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Chapter 24 - The Universe


Universe is a vast space and all the heavenly bodies that lie in it. It is hard to determine the size and limitation of the universe. The universe consists of countless galaxies in it and galaxies has numerous stars.

Light year
A unit of astronomical distance equivalent to the distance that light travels in one year, which is 9.4607 × 1012 km (nearly 6 million million miles).
Light travels in space at a speed of 300,000 km each second. A light year is the distance light travels in one year. Light travels 9.46 x 1012 km in one year. So, one light year is equivalent to 9.46 x 1012 km nearly equals to 9.5 x 1012 km.

Solar system: A solar system is a group of different heavenly bodies that revolve around the sun including the sun itself. It includes many smaller objects that revolve around the sun. All eight planets and their satellites as well as asterois, comets, meteoroids, etc revolve around the sun. Sun contains 99.8 % of the total mass of solar system. Life on earth is made possible due to the heat and light energy of the sun and sun mainly contains hydrogen and helium. The distance of sun from earth is 150,000,000 km. contains hydrogen and helium

Motion of the earth:

The earth spins around its axis once in 24 hours is called rotation of the earth. This is equal to one day. It goes around the sun in fixed path or orbit i.e. once in 365 days and it is called revolution or annual motion.

Sun, earth and moon

Moon is the nearest neighbour of the earth. It is only the satellite of the earth. We know tha earth goes around the sun in its orbit once in a year. Similarly, the moon also revolves around the earth in its orbit. The revolution period of the moon around the earth is named differently as sideral month and synodic month.

Sideral month: The moon makes a complete revolution around the earth once in a 27.33 days, it is called sideral month. It's other name is Nakshyatramaas.

Synodic month: The duration for the moon to move from M1 position to M3 position is called synodic month. or, The duration between two successive phases of the moon which is 29.5 days is called synodic month. It's other name is Chandramaas.

Umbra and Penumbra:

From above figure,
From the region 'a' of the shadow a part of the light of the lamp is seen but from 'b' light is not seen at all. The region of complete darkness where all the light from the source is blocked is called umbra. Here from figure 'b' is umbra.

The region of partial darkness, where the light rays are partially blocked is called penumbra.

Lunar eclipse:
When the earth passes between sun and moon the shadow of the earth may fall on the moon and causes the partial or complete darkness of the moon's surface. It is called lunar eclipse. And if the moon's surface is completely darkened it is called total lunar eclipse and if it's surface is darkened partly it is called partial lunar eclipse.

Solar eclipse:
When the moon passes between the sun and the earth and the sun is block, a shadow is cast on the earth. The sun is partly or wholly darkened.

Annual Solar eclipse:
If the umbral cone formed by the moon's shadow meets before it reaches the surface of the earth due to the increase in the distance between the moon and the earth, the bright part of the sun is seen as a bright ring called annualar eclipse.

Eclipses donot occur on every full moon or new moon day.

Some questions from this lesson:
1. Difference between Solar eclipse and lunar eclipse.
2. What is the effect of diurnal motion?
3. What is annual eclipse? When does it occur?
4. How does lunar eclipse occur? Explain with diagram.
5. What is light year? How many kilometres make one light year?

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