The earth goes around and sun once each year, and the plane of its motion is called the plane of the ecliptic. The earth rotates once each day; and the axis of its rotation lacks 23.5 degrees of being perpendicular to the plane of the ecliptic. This angle of 23.5 degrees is called the inclination of the earth's axis, and is one of the most important facts concerning the earth's motion, since it causes the seasons. If the axis of the earth were perpendicular to the plane of the ecliptic the climate at any given point on the earth would remain the same the year around.
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Boon, John D.
"The Tilt Of The Earth's Axis And The Consequences Thereof,"
Field and Laboratory: Vol. 13
Available at: https://scholar.smu.edu/fieldandlab/vol13/iss1/2