Earth, Moon & Sun
Section: Space Physics | Syllabus: Cambridge AS Level Physics 9702
Day and Night The Earth rotates on its axis - an imaginary line through the North and South Poles - once every approximately 24 hours . Because light travels in straight lines, only the half of the Earth facing the Sun receives sunlight at any one time, creating day and night.
Apparent Daily Motion of the Sun The Sun appears to rise in the east and set in the west. This is because the Earth rotates from west to east - not because the Sun moves around the Earth. At sunrise, a location on Earth is just rotating into sunlight - the Sun appears on the eastern horizon At midday, the Sun appears directly overhead (highest in the sky) At sunset, the location rotates out of sunlight - the Sun appears on the western horizon Figure: Day and Night The Earth shown with sunlight illuminating one half.
The half facing the Sun is labelled "Day" and the opposite half "Night". An arrow on the Earth shows the direction of rotation (west to east, anticlockwise when viewed from above the North Pole). Sunrise and sunset positions are marked.
The Sun is shown as a large yellow sphere to the right. Key Fact The Earth's axis is tilted at approximately 23.5° relative to its orbital plane around the Sun. This tilt causes the length of day and night to vary with season and latitude.
Seasons The Earth orbits the Sun once in approximately 365 days (one year). Because the Earth's axis is tilted, different hemispheres receive different amounts of sunlight at different times of year - this causes the seasons .
When a hemisphere is tilted towards the Sun : longer days, more direct sunlight → summer When a hemisphere is tilted away from the Sun : shorter days, less direct (more spread-out) sunlight → winter When the northern hemisphere has summer, the southern hemisphere has winter, and vice versa Countries at or near the Equator receive sunlight at roughly the same angle all year → no significant seasons The further from the Equator, the more extreme the seasonal differences Figure: The Seasons The Earth shown at four positions in its orbit around the Sun.
The Earth's axis is tilted consistently in the same direction throughout the orbit. Position 1: northern hemisphere tilted towards Sun (Northern Summer / Southern Winter). Position 3 (opposite): northern hemisphere tilted away from Sun (Northern Winter / Southern Summer).
Positions 2 and 4: axis roughly perpendicular to Sun direction (Spring and Autumn). Phases of the Moon The Moon orbits the Earth once in approximately 27.5 days . The Moon does not produce its own light - it reflects sunlight.
As the Moon orbits the Earth, we see different fractions of its illuminated side, creating the phases of the Moon . Phase Description New Moon Moon is between Earth and Sun - lit side faces away from Earth; Moon appears dark Waxing Crescent Small sliver of the right side visible; growing First Quarter Right half of the Moon is visible Waxing Gibbous More than half visible, still growing Full Moon Earth is between Sun and Moon - entire lit side faces Earth Waning Gibbous More than half visible, now shrinking Last Quarter Left half of the Moon is visible Waning Crescent Small sliver of the left side visible; returning to new Moon Figure: Phases of the Moon A circular diagram showing the Moon at 8 equally-spaced positions in its orbit around the Earth.
The Earth is at the centre. Sunlight arrives from the right as parallel horizontal arrows. For each position, a small circle shows how the Moon appears from Earth (the illuminated portion visible to an observer).
Position 1 (right, between Earth and Sun): New Moon - the sunlit side faces away from Earth, the Moon appears as a dark circle. Position 2 (lower-right): Waxing Crescent - a small right-hand sliver is lit.
Position 3 (bottom): First Quarter - right half of the Moon is lit. Position 4 (lower-left): Waxing Gibbous - more than half lit. Position 5 (left, opposite the Sun): Full Moon - entire face is lit as seen from Earth.
Position 6 (upper-left): Waning Gibbous - more than half lit, left side. Position 7 (top): Last Quarter - left half is lit. Position 8 (upper-right): Waning Crescent - small left-hand sliver lit. Labels name each phase.
Key Definitions A day = time for the Earth to rotate once on its axis (~24 hours) A month = time for the Moon to orbit the Earth (~27.5 days) A year = time for the Earth to orbit the Sun (~365 days) Orbital Speed Since planetary orbits are approximately circular, we can calculate the average orbital speed of an object from the circumference of its orbit and the time it takes to complete one orbit.
Average Orbital Speed Equation v = 2πr / T v = average orbital speed (m/s) r = average orbital radius - average distance from the object being orbited (m) T = orbital period - time for one complete orbit (s) The distance travelled in one complete orbit is the circumference of a circle: 2πr .
Dividing by the time taken (T) gives the average speed. Figure: Orbital Speed A circle representing a planet's approximately c…
Interactive revision notes, videos and practice questions load below.