This chapter explains the motions of the Earth and how they, together with the Moon and the Sun, produce day and night, seasons, and eclipses. The Earth spins (rotates) on its own axis, an imaginary line passing through the North and South Poles, completing one rotation in about 24 hours. Viewed from above the North Pole, it rotates anti-clockwise, from West to East, which is why the Sun appears to rise in the East and set in the West and why we have a day-night cycle — the half facing the Sun has day, the other half has night. The same rotation makes the stars and Moon appear to move; the Pole Star stays nearly still because the Earth's axis points close to it. The Earth also revolves around the Sun along a nearly circular orbit, completing one revolution in about 365 days and 6 hours, which changes the stars seen at night through the year. Because the Earth's axis is tilted (not upright) with respect to its orbit, and the Earth is spherical, different hemispheres receive sunlight more directly and for longer at different times — this tilt, not the distance from the Sun, causes the seasons. Two important blockages of light are studied: a solar eclipse occurs when the Moon comes between the Sun and the Earth, blocking the Sun's light (the Moon can do this because, though much smaller, it is much closer, so its apparent size matches the Sun's); it must never be viewed directly. A lunar eclipse occurs when the Earth comes between the Sun and the Moon, casting its shadow on the full Moon, and can be safely watched with the naked eye.
Rotation of the Earth and day and nightApparent motion of the Sun, Moon, and starsRevolution of the Earth and changing night skyTilt of the axis and the seasonsSolar and lunar eclipses
Key terms
Rotation
The spinning of the Earth on its own axis, completing one turn in about 24 hours and causing day and night.
Axis of rotation
The imaginary line passing through the Earth's North and South Poles about which it rotates; it is tilted, not upright.
Revolution
The motion of the Earth around the Sun along its orbit, completing one revolution in about 365 days and 6 hours.
Tilt of the axis
The slanting of the Earth's axis with respect to its orbit, which, together with the Earth's spherical shape, causes the seasons.
Solar eclipse
The event when the Moon comes between the Sun and the Earth and blocks the Sun's light from reaching the Earth.
Lunar eclipse
The event when the Earth comes between the Sun and the Moon, casting its shadow on the Moon.
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