Like all members of the Solar System, the motion of the Earth in space is dependent on the Sun. Our planet is compelled by the powerful solar gravity to describe a slightly elliptical orbit, with a radius of approximately 150 million km.

One full orbit takes approximately 365 days (more accurately 365.256 days). In order to keep our calendar closely regulated by the Sun, we normally have three "common" years of 365 days followed by a "leap" year of 366 days in each four-year interval.

In the Northern Hemisphere winter, the distance from the Sun is least - approximately 147.1 million km. Six months later, the distance has increased to 152.1 million km. As a result of this ellipticity, the solar radiation falling on the Earth in the Southern Hemisphere summer is about 7 per cent greater than in the northern summer.

Even such a large difference has little effect on the Earth's climate, however, because this is dominated by the distribution of continents and oceans in the two hemispheres.