People in the ancient times believed that the Earth is the centre of the Universe and all the other planets, such as the Sun and the Moon rotate around it. However, the astronomers of that time observed that instead of following a straight path, the planets made backward and forward movements. The Greek astronomers explained that the tiny spheres of the planets made little circles as they orbited around the Earth. However, in the 16th century, the Polish mathematician and astronomer Nicolaus Copernicus, through his heliocentric model of the solar system, presented that the Earth and the other planets orbited around the Sun. Galileo supported his observations when he discovered that Jupiter has moons of its own and Venus had phases like the Moon. After few years, the idea was considered and the scientific establishment agreed that Earth is just another planet orbiting around the Sun and is not the centre of the Universe.
The third planet from the Sun between Venus and Mars, the Earth takes about 3651/4 days to complete its orbit around the Sun. One complete year on the Earth amounts to 365 days but 1/4th is added for one extra day, every four years, that is, on the 29th of February. Its diameter is about 12,760 km with a mass of about 5.97219 x 1024 kg, the Earth’s orbital speed is about 29.8 km per second. Its minimum distance from the Sun is about 146 million km. Due to such distance, it takes about 8 minutes and 19 seconds for light to reach the Earth.
The atmosphere of the Earth is mainly composed of nitrogen (78%), oxygen (21%), argon (0.93%) and carbon dioxide (0.03%). The fifth largest planet in the solar system, the atmosphere of the earth is divided into 5 sections namely Troposphere, Ozone layer, Stratosphere, Mesosphere and Thermosphere. The atmosphere is 483 km thick, but most of it is within 16 km of the surface of the earth. As it rises, the atmosphere gets thinner and thinner until it merges with outer space. The planet has an average temperature of about 13°C but the temperature rises to 35°C due to the greenhouse effect. From its surface, the earth has several layers with unique properties such as the crust, upper mantle, transition region, lower mantle, D layer, the outer core and the inner core. The surface of the Earth is fragile and can be split into plates, its crust and its hot, liquid core. Along these tectonic plates, earthquakes, volcanoes and mountain building takes place. Due to such geological activity, there are fewer craters on the surface of the Earth and these craters have been sunk down or worn away by wind and rain over millions of years.
Several elements make up the crust of the earth such as iron (32%), oxygen (30%), silicon (15%), magnesium (14%), sulphur (3%), nickel (2%), and traces of calcium (1.5%) and aluminium (1.4%) and other elements. Huge plates float on the mantle, which is divided by the crust.
The Earth has only one Moon and natural satellite, the Moon. It is a relatively large, planet-like satellite and the largest moon in the solar system. It has a diameter of about one-quarter of that of the Earth. Tides are caused on the Earth and tidal locking is caused on the Moon due to gravitation between the Earth and the Moon. This tidal interaction has led the Moon to recede from the earth at the rate of approximately 38 mm/year.
The early Greeks believed that Gaea or Mother Earth was the great goddess and was worshipped as the universal mother. According to the Greek mythology, she created the Universe and gave birth to the first race of gods as well the first humans.