A watery living world

Earth is the third planet from the Sun, with an average orbital radius of 150 million kilometres. This distance is also known as one astronomical unit (1 AU).

Earth is the fifth largest planet, with a diameter of 12 756 km. This makes Earth slightly larger than Venus. Earth orbits the Sun between Venus and Mars, and is 1.4 times further from the Sun than is Venus.

Earth is thought to have formed at the same time as the other planets in the solar system, about 4.5 billion years ago. Scientists know the age of the Earth because the oldest rocks ever discovered are 4.3 million years old (determined from radioactive dating). Earth is the largest of the four rocky or terrestrial planets (Mercury, Venus, Earth and Mars). Like Mercury and Venus, Earth formed from a hot and molten state before it cooled to become a solid planet. Even though it has cooled since formation, Earth is currently the most geologically active planet, and its interior is still hot. The main feature that separates Earth from the other planets in the solar system is that it is the only planet to contain water in the liquid state.

Figure 6.1 The Earth as seen from a satellite about 36 000 km out in space. Water covers about 70 per cent of the Earth. Land covers only about 30 per cent. (Photo: NASA)

Table 6.1 Details of Earth

Distance from Sun

149 600 000 km (1.0 AU)


12 756 km


5.97 x 1024 kg


5.52 g/cm3 or 5520 kg/m3

Orbital eccentricity

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