Our rotating globe Copernican System The length of day and night On an angle Summer and Winter Solstice

Our Solar System has one central star, the Sun. This makes it very easy to define day and night:

• Day occurs when the part of the Earth you are on faces the Sun;
  
• Night therefore occurs when your part of the Earth faces away from the Sun.

This is fairly simple, but why then does the length of day and night vary at different times of the year and in different places on the Earth's surface? Another question to ponder is why one person's day in Europe is our night in Australia?

Our rotating globe
Day and night is caused by the rotation of the Earth on its polar axis once every 24 hours, hence the combined length of day and night is always 24 hours.

Day and night

Day and night are caused by the rotation of the Earth on its axis. There are two basic types of day. The Sidereal Day is the time it takes for the Earth to rotate in relation to the distant stars; its length is 23 hours 56 minutes and 4 seconds.

The Solar Day is the time it takes for the Earth to return to the same orientation with respect to the Sun; its length averages exactly 24 hours.

The difference arises because the Earth moves about one degree around its orbit over the course of a day, and must rotate a little more for a given point on Earth to exactly face the Sun again. In the remainder of this section, we shall use the term day to indicate the 24-hour solar day.

Copernican System
Long ago, it was generally believed that the Earth was the centre of all things, with the Sun and other planets orbiting around it. Models based on this idea were developed by ancient Greek philosophers such as Aristotle and Ptolemy. However, as we know today, the Earth and other planets revolve around (or orbit) the Sun and the Earth turns (rotates) on its own axis.

This model was put forward by the Polish Astronomer Nicolas Copernicus. He published details of the Copernican System in 1543, many years after writing his ideas down - and then only when he was dying. To the Earth-centric church of the middle ages, such ideas were heresy, and Copernicus may not have lasted until 1543 if he'd tried to publish them earlier!

The length of day and night
Although the Earth rotates once every 24 hours on its polar axis through the North and South Poles, day and night do not share the 24 hours equally - except on two dates, around 21^st September and 21^st March, known as the Equinoxes. On these two dates, day and night are each exactly 12 hours long all over the Earth, except at the North and South Poles where night and day exchange places on these dates.

From 22^nd September to 20^th March, days are longer than nights in the Southern Hemisphere and correspondingly shorter in the Northern Hemisphere. From 22^nd March to 20^th September, days are longer than nights in the Northern Hemisphere and correspondingly shorter in the Southern Hemisphere. This is taken to the extreme at the North and South Poles, where daylight lasts for 24 hours for the summer half the year and nights for 24 hours for the winter half year. That is, a day lasts 6 months followed by a 6 month night.

On an angle
The variation in the length of day over a year is due to the angle, or tilt, at which the Earth rotates in relation to its orbit around the Sun. If the Earth's rotation was perpendicular to its orbit, then day and night would be 12 hours each for the whole year. Because the Earth is tilted at an angle of 23.5° to this direction, we have more daylight in summer than we do in winter and the opposite during winter.

Carefully watch the observer in the following animations as the Earth rotates once to see the difference in the length of a day between summer and winter.

Summer day

Winter day

Summer and Winter Solstice
The longest day of the year, the summer solstice, occurs on 22^nd December in the Southern Hemisphere. In the Northern Hemisphere, this day corresponds to the shortest day of the year or winter solstice.

Six months later on, 22^nd June, the shortest day occurs in the Southern Hemisphere and the longest day in the Northern Hemisphere. Both these days are three months from the Equinoxes.

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