Motions of the Earth

The Earth is never still. It carries out two separate movements at the same time. Together they explain two of the most basic rhythms of our lives: the daily change from day to night, and the yearly cycle of seasons. The two motions are called rotation and revolution. Keeping them clearly apart (what each one is, how long it takes, and what it causes) is the heart of this topic.

Rotation is the spinning of the Earth on its own axis, the imaginary line through the two poles. The Earth turns from west to east, which is why the Sun appears to rise in the east and set in the west. One complete rotation takes about 24 hours, which we call an earth day.

Revolution is the movement of the Earth around the Sun along a fixed path called its orbit. The Earth completes one revolution in about 365 and a quarter days, which we call an earth year. As it travels this orbit, the Earth's axis stays tilted at a constant angle.

The key contrast to remember: rotation gives us the day, while revolution gives us the year.

Neither the spin nor the axis is perfectly fixed. Scientific studies show that the Earth's rotation and the direction of its axis are slowly shifting. The main culprit is melting polar ice. As the ice sheets melt, the meltwater flows away from the poles and moves towards the equator. This shifts a huge amount of mass across the planet and changes its moment of inertia, the way the Earth's mass is spread around its spin axis. A spinning body responds to such redistribution, so the axis drifts and the rotation alters slightly. Mass redistribution from melting ice therefore explains the observed axial shift.

One distractor is worth clearing up. Solar flares and coronal mass ejections do strike the Earth's outermost atmosphere with enormous energy. They can disturb satellites, radio signals and power grids. But they do not cause the shift in the Earth's axis. The axial drift comes from how mass moves on and within the Earth itself, not from energy arriving from the Sun.

Because the Earth is a sphere, the Sun can light only one half of it at any moment. The half that faces the Sun has day, while the half turned away has night. As the Earth rotates, places move from the lit side to the dark side and back again, so day and night follow each other in a steady cycle.

The imaginary line that separates the lit half from the dark half is called the circle of illumination. It does not pass through the poles, because the Earth's axis is tilted rather than upright. If the Earth did not rotate, the half facing the Sun would have endless day and scorching heat. The other half would suffer endless freezing night. Life as we know it could not survive.

Here lies the most important and most tested idea of the topic. The Earth's axis is not upright. It is tilted at an angle of 23.5 degrees from the vertical. The seasons are caused by this tilt working together with the revolution, not by the Earth being nearer to or farther from the Sun.

As the Earth moves around the Sun, the tilt makes different parts lean towards the Sun at different times of the year. The half leaning towards the Sun receives more direct rays and has longer days. That half enjoys summer. At the same time, the other half leans away. It receives slanting rays, has shorter days, and experiences winter. Six months later the situation is reversed. This is why the Northern and Southern Hemispheres always have opposite seasons.

Four positions of the Earth in its orbit mark the turning points of the year, and their dates are worth remembering.

On about 21 June, the Northern Hemisphere is tilted towards the Sun. The Sun's rays fall directly on the Tropic of Cancer. The Northern Hemisphere has its longest day and shortest night, and it is summer there. This position is the summer solstice.

On about 22 December, the situation is exactly reversed. The Southern Hemisphere is tilted towards the Sun, and the Sun's rays fall directly on the Tropic of Capricorn. Now the Southern Hemisphere has its longest day, while the Northern Hemisphere has its shortest day and its winter. This is the winter solstice.

On about 21 March and 23 September, neither pole is tilted towards the Sun. The Sun's rays fall directly on the Equator, and day and night are equal all over the world. Each of these positions is an equinox (the word means "equal night"). The March date is the spring equinox and the September date the autumn equinox for the Northern Hemisphere.

A small but neat consequence of revolution is the leap year. The Earth actually takes 365 days and about 6 extra hours to go once around the Sun. For convenience we count an ordinary year as 365 days and ignore the spare 6 hours.

Those 6 hours, however, add up. Over four years they make a full day (6 hours times 4 equals 24 hours). So every fourth year we add one extra day to the calendar, on 29 February, making that year 366 days long. A year with this extra day is called a leap year.