Science [2 Mark Questions]

Since Earth also rotates once in 24 hours, a satellite stays in a fixed position relative to a point on Eqrth’s surface. Because the satellite stays over the same spot all the time, this kind of orbit is called ‘geostationary’.

Artificial satellites are made to revolve in an orbit at a height of few hundred kilometres. At this altitude, the friction due to air is negligible. The satellite is carried by a rocket to the desired height and released horizontally with a high velocity, so that it remains moving in a nearly circular orbit.

Stars are the fundamental building blocks of galaxies. Stars were formed when the galaxies were formed during the Big Bang. Stars produce heat, light, ultraviolet rays, x-rays, and other forms of radiation. They are largely composed of gas and plasma (a superheated state of matter).
The brightness of a star depends on their intensity and the distance from the Earth. Stars also appear to be in different colours depending on their temperature. Hot stars are white or blue, whereas cooler stars are orange or red in colour.

The Earth where we live is the only planet in the solar system which supports life. Due to its right distance from the Sun it has the right temperature, the presence of water and suitable atmosphere and a blanket of ozone. All these have made continuation of life possible on the Earth. The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth.

In the early 1600s, Johannes Kepler proposed three laws of planetary motion.

• First Law – The Law of Ellipses
  The path of the planets about the Sun is elliptical in shape, with the center of the Sun being located at one of the foci.
• Second Law – The Law of Equal Areas
  An imaginary line drawn from the center of the Sun to the center of the planet will sweep out equal areas in equal intervals of time.
• Third Law – The Law of Harmonies
  The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their semi major axis from the Sun.

Comets are lumps of dust and ice that revolve around the Sun in highly elliptical orbits. Their period of revolution is very long. When approaching the Sun, a comet vaporizes and forms a head and tail. Some of the biggest comets even seen had tails 160 million (16 crores) km long. Many comets are known to appear periodically. One such comet is Halley’s Comet, which appears after nearly every 76 years. It was last seen in 1986. It will next be seen in 2062.

The planets are spaced unevenly. The first four planets are relatively close together and close to the Sun. They form the inner solar system. The four planets grouped together in the inner solar system are Mercury, Venus, Earth and Mars. They are called inner planets.
They have a surface of solid rock crust and so are called terrestrial or rocky planets. Their insides, surfaces and atmospheres are formed in a similar way and form similar pattern. Our planet, Earth can be taken as a model of the other three planets.