Earth information 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 the imaginary red line. 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. Ever since the impact around 4.5 billion years ago, 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' be
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