class 8 Stars And The Solar System Short notes science chapter 17

Celestial Objects: Things like stars, planets, moons, etc. which are present in the space are known as celestial objects.

The Moon:

1. Phases of the moon:The different shapes of moon visible during various days in a month are called as phases of the moon.

2. Full moon day: It is the day on which the full moon is visible.

3. New Moon day: After the full moon day, the moon appears to become thinner and thinner. On the fifteenth day the moon is not visible. This day is known as the new moon day.

4. Why does Moon have different phases?
(i) Moon does not have its own light and actually it reflects the light coming from sun which is responsible for various phases.
(ii) After new moon day, the visibility size of illuminated part of moon seen from Earth keeps increasing and after full moon day it keeps decreasing.

The Stars:
(i) These are objects which twinkle in the sky at night.
(ii) Sun is the nearest star to our planet which is approx 150 million km away from Earth.

Light Year:
(i) It is defined as the distance travelled by light in one year. And it is the unit to measure large distances.
(ii) The speed of light is about 300,000 km per second.
(iii) The distance of the Sun from the Earth may be said to be about 8 light minutes

Pole Star: It is the star which is situated in the direction of the earth’s axis.

Constellations:

 1. Ursa Major:

2. Orion:
(i) It is another well known constellation visible during late evenings in winter.
(ii) It also consists of seven or eight bright stars.  The three middle stars in Orion represent the belt of the hunter and the remaining four bright stars are arranged in the form of a quadrilateral.
(iii) Orion is also called the Hunter.

3. Sirius:
(i) It is the brightest star in the sky.
(ii) To find the location of Sirius, imagine a straight line passing through the belt of three middle stars of Orion.

4. Cassiopeia:
(i) It is another popular constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M.

The Solar System:
1. It consists of the Sun and other celestial objects present in the space revolving around the sun.The gravitational attraction between the Sun and these objects keeps them revolving around it. The various entities like planets, meteors, comets, etc. are part of our solar system.
2. The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

2. The Planets: The planets are like stars, but they do not have light of their own. They merely reflect the sunlight that falls on them.  The planets keep changing their positions with respect to the stars.
These are the objects revolving around the sun in fixed path. This fixed path is termed as an orbit.
1. Period of revolution: It is the time taken by planet to complete full revolution around the sun.
2. Period of rotation: It is the time taken by planet to complete one rotation around its own axis.
3. Satellite: These are the celestial bodies present around other celestial body. Example Moon is a satellite of the Earth.
4. Artificial satellite: These are the man-made satellite present around the Earth.

Planets Revolving Around the Sun:

1. Mercury (Budh)

(i) It is nearest planet to the Sun and the smallest planet present in our solar system.

(ii)  It is very difficult to observe it, as most of the time it is hidden in the glare of the Sun.

(iii) Mercury has no satellite of its own.

2. Venus (Shukra):

(i) It is nearest planet to Earth and It is the brightest planet visible in the night sky.

(ii) It is also called a morning or an evening star, although it is not a star.

(iii) Venus rotates on its axis from east to west.

(iv) Venus has no moon or satellite of its own.

3. The Earth:

(i) It is the only planet in the solar system on which life exists.

(ii) Earth appears blue green due to the reflection of light from water and landmass on its surface.

(iii) 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.

(iv) It has only one moon or satellite of its own.

4. Mars (Mangal):

(i) It looks somewhat reddish and, so, it is also known as the red planet.

(ii) Mars has two small natural satellites.

5. Jupiter (Brihaspati):

(i) It is the largest planet of the solar system. It rotates very rapidly on its axis.

(ii) Jupiter has a large number of satellites. And also has faint rings around it.

(iii) It is about 1300 times larger than that of earth and its mass about 318 times than that of our Earth

6. Saturn (Shani):

(i) It is yellowish colored planet.

(ii) There are beautiful rings around this planet, which are not observed in any other planets.

(iii) It is the least dense among all the planets. Its density is less than that of water.

7. Uranus:

(i) It is the one of outermost planet of the solar system.

(ii) The most distinctive feature of Uranus is that it has highly tilted rotational axis.

8. Neptune:

(i) It is the outermost planet of the solar system.

Some Other Celestial Members of the Solar System:

1. Asteroids:

(i) The small rocks like substances that are orbiting around the Sun are called asteroids.

(ii) It is found in large gap between the orbits of Mars and Jupiter.

(ii) They are visible only through large telescopes.

2. Comets:

(i) They revolve around the Sun in highly elliptical or circular orbits.

(ii) It looks like as if it has a bright head with a long tail and it is been seen that the tail keeps growing in size as it keeps approaching the sun.

 

3. Meteors:

(i) These are small objects that sometimes enter the Earth’s atmosphere.

(ii) It occasionally enters the earth’s atmosphere with very high speed. Due to friction of earth's atmosphere, It glows then evaporates quickly. That is why the bright steak lasts for a very short time.

4. Meteorite:

(i) These are the leftover object that reaches the Earth.

(ii) Meteorites help scientists in investigating the nature of the material from which the solar system was formed

5. Artificial Satellites:

(i) These are the man-made satellites that revolve around the Earth.

(ii) Aryabhatta was the first Indian satellite. Some other Indian satellites are INSAT, IRS, Kalpana-1, EDUSAT, etc.

(iii) They are used for forecasting weather, transmitting television and radio signals. They are also used for telecommunication and remote sensing.

 

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