11.4 OUR PLANET EARTH

Our planet Earth isthe largest of the inner planets of the solar system. It is the third planet out from the sun.The photograph on the right was taken from a rocket out in space. The Earth is almost a perfect sphere but it is slightly flattened at the north and south poles. It takes exactly one year (365¼ days) to complete its orbit around the sun and it spins from left to right (west to east) on an axis through the poles.It takes exactly one day (24 hours) to complete each rotation.

More than 70% of the Earth’s surface is covered by the oceans and there are large areas of land that we call continents. On this photo, you can see North America and South America, and on the photo in Module 11.1 you can see Africa and Antarctica. The other continents are Europe, Asia and Australia.(Sometimes Europe and Asia together are called Eurasia, and sometimes the name Oceania is used to include Australia, New Zealand and the islands of the Pacific Ocean).

The equator is an imaginary line around the Earth, half way between the north and south poles. Some basic facts about the Earth are summarised in the table below.

Some basic facts about planet Earth
Diameter at equator / Circumference at the equator / Mass / Mean distance from sun / Time for 1 rotation / Time for 1 orbit
12,756 km / 40,075 km / 5.97 x 1024 kg / 149.6 million km / 24 hours / 365¼ days

Day and night: The axis of rotation of the Earth is an imaginary line passing through the north and south poles. The Earth rotates around this axis once every day (24 hours) and this rotation causes day and night. In the photo above, the sun is shining from behind youon your left side. The side of the Earth facing the sun is having daylight. As the Earth rotates from left to right (west to east), you can see that South America will soon pass from daylight into night.

Of course, to people on the Earth, it looks as if the sun rises in the east, moves across the sky, and sets in the west. We are not usually aware that the cycle of day and night is actually caused by the earth spinning, not by the sun moving!Try the ‘thought experiment’ in the box using the simple model below.

  • 1. How do you think we can define an hour?
  • 2. How many times does the Earth spin on its axis in a year?
  • 3. On Earth, how long are the longest day and the longest night?

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