Earth, The Sun & Moon

The Sun, Earth and Moon form a system that explains day and night, the seasons, and the Moon’s changing shape in the sky.

The Sun

• The Sun is a star made mostly of hydrogen and helium.
• It releases energy by nuclear fusion and shines mainly in visible, infrared and ultraviolet light.
• Its gravity holds the Solar System together.

Earth’s Rotation: Day and Night

• Earth spins (rotates) once about its axis in about 24 hours.
• The axis is tilted by about $23.5^\circ$.
• Because Earth spins from west to east, the Sun appears to move across the sky from east to west.
• It is daytime for the half of Earth facing the Sun and night for the half facing away.

Earth’s Orbit: The Seasons

• Earth orbits the Sun once in about 365 days.
• Seasons are caused by Earth’s tilted axis, not by changing distance from the Sun.
• When the Northern Hemisphere is tilted towards the Sun (around June), it has longer days and more direct sunlight (summer). At the same time, the Southern Hemisphere is tilted away (winter), and vice versa in December.
• Around March and September (equinoxes), day and night are roughly equal in length worldwide.

The Moon’s Orbit and Phases

• The Moon orbits Earth in about one month. The cycle of phases (from new Moon back to new Moon) takes about 29.5 days.
• Phases happen because we see different portions of the Moon’s sunlit half as the Moon moves around Earth.
• Main phases in order: new Moon → crescent → first quarter → gibbous → full Moon → gibbous → last quarter → crescent → new Moon.
• The Moon rotates once in the same time it orbits Earth, so the same side faces us.

Gravity and Orbits

• Gravity from the Sun keeps Earth in orbit. Gravity from Earth keeps the Moon in orbit.
• We see regular, repeating cycles because these motions are steady and periodic.

Common Misconceptions

• Seasons are not caused by Earth being nearer to the Sun; they are caused by the tilt changing sunlight angle and day length.
• Moon phases are not Earth’s shadow. Earth’s shadow causes rare lunar eclipses; phases are due to viewing angles of sunlight on the Moon.