Axial tilt refers to the angle between the planet's rotational axis and its orbital axis. The orbital axis is perpendicular, as a rule, to the ecliptic or orbital plane. When you picture the planets orbiting the sun, the plane the planets rest on is the orbital plane. Axial tilt, or obliquity, impacts a planet in many ways. Earth's obliquity varies between 22.1 and 24.5 degrees, and is currently 23.5 degrees, and is the reason for the changing seasons and other cycles on Earth.

Milankovitch cycles, named for the Serbian mathematician and geophysicist who worked on the topic, are three ways the Earth's orbit around the Sun changes over the course of tens of thousands of years. Eccentricity, axial tilt, and precession influence how much solar energy hits the Earth, and that quantity in turn influences periods of glaciation and climate seasons. The Milankovitch cycles can predict the advancement and retreat of Earth's glaciers through time.

But what caused obliquity in the first place? A lunar origin theory that originated in the 1970's, the "Big Whack" theory or giant-impact hypothesis, explains both the slight tilt of the Earth's rotational axis and also the low density and tiny core of our moon. The moon would, according to the theory, be comprised of 2/3rds original impactor materials and only 1/3 Earth mantle. This original impactor, Theia, was likely a Mars sized planet that collided with Earth some 4.5 billion years ago. Some evidence has been found below the Earth's surface that pieces of Theia still are embedded within the rocky mantle near the Earth's molten core.