This topic gives an overview of;
The Sun and the celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational attraction between the Sun and these objects keeps them revolving around it.
The Earth, as you know, also revolves around the Sun. It is a member of the solar system. It is a planet. There are seven other planets that revolve around the Sun. The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars ,Jupiter, Saturn, Uranus and Neptune.
Till 2006 there were nine planets in the solar system. Pluto was the farthest planet from the Sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto does not fit this definition. It is no longer a planet of the solar system.
The Sun is the nearest star from us. It is continuously emitting 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 look like stars, but they do not have light of their own. They merely reflect the sunlight that falls on them. The simplest method of identifying planets from stars is that stars twinkle, whereas planets do not. Also the planets keep changing their positions with respect to the 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 increases from the sun.
Go out into the playground with four or five of your friends. Draw four circles of radii 1m, 1.8m, 2.5m and 3.8m, all having a common centre Ask one of your friends to stand in the centre and represent the Sun. Your other four friends may represent Mercury, Venus, Earth and Mars. Ask your friends to move around the Sun in anti-clockwise direction in their own orbits.
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.
Some planets are known to have moons/satellites revolving round them. Any celestial body revolving around another celestial body is called its satellite. The Earth revolves around the Sun. The Earth can be said to be a satellite of the Sun, though generally we call it a planet of the Sun. We use the term satellite for the bodies revolving around planets. Moon is a satellite of the Earth. There are many man-made satellites revolving round the Earth. These are called artificial satellites.
The planet mercury is nearest to the Sun. It is the smallest planet of our solar system. Because Mercury is very close to the Sun, it is very difficult to observe it, as most of the time it is hidden in the glare of the Sun. However, it can be observed just before sunrise or just after sunset, near the horizon. So it is visible only at places where trees or buildings do not obstruct the view of the horizon. Mercury has no satellite of its own.
Venus is earth’s nearest planetary neighbour. It is the brightest planet in the night sky.
Sometimes Venus appears in the eastern sky before sunrise. Some times it appears in the western sky just after sunset. Therefore it is often called a morning or an evening star, although it is not a star. Try to locate Venus in the night sky.
Find out from some newspaper or from an almanac the time when Venus is visible in the sky. You can easily recognise Venus by its brightness. Remember that Venus cannot be seen very high in the sky. You must try to observe Venus either 1-3 hours before sunrise or 1-3 hours after sunset.
Venus has no moon or satellite of its own. Rotation of Venus on its axis is somewhat unusual. It rotates from east to west while the Earth rotates from west to east.
If you get a chance, try to observe Venus through a telescope. You will observe that Venus shows phases just like the moon
The Earth is the only planet in the solar system on which life is known to exist. Some special environmental conditions are responsible for the existence and continuation of life on the Earth. These include just the right distance from the Sun, so that it has the right temperature range, the presence of water and suitable atmosphere and a blanket of ozone.
From space, the Earth appears blue-green due to the reflection of light from water and landmass on its surface.
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. The Earth has only one moon.
You are familiar with the equator of the Earth. The plane of the equator is called the equatorial plane. The plane in which the Earth revolves round the Sun is called the orbital plane of the Earth. These two planes are inclined to each other at an angle of 23.5º. This means that the axis of the Earth is inclined to its orbital plane at an angle of 66.5º.
The next planet, the first outside the orbit of the Earth is Mars. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites.
Jupiter is the largest planet of the solar system. It is so large that about 1300 earths can be placed inside this giant planet. However, the mass of Jupiter is about 318 times that of our Earth. It rotates very rapidly on its axis.
If you take a large ball which can accommodate about 1300 peas, then the ball will represent Jupiter and one pea will represent the Earth. Jupiter has a large number of satellites. It also has faint rings around it. You can easily recognise Jupiter as it appears quite bright in the sky. If you observe it with the help of a telescope, you can also see four of its large moons.
Beyond Jupiter is Saturn which appears yellowish in colour. What makes it unique in the solar system is its beautiful rings. These rings are not visible with the naked eye. You can observe them with a small telescope. Saturn also has a large number of satellites.
One interesting thing about Saturn is that it is the least dense among all the planets. Its density is less than that of water.
These are the outermost planets of the solar system. They can be seen only with the help of large telescopes. Like Venus, Uranus also rotates from east to west.The most remarkable feature of Uranus is that it has highly tilted rotational axis. As a result, in its orbital motion it appears to roll on its side.
The first four planets, Mercury, Venus, Earth and Mars are much nearer the Sun than the other four planets. They are called the inner planets. The inner planets have very few moons.
The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much farther off than the inner planets. They are called the outer planets. They have a ring system around them. The outer planets have large number of moons.
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