Day and night

The Earth is almost perfectly spherical in shape. The parts of the Earth that face the Sun are lit by it and experience day, the parts of the Earth that face away from the Sun are in darkness and experience night. The day and night cycle is caused by the rotation of the Earth. Because the Earth always rotates the same way (anticlockwise as you look down on the North Pole), the Sun always appears to rise at dawn generally towards the east and set at dusk generally towards the west. The Earth rotates on an imaginary axis passing through its poles once every 23 hours and 56 minutes. As the Earth rotates, it also moves in its year long orbit around the Sun. As a result, the time from one ‘sunrise’ to the next, the day and night cycle, is 24 hours.

Standing on the surface of the Earth, its shape is not at all obvious. Clues that the Earth is at least curved and not flat come from observing ships at sea disappearing over the horizon and the shadow of the Earth on the Moon during a lunar eclipse. World-wide travel and images of the Earth from space are enough to convince most sceptics. Knowing that the Earth rotates is not at all obvious either. One of the first real proofs involved a swinging pendulum. Imagine watching a pendulum swinging back and forth over the North Pole. Under ideal conditions, the pendulum will continue to swing in the same plane unless acted upon by another force. If we watched the pendulum for an hour or so we would notice the swing of the pendulum appear to change position relative to the surface of the Earth. Because we know that there are no other forces acting on the pendulum as it swings, what we are actually observing is the Earth itself moving beneath the pendulum. This experiment is still demonstrated in many museums today. Other proofs, including the Earth’s equatorial bulge, the Coriolis effect, observations of the Earth from space and world-wide communications, are equally convincing.

Contents

The Universe
Galaxies
The Milky Way Galaxy
Stars
Constellations, Surveying the Solar System
The Sun
Planets and their moons
Asteroids, comets and meterorites, Exploring the Earth-Sun-Moon System
Seasonal change
The Moon's phases and eclipses
Further Reading
Self assessment(1)
Self assessment(2)
Self assessment(3)