar system, which accounts for the each planet having a different axial tilt.