The Earth and Its Axis
Earth, the third planet from the Sun is the fifth largest planet in the solar system. Its average distance from the Sun is 150 million km. Its circumference is about 40,000 km and its diameter is roughly 12,756 km across the equator and 12,713 km across the poles. Total surface area of the Earth is about 510 million sq km. In early days people believed that the surface of the Earth is flat. But several simple observations indicate that the idea was wrong. Firstly, the Sun does not rise at the same time in every country. It rises in Kolkata about 41/2 hours earlier than in London. If the Earth were flat, the rising Sun would be visible at the same time at all the places on Earth. Secondly, if we watch the approach of a ship with a telescope, we would first see the smoke, then the funnels, and then the mast and then the hull one by one. If the Earth ware flat, we would see the whole of the ship at one time. Thirdly, in the Bedford Level Experiment, three vertical poles of equal height were set on the surface of the Bedford Canal, at an interval of about 5 km and an observer using a telescope, looked from the top of the first pole towards the third. It was observed that the middle pole is about 1.8 meters above the other two. If the surface of the Earth were flat, the middle pole would not have appeared higher.
It is now confirmed that the shape of the Earth is almost spherical, not perfectly spherical. As the equatorial diameter of the Earth is 43 km more than its polar diameter, it is bulged at the Equator and slightly flattened at each pole. It is rather an ‘oblate spheroid’ whose spin causes it to be flattened at its poles and bulged at the equator. In fact, the shape of the Earth is unique and cannot be compared with anything else. Therefore, the Earth is often referred to as a ‘Geoid’, which simply means ‘earth shaped’. The shape of the Earth can easily be confirmed by the circular shadow of the Earth on the Moon, during a Lunar Eclipse. Further, the pictures of the Earth taken from the Moon by the Apollo astronauts clearly show that the almost spherical shape of the Earth.
While orbiting the Sun, the Earth spins on an imaginary line called an axis that runs from the North Pole to the South Pole. The axis of the Earth is not vertical, it is tilted. Its axis is inclined at an angle of 23½º to the vertical or 66½º to the plane of the orbit. The plane of the orbit is the surface of the orbit or the path in which the Earth revolves round the Sun. The axis of the Earth always points to the Pole Star and remains parallel at every position of the Earth on its orbit. This means, the Earth revolves round the Sun, while spinning on its tilted Axis. The tilting of the Earth’s axis is also called the inclination of the earth’s axis.