What Is Earth's Axial Tilt or Obliquity?
When an object the size of Mars crashed into the newly formed planet Earth around 4.5 billion years ago, it knocked our planet over and left it tilted at an angle.
Earth's Axis Is Imaginary
In astronomy, an axis refers to the imaginary line that an object, usually a planet, rotates around.
Earth's rotational axis is an imaginary straight line that runs through the North and South Pole. In our illustrations, Earth's axis is drawn as a straight red line.
Giant Impact Hypothesis
The impact around 4.5 billion years ago is described in the Giant Impact Hypothesis, which is the current prevailing theory on how the Moon was formed and how Earth got its tilt.
Ever since this impact, Earth has been orbiting the Sun at a slant. This slant is the axial tilt, also called obliquity.
Earth's obliquity angle is measured from the imaginary line that runs perpendicular to another imaginary line; Earth's ecliptic plane or orbital plane (see illustration).
At the moment, Earth's obliquity is about 23.4 degrees and decreasing. We say 'at the moment' because the obliquity changes over time, although very, very slowly.
Earth's Obliquity Today
Today, on October 14, 2019 at noon, Earth's axial tilt, or mean obliquity was 23.43671° or 23°26'12.1".
Earth's mean obliquity today is about 0.00001°, or 0.04", less than 30 days ago.
The Arctic and Antarctic circles today are 1.2 m (4 ft) closer to the poles, and the Tropic of Cancer and Tropic of Capricorn are equally closer to the equator than 30 days ago.
The Tilt Changes
Earth's axial tilt actually oscillates between 22.1 and 24.5 degrees. The reason for this changing obliquity angle is that Earth's axis also wobbles around itself. This wobble motion is called axial precession, also known as precession of the equinoxes. It is caused by the gravitational force from the Sun, the Moon, and other planets.
Acts Like Two Spinning Tops
Axial precession can be described as a slow gyration of Earth's axis about another line intersecting it. A complete wobble of Earth's axis takes around 26,000 years. It outlines the shape of a pair of cones or two spinning tops connected at the tips, which would be at the center of Earth.
Greek astronomer Hipparchus of Nicea is historically credited as the man who first proposed that Earths axis gradually shifts, though very slowly. Hipparchus made his discovery around 130 BCE, based on comparisons of astronomical observations more than a century apart.
Tilt Causes Seasons
Because Earth orbits the Sun at an angle, the solar energy reaching different parts of our planet is not constant, but varies during the course of a year.
This is the reason we have different seasons and why the seasons are opposite in the Northern and Southern Hemispheres.
At the same time, the Southern Hemisphere tilts away from the Sun, resulting in shorter days.
From the September equinox to the next March equinox, the days are longer south of the equator and shorter north of the equator.
Illustrator's Point of View
Different illustrators may vary which direction they incline the axis in their images. Some draw it tilted left-to-right, others right-to-left. These illustrations can both be accurate; the only difference is that the artist has chosen the opposite side of the Sun as the point-of-view.