The imaginary line that passes through Earth's center and the North and South poles.
The spinning of Earth on its axis.
The movement of one object around another.
Earth's path that is a slightly elongated circle (ellipse).
A system of organizing time that defines the beginning, length, and divisions of a year.
The days when the sun appears farthest north of the equator once a year and farthest south once a year.
Means "equal night". During this time day and night are each about 12 hours long everywhere.