PLANET EARTH: SHAPE, SIZE AND UNIQUENESS IN THE SOLAR SYSTEM

 PLANETS

The word 'planet' originates from a Greek word meaning 'wanderer'. Planets are bodies that

• revolve around the sun,
• rotate on their own axes (plural of axis),
• do not have heat or light of their own and
• shine only because they reflect sunlight

Planet Earth, as seen from the moon. Although the Apollo 11 team spent only two and a half hours on the moon and went only 60 m from their landing capsule, they brought back information that has helped many years of subsequent research.

Apart from the earth, seven other major planets revolve around the sun in their respective orbits. The eight planets in order of their distance from the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Earth is the only planet known to support life. 

The planets orbit the sun in the same direction; the closer they are to the sun, the faster they revolve. The inner planets (Mercury, Venus, Earth, Mars) have a shorter period of revolution.

Modern science has revolutionized astronomy and the study of the Earth. Advanced technology has equipped us with optical and radio telescopes and space telescopes. Several unmanned and unmanned missions have also been sent out in spacecraft. Space stations were established to enhance our knowledge about the universe. Thousands of spacecraft, hundreds of artificial satellites, and probes have gone out on various space missions after the first artificial satellite, Sputnik, was launched in 1957. The satellite pictures help us to know, forecast, and track the weather conditions. They facilitate quick telephone TV and radio communications.

SIZE OF THE EARTH

Eratosthenes, an ancient Greek mathematician, estimated the circumference of the earth to be approximately 46,250 km. The figure is very close to modern calculations based on the assumption that the earth is a sphere and thus as a complete circle with an angular distance of 360°, its circumference is (360°/7°12' ×800) 40,000 km.

Further studies revealed that the earth's equatorial diameter is slightly greater than the polar diameter. The difference between the two is 42 km. This is the result of the centrifugal force created by the rotation of the earth.

MOON: EARTH'S COMPANION

The moon is the earth's only natural satellite. Its diameter is 3,475 km, about one-fourth of that of the earth. The moon and the earth revolve around a common center of gravity located about 40,000 km inside the earth. This gravitational pull and the gravitational pull of the earth keep the moon rotating on the axis.

The moon completely lacks an atmosphere. On the moon's surface, the force of gravitation is about one-sixth of that of the Earth. The moon revolves around the sun in 27 days and it takes a rotation in 29 days, therefore, we see the same side of the moon.

OUR EARTH- A UNIQUE PLANET

Our Earth when viewed from space, gives us a  magnificent perspective. When the astronauts first landed on the moon they saw our Earth as no one had ever seen it before. Our Earth is the third planet with respect to the distance from the sun and the fourth-largest in size in the family of the sun. It is indeed unique because in the vast universe it is the only planet known so far to have life in it.

The following factors are responsible for sustaining life on Earth.

• The earth is located at an optimum distance from the sun. It is thus neither as hot as Mercury and Venus nor as cold as Neptune. The average temperature is over 14° C.

• The presence of water in oceans, seas, lakes, rivers, ponds sustains all kinds of life on the Earth and makes our planet unique. Life first began on the earth 3,000 million years ago. The development of life forms, in turn, helped to create an atmosphere mainly compromised of nitrogen and oxygen.
• In its atmosphere, there are gases like oxygen that support life.
• The atmospheric layer that envelops the earth regulates the temperature on the planet. It allows regulates the temperature on the planet. It allows sunlight to pass through but resists harmful radiations like ultraviolet rays. It also burns up and consumes the meteors that hurtle down when pulled in by the gravitational force of the earth.
• Earth's uniqueness, as mentioned, is because of the ability to support life. It has four realms, the lithosphere (land), hydrosphere(water), atmosphere(air), and biosphere(living sphere where life exists).

Four realms of the Earth

The lithosphere has soils and rocks that provide mineral nutrients and habitat for animals and plants directly or indirectly. The hydrosphere is available in three states- solid, liquid, and gas. It helps to dissolve and transport nutrients from the soil for the plants to enable them to synthesize their food. Water is an important constituent of the bodies of all animals including humans. All chemical reactions in their bodies are carried out with the help of water. The atmosphere constitutes an ideal mix of gases. Air and water along with the heat from the sunset the water cycle in motion. This does not happen on other Planets. The biosphere is the zone where all the three other planets. The biosphere is the zone where all the three realms meet, different reactions and interactions enable life to occur and therefore life exists.