G-6: SOLAR SYSTEM

We can see the full moon only once in about a month’s time. It is Full moon night or Poornima.
A fortnight later, you cannot see it at all. It is a New moon night or Amavasya. On this day, you can watch the night sky best, provided it is a clear night.

We can’t we see the moon and all those bright tiny objects during day time because of the very bright light of the sun that does not allow us to see all these bright objects of the night sky.
The sun, the moon and all those objects shining in the night sky are called celestial bodies. Some celestial bodies are very big and hot. They are made up of gases. They have their own heat and light, which they emit in large amounts. These celestial bodies are called stars.
We may notice various patterns formed by different groups of stars. These are called constellations. Ursa Major or Big Bear is one such constellation. One of the most easily recognisable constellation is the Saptarishi (Sapta seven, rishi-sages). It is a group of seven stars that forms a part of Ursa Major Constellation.
In ancient times, people used to determine directions during the night with the help of stars.

The North star indicates the north direction. It is also called the Pole Star. It always remains in the same position in the sky. We can locate the position of the Pole Star with the help of the Saptarishi.
Some celestial bodies do not have their own heat and light. They are lit by the light of the stars. Such bodies are called planets.
Satellite is a celestial body that moves around the planets in the same way as the planets move around the sun.
Human-made Satellite is an artificial body. It is designed by scientists to gather information about the universe or for communication. It is carried by a rocket and placed in the orbit around the earth. Some of the Indian satellites in space are INSAT, IRS, EDUSAT, etc.

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- M – Mercury (बुध)
- V – Venus (शुक्र)
- E – Earth (पृथ्वी)
- M – Mars (मंगल)
- J – Jupiter (बृहस्पति)
- S – Saturn (शनि)
- U – Uranus (अरुण)
- N – Neptune (वरुण)
The sun, eight planets, satellites and some other celestial bodies known as asteroids and meteoroids form the solar system.
Sun is in the centre of the solar system. It is huge and made up of extremely hot gases. It provides the pulling force that binds the solar system. The sun is the ultimate source of heat and light for the solar system. The sun is about 150 million km away from the earth.
All the 8 planets of the solar system move around the sun in fixed paths which are elongated. and are called orbits.
Mercury is nearest to the sun. It takes only about 88 days to complete one round along its orbit.
Venus is considered as ‘Earth’s-twin’ because its size and shape are very much similar to that of the earth.
Till August 2006, Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (Ceres, 2003 UB313) discovered in recent past may be called dwarf planets.

The Earth is the third nearest planet to the sun. In size, it is the fifth largest planet. It is slightly flattened at the poles. That's why, its shape is described as a Geoid. Geoid means an earth-like shape.
Conditions favourable to support life are probably found only on the earth. The earth is neither too hot nor too cold. It has water and air, which are very essential for our survival. The air has life-supporting gases like oxygen. Because of these reasons, the earth is a unique planet in the solar system. From the outer space, the earth appears blue because its two-thirds surface is covered by water. It is, therefore, called a blue planet.

Our earth has only one satellite, that is, the moon. Its diametre is only one-quarter that of the earth. It appears so big because it is nearer to our planet than other celestial bodies. It is about 3,84,400 km away from us.
The moon does not have conditions favourable for life. It has mountains, plains and depressions on its surface.
The moon moves around the earth in about 27 days. It takes exactly the same time to complete one spin. As a result, only one side of the moon is visible to us on the earth.

Apart from the stars, planets and satellites, there are numerous tiny bodies which also move around the sun. These bodies are called asteroids. They are found between the orbits of Mars and Jupiter.

The small pieces of rocks which move around the sun are called meteoroids. Sometimes these meteoroids come near the earth and tend to drop upon it. During this process due to friction with the air they get heated up and burn. It causes a flash of light. Sometimes, a meteor without being completely burnt, falls on the earth and creates a hollow.

A whitish broad band, look like a white glowing path across the sky on a clear starry night is a cluster of millions of stars. This band is the Milky Way galaxy. Our solar system is a part of this galaxy.

In ancient India, it was imagined to be a river of light flowing in the sky. Thus, it was named Akash Ganga.
A galaxy is a huge system of billions of stars, and clouds of dust and gases. There are millions of such galaxies that make the Universe.

Jupiter, Saturn and Uranus have rings around them. These are belts of small debris. These rings may be seen from the earth with the help of powerful telescopes.
‘Sol’ in Roman mythology is the ‘Sungod’. ‘Solar’ means ‘related to the sun’.
Geography has its origin in Greek, which relates to the description of the earth. It is made of two Greek words, ‘ge’ meaning’ ‘earth’ and ‘graphia’ meaning ‘writing’.
Similarly, logy -logia(study of the earth) in geology, metry -metria(measurement of the earth) in geometry, oid -oeides(resembles the shape of the earth).
Those who study the celestial bodies and their movements are called astronomers. Aryabhatta was a famous astronomer of ancient India. He said that the moon and the planets shine due to reflected sunlight.
Light travels at the speed of about 300,000 km per second. Even with this speed, the light of the sun takes about 8 minutes to reach the earth.
Neil Armstrong was the first man to step on the surface of the moon on 20 July 1969.

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