23.1 - Days, Months & Seasons

The Earth's rotation causes day and night

• The Earth spins on its axis - the imaginary line running through the North and South Poles.
• One complete turn about this axis takes approximately 24 hours, which defines a day.
• The Sun appears to move across the sky because the Earth's surface is turning beneath it; the Sun isn't actually moving.

Day and night

• As the Earth rotates, the side facing the Sun has daytime.
• The side facing away into shadow has night-time.

The Earth's orbit and the year

• The Earth travels in an orbit around the Sun.
• One complete orbit takes approximately 365¼ days, which defines a year.

Leap years

To account for the extra quarter-day each year, an extra day is added to the calendar every four years on the 29th of February - a leap year.

The seasons are caused by the axis tilt

• The Earth takes about 365¼ days to make one orbit of the Sun, and its axis isn't upright relative to that orbit - it's tilted.
• The seasons are caused by this tilt, not by the Earth moving closer to or further from the Sun.

Opposite seasons in the two hemispheres

• The axis stays tilted in the same direction as the Earth orbits, so the two hemispheres take turns leaning towards the Sun.
• When the northern hemisphere is tilted towards the Sun, it's summer in the north and winter in the south.
• Half an orbit later, the situation is reversed: the southern hemisphere tilts towards the Sun and has summer, while the north has winter.

Why summer and winter feel different

The tilt changes the heating of a hemisphere through day length and the angle of the Sun's rays.

Summer - Hemisphere tilted towards the Sun

• Days are longer than nights, so there are more hours of sunshine to warm the ground.
• The Sun is high in the sky, so its rays strike steeply and are concentrated onto a smaller area of land, causing stronger heating.

Winter - Hemisphere tilted away from the Sun

• Days are shorter than nights, so there are fewer hours of sunshine to warm the ground.
• The Sun is low in the sky, so its rays strike at a shallow angle and spread out over a larger area of land, causing weaker heating.

The Moon's orbit and phases

• The Moon orbits the Earth. One complete orbit takes approximately 28 days (one month).
• The Moon produces no light of its own - it's visible only because it reflects light from the Sun.
• At any moment, only the half of the Moon facing the Sun is lit; the other half is in shadow.

The phases of the Moon

• As the Moon orbits, we view its lit half from different angles, so the amount of lit surface visible from Earth changes. These shapes are the phases of the Moon.
• A full moon occurs when the entire lit half faces the Earth.
• A new moon occurs when the dark half faces the Earth.