The sun, together with the celestial bodies that revolve around it, forms the solar system. The gravitational attraction between the sun and these objects keeps them revolving around it.

The solar system consists of a large number of bodies such as:

1. Planets
2. Comets
3. Asteroids
4. Meteors

Our solar system has two major components:

1. The Sun
2. The Planets.

The Sun

The sun is the nearest star to earth. It continuously emits huge amounts of heat and light. The sun is the source of almost all energy on the earth. In fact, the sun is the main source of heat and light for all the planets.

The Planets

Planets are the major components of our solar system. They look like stars but they do not have a light of their own. They only reflect the sunlight that falls on them. They do not twinkle like stars. They keep changing their position with respect to stars. A planet has a definite path in which it revolves around the sun. This path is called an orbit. The time taken by a planet to complete one revolution is called its period of revolution.

The period of revolution increases as the distance of the planet from the sun increases. All the planets revolve around the sun in their own orbits, so they don't collide with each other. Besides revolving around the sun, a planet also rotates on its own axis like a top. The time taken by a planet to complete one rotation is called its period of rotation.

How many planets are there in our solar system? There are eight planets in the solar system:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune.

Earth: The earth is the only planet in the solar system on which life exists because it has the right distance from the sun, the right temperature range, suitable atmosphere, a blanket of ozone gas and it has the presence of water.

Earth appears blue and green due to the reflection of light from water and landmass on its surface. The earth's Equatorial plane and orbital plane are not perpendicular to each other; in fact, these are inclined to each other at an angle of 23.5 degrees. This tilt is responsible for the change of season on the earth. The earth has one natural satellite i.e. moon.

Venus: Venus is earth's nearest planet. It is the brightest planet. Sometimes Venus appears in the eastern sky before sunrise and sometimes in the western sky after sunset. Therefore it is often called morning or evening star, although it is not a star.   It rotates from east to west in opposite direction to that of the earth. It also shows phases like the moon. It has no satellite.

Mercury: Mercury is the nearest planet to the sun. It is difficult to observe mercury as most of the time it is hidden in the glare of the sun. Mercury can be observed just before sunrise or just after sunset, near the horizon. It is the smallest planet in the solar system. It has no satellite of its own.

Stars

There are large numbers of stars in the sky. The distance of most of these stars is millions of times the distance of the earth from the sun. Therefore stars appear to us like points.

Sun is also a star but comparatively near to earth so it looks bigger than other stars such as the football near to us looks bigger than the one placed away. Why don't we see stars during the day? Although the stars are present in the sky during the daytime, they are not visible in day time due to bright sunlight.

Don't forget that the sun is also a star. Like our sun all the other stars appear to rise in the east and move towards the west. Do the stars move from east to west? In fact, the earth rotates from west to east and it appears as the stars are moving from east to west.

It is just like as if you are sitting in a fast-moving train and all other objects outside the train seem to be running in the opposite direction Or when one is standing in the center of a big room and rotating fast, the object appears to rotate in the opposite direction. But there is one star that does not change its position i.e. the pole star.

The Pole Star

Look at the two stars at the end of Ursa major and imagine a straight line passing through these stars as shown. If you extend this towards the north direction, this line will lead to a star. This star is a very bright star. This is the Pole Star.

Pole star has the characteristics of remaining stationary. If one observes Ursa major 3-4 times at an interval of 2 or 3 hours, during a summer night, Ursa major appears to move from east to west and appears to revolve around the pole star. In fact, all stars appear to move around the pole star. The pole star is not visible from the southern hemisphere of the earth.

The Moon

We all see a big bright heavenly body at night in the sky, which changes its shapes daily. Someday it is in full shape while on some other day it is not visible even in the clear sky. This is the moon.
Moon does not have its own light. It does not have an atmosphere like earth, so life is not possible on the moon. Moon is a natural satellite of earth and it revolves around the earth

The moon revolves around the earth and also rotate on its axis as the earth goes around the sun
The moon completes one revolution around the earth in 29.5 days and takes 27.8 days to complete one rotation around its axis. As the revolution and rotation time periods of the moon are approximately the same therefore we are unable to see the backside of the moon from the earth.
On July 21, 1969, the American Astronaut, Neil Armstrong landed on the moon for the first time. He was followed by Edwin Aldrin.

They found that the moon's surface is dirty and barren. They found many craters of different sizes on the surface of the moon. They found a large number of steep and high mountains. It has no atmosphere and no water. There is no medium so we can't hear any sound on the moon. Now, the moon is not a distant story. Scientists are exploring it more and more to get more information about it.

Satellites

A satellite is an object that revolves around a planet in a circular or an elliptical path. The moon is the Natural satellite of the earth.

There are many manmade satellites that are launched from the earth that revolve around the earth much closer than the earth's natural satellite. These are called artificial satellites. They revolve around the Earth much closer than the earth's natural satellite, the moon.

Advantages of Artificial Satellites: Artificial satellites have many practical applications.

They are used for:
-Forecasting weather
-Transmitting television signals
-Transmitting radio signals
-Telecommunication & remote sensing

Meteorites

Have you ever seen a bright streak of light in the sky at night when the sky is clear and the moon is not there? Yes, You call them shooting stars but they are not stars. They are meteors. Meteors are members of our solar system. A meteor is usually a small object that occasionally enters the earth's atmosphere.

While entering the earth's atmosphere it has a very high speed. When it enters the earth's atmosphere, it gets heated up due to friction offered by the earth's atmosphere and it glows and we see a bright streak. But it evaporates quickly therefore we see it for a very short time. Some meteors are large and thus they are able to reach the earth's surface before they evaporate completely

The body that reaches the earth is called a meteorite. Large meteorites (meteoroids) may strike the ground with a significant fraction of their cosmic velocity and leave behind a crater.

Few craters formed by meteorites are:-

1) Barringer Meteor crater
2) Odessa Meteor crater
3) Wabar craters
4) Wolfe creek crater
5) Shiva crater (beneath the Indian ocean west of Mumbai)

Meteorites help scientists in investigating the nature of the material from which the solar system is formed. Sometimes, when the earth crosses the tail of a comet, a large number of meteors are seen. This celestial phenomenon is known as a meteor shower.

Constellations

When we look at the sky we see some groups of stars forming different shapes. These shapes resemble familiar objects. The stars forming a group that has a recognizable shape is called a constellation. Some examples of constellations are:-

Ursa major, also known as Big Dipper, the great bear or the saptrishi, Orion, Cassiopeia, etc

In Ursa major, or saptrishi, there are seven prominent stars. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl.

If you try to see the constellation for a few hours, it appears to move in the sky from east to west. We can see Ursa Major in the northern part of the sky during summer nights. Ursa major is a northern constellation so may be visible from some regions in the southern hemisphere

Orion can be seen during the winter season in the late evenings. It also has 7 or 8 bright stars, and due to its shape, it is also called the Hunter. The four bright stars appear to be arranged in the form of a quadrilateral. The bright star in the sky, Sirius is located close to Orion. On Imagining a straight line passing through the three middle stars of the Orion to the east, one will find the star Sirius.

Cassiopeia is visible during the winter season in the early part of the night and looks like a distorted W or M.

Space Facts

1. First Artificial satellite was Sputnik, launched by the Soviet Union on 4^th October 1957.

2. India has also built and launched several artificial satellites. Aryabhatta was the first Indian artificial satellite. Some other Indian artificial satellites are:-

• INSAT
• IRS
• KALPANA-1
• DUSATY

3. The most spectacular meteor shower is probably the Leonids which peaks on a day near 17 November. Approximately every 33 years, the Leonid shower produces a meteor storm.

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