Celestial Bodies and Solar System : NCERT / CBSE Revision Notes

Celestial Bodies

Stars are always fascinating to mankind. They are the most beautiful celestial bodies, about which mankind contemplates. All natural bodies visible in the sky, outside the earth's atmosphere, constitute the celestial bodies, e.g. stars, planets, their moons, comets, asteroids, meteors, etc. The moon is the celestial body closest to us. The moon is a natural satellite of the earth, and it reflects the sunlight incident on it. Due to its revolution around the earth, when it is at different positions in its path, the apparent disc of the moon changes, which gives rise to its phases.

 Celestial Bodies and Solar System, Science NCERT / CBSE revision Notes

When the moon is positioned between the sun and the earth, the illuminated portion of the moon is away from the earth, and we are not able to see the moon. We call this day as the new moon day. With time, the position of the moon changes and the illuminated portion of the moon exposed to the earth gradually increases. Thus, the size of the apparent disc of the moon increases gradually from a crescent to a full round when the earth lies between the moon and the sun. We call this day the full moon day.

The duration from one new moon day to the succeeding new moon day is the lunar month. If the moon is observed closely, we find craters, depressions on the surface of the moon, which might have been formed by the collision of some heavenly body like a meteorite with the moon. Even before astronauts landed on the moon, we were able to find information about the universe by celestial objects like meteorites.

A meteor is a small body of matter from outer space that enters the earth's atmosphere, which burns as a result of friction and appears as a streak of light. If the size of the meteor is large, a lump of it may be left without getting destroyed in the course of reaching the earth. This part of the meteor is called a meteorite.

The huge distances between the earth and other celestial bodies are measured in light years. A light year is the distance covered by light in one year.

Solar System

The sun is a major source of heat and light for all the planets in the solar system. Planets reflect sunlight that is incident on them. They have no light of their own, so they don’t twinkle like the stars. Planets have definite paths called orbits in which they revolve around the sun. The time taken by a planet to complete one full revolution around the sun is called its period of revolution. The time taken by a planet to rotate a full 360 degrees on its axis is called its period of rotation. A celestial body that revolves around another celestial body is called a satellite

  1. Mercury is the smallest planet in the solar system as well as the closest to the sun. It takes about 88 days to complete one revolution around the sun. 
  2. Venus is the second closest planet to the sun. It takes about 225 days to complete one revolution around the sun. It has no satellites or moons. Venus rotates from east to west.
  3. Earth is the only planet on which life is known to exist. It takes 365 days for the earth to complete one revolution around the sun. 
  4. Mars completes one revolution around the sun in about 687 days. It has two moons of its own.
  5. Jupiter is the largest planet in the solar system. Jupiter’s four larger moons are called Io, Europa, Callisto and Ganymede. It rotates the fastest among all planets. 
  6. The rings of Saturn are made of ice particles and dust. Saturn is the only planet that is lighter than water. The largest of Saturn’s moons is Titan. 
  7. Uranus is the coldest planet. 
  8. Neptune was discovered through mathematical calculation. 

Mercury, Venus, Earth and Mars are called the inner planets. Jupiter, Saturn, Uranus and Neptune are called the outer planets. The outer planets have several moons and a system of rings

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