Science Class Eight  

Stars And The Solar System

Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes: the satellites of the planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium.

The moon is the brightest object in the night sky. The stars, the planets, the moon and many other objects in the sky are called celestial objects.

solar system

The Moon

The Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is 384,403 km, about thirty times the diameter of the Earth. The common centre of mass of the system is located about 1,700 km—a quarter the Earth's radius beneath the surface of the Earth. The Moon makes a complete orbit around the Earth every 27.3 days (the orbital period), and the periodic variations in the geometry of the Earth–Moon–Sun system are responsible for the lunar phases that repeat every 29.5 days.

The Moon is the only celestial body to which humans have traveled and upon which humans have performed a manned landing.


Eclipses can occur only when the Sun, Earth, and Moon are all in a straight line. Solar eclipses occur near a new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur near a full moon, when the Earth is between the Sun and Moon.

Because the Moon's orbit around the Earth is inclined by about 5° with respect to the orbit of the Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.

The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by the Earth, is described by the soars cycle, which has a period of approximately 6 585.3 days (18 years 11 days 8 hours).

The Stars

A star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun.

For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature.


A constellation is what astronomers call an asterism: a group of celestial bodies (usually stars) that appear to form a pattern in the sky or appear visibly related to each other. Examples are Orion (which appears like a human figure with a belt, often referred to as "The Hunter"), Leo (which contains bright stars that outline the form of a lion), Scorpius (which can seem reminiscent of a scorpion), and Crux (a cross).

Constellations are names for groups of stars that appear to form shapes in the sky. They were given their names many hundreds of years ago to help us remember which stars are which. We use constellations to divide up the sky; finding one can help us find another because constellations move so slowly that, in our lifetime, they will always be found in about the same place.

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The Big Dipper (Ursa Major or the Great Bear):

The best known group of stars is the Big Dipper. The Big Dipper is a group of seven bright stars, 3 which form a handle and 4 which form a bowl. The Big Dipper is not actually a constellation but is part of the constellation Ursa Major, or Great Bear.

Little Dipper (Ursa Minor or the Little Bear):

The Little Dipper is a mirrored and smaller version of the Big Dipper. It can be found by looking directly up from the two outermost stars which form the bowl on the Big Dipper. These two stars point us to Polaris, the Pole Star (indicating the North Pole), which is at the end of the Little Dipper's handle.


Another well-known constellation is Orion. Orion was an ancient Greek hunter and warrior and the constellation resembles this figure, with a club and a shield, and a sword dangling from his belt. The belt is usually the easiest part of Orion to spot, with three bright stars in a row. Can you find his belt in the picture below? Orion has more bright stars than any other constellation; the two brightest are Betelgeuse (shoulder) and Rigel (foot).


Cassiopeia is found next to the Big Dipper and Orion. Its shape is easy to remember, a neat W, or M, formed by 5 bright stars. Cassiopeia is the mythological Queen of Ethiopia.

The Pleiades (Seven Sisters)

The Seven Sisters were said to be the daughters of Atlas and the objects of Orion's affection. As the name Seven Sisters implies, there are seven stars in this constellation but most people can only see six. The Pleiades is located between the constellations of Taurus and Perseus.

The Solar System

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 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.

The Sun

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

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.

Some planets are known to have moons/satellites revolving round them. Any celestial body revolving around another celestial body is called its satellite.

Mercury (Budh)

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 (Shukra)

Venus is earth’s nearest planetary neighbor. 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. 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.

The Earth

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.

Mars (Mangal)

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 (Brihaspati)

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. Jupiter has a large number of satellites. It also has faint rings around it.

Saturn (Shani)

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. Saturn also has a large number of satellites. Saturn is the least dense among all the planets. Its density is less than that of water.

Uranus and Neptune

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.

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.

Some Other Members of the Solar System

There are some other bodies which revolve around the Sun. They are also members of the solar system.


There is a large gap in between the orbits of Mars and Jupiter. This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids. Asteroids can only be seen through large telescopes.


Comets are also members of our solar system. They revolve around the Sun in highly elliptical orbits. However, their period of revolution round the Sun is usually very long. A Comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun. Many comets are known to appear periodically. One such comet is Halley’s Comet, which appears after nearly every 76 years.

Meteors and Meteorites

A meteor is usually a small object that occasionally enters the earth’s atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly. That is why the bright steak lasts for a very short time. Some meteors are large so that they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite.

Artificial Satellites

India has built and launched several artificial satellites. Aryabhatta was the first Indian satellite. Some other Indian satellites are INSAT, IRS, Kalpana-1, EDUSAT, etc. Artificial satellites have many practical applications. They are used for forecasting weather, transmitting television and radio signals. They are also used for telecommunication and remote sensing.


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