The Earth is the third planet from the Sun. For the innumerable living inhabitants of this sphere, including more than six billion humans, it is the equivalent of a source of life.

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How old is Earth? Between 4.5 and 4.6 billion years, say those who use the estimated age of the solar system as the reference point.

We did not always have a clear notion of “planet”. In the past there were numerous interpretations about the characteristics of Earth, with some believing it was flat, and others denied it had any relationship with its surroundings.

But at different points around the Earth, the first scrutinizers and investigators of the cosmos found the key. Now we know much more. For example, its mass is 6×1,021 tons and its diameter is 12,756.3 km.

Information about the Earth abounds on the Internet. Many websites are dedicated to “Earth Sciences”, and some of the most informative are about geology, defined by one site as the study of the process of the planet's formation, the materials of which it is composed, its history and its changes over time.

There are also educational sites about our planet, its role in the solar system, and there are a lot of good resources from NASA, including incredible images.