- When we look at the sky in the night, we notice countless shining objects. The whole sky is filled with tiny shining objects- some of these shining objects are bright while others are dim.
- But all of these shining objects do not twinkle. Some of them simply glow without any flicker.
- Along with these bright objects, we can also see the moon. The moon appears at different times, in different shapes, and at different positions. We can see the full moon only once in a month on Poornima. Fifteen days after the Poornima comes Amavasya or New moon night when the moon is not visible at all.
- These bright objects seen in the night sky are not visible in the day time because of the very bright light of the sun.
- The sun, the moon, and all those shining objects in the sky are called celestial bodies.
- Stars are also celestial bodies. They are made up of gasses. They have their own heat and emit light.
- The sun is also a star. The other stars appear small and we do not feel the heat because they are very far from us.
- Constellations are various patterns formed by different groups of stars. The International Astronomical Union lists 88 constellations — a list that has been in use since 1922 and encompasses all the night sky around the world.
Some of the important constellations are:
1. Hydra or the sea serpent: Hydra as the largest constellation in the sky. Its total area in square degrees, according to the official boundaries established by the International Astronomical Union (IAU). Hydra covers 1,303 square degrees or 3 percent of the celestial sphere.
2. Virgo: Virgo constellation lies in the southern sky. Its name means “virgin” in Latin. Virgo is the second-largest constellation in the sky, occupying an area of 1294 square degrees.
3. Ursa Major or the Great bear: Ursa Major constellation lies in the northern sky. Its name means “the great bear,” or “the larger bear,” in Latin.
Ursa Major is the largest northern constellation and third largest constellation in the sky. Its brightest stars form the Big Dipper asterism, one of the most recognizable shapes in the sky, also known as the Plough.
- In ancient times people used to determine directions during the night with the help of stars.
- The North Star also called Polar star or Polaris always remains in the same position in the sky and it indicates the north direction. It belongs to the Ursa Minor constellation.
- Sirius, also known as the Dog Star or Sirius A, is the brightest star in Earth's night sky. The name means "glowing" in Greek.
- Planets are also celestial bodies. Planets do not have their own light. They are lit by the light of the stars. The word ‘Planet’ comes from the Greek word ‘Planetai’ which means ‘wanderers’.
The Solar System
- The Sun, eight planets, and their satellites and some other celestial bodies known as asteroids and meteoroids from the solar system.
- ‘Sol’ in Roman mythology is the ‘sun god’. ‘Solar’ means ‘related to the sun’.
- The Sun: The sun is in the center of the solar system. It is made up of extremely hot gasses. It provides the pulling force that binds the solar system. The sun is the source of light and heat for the whole solar system.
- The sun is about 150 million km away from earth.
- The light from the sun rakes about 8 minutes to reach earth.
- There are eight planets in our solar system. They are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
- Mercury, Venus, Earth, and Mars are called Inner planets. Jupiter, Saturn, Uranus, and Neptune are called Outer planets.
- Venus is considered as Earth’s twin because its size and shape are very much similar to that of the earth.
- Till August 2006, Pluto was also considered as a planet. But the International Astronomical Union (IAU) downgraded the status of Pluto to that of a dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet.
The three criteria of the IAU for a full-sized planet are:
(i) It is in orbit around the Sun.
(ii) It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
(iii) It has “cleared the neighborhood” around its orbit.
Pluto meets only two of these criteria, losing out on the third. In all the billions of years it has lived there, it has not managed to clear its neighborhood
- All the planets in the solar system move around the sun in fixed path known as orbits. They also rotate in the axis.
- Jupiter's moon Ganymede is the largest in the solar system. Larger than Mercury and Pluto, and only slightly smaller than Mars.
- Jupiter, Saturn, Uranus, and Neptune have rings around them. These rings are made of small debris. These rings can be seen from earth with the help of powerful telescopes.
- The earth is the third nearest planet to the sun. it is the fifth-largest planet in the solar system.
Size of the Eight Planets
(a) According to NASA, this is the estimated radii of the eight planets in our solar system, in order of size.
- Jupiter - 1,120% the size of Earth
- Saturn - 945% the size of Earth
- Uranus - 400% the size of Earth
- Neptune - 388% the size of Earth
- Earth (Radius = 6,378 km / 3,963 miles)
- Venus - 95% the size of Earth
- Mars - 53% the size of Earth
- Mercury - 38% the size of Earth
(b) The shape of the earth is not a perfect circle. Earth is slightly flattened at the poles and bulged in the equator. That’s why; its shape is described as Geoid. Geoid means an earth-like shape.
(c) Earth is a unique planet in the solar system because conditions favorable to support life are found only on earth.
(d) Two- thirds of the earth’s surface is covered by water. From outer space, the earth appears blue and hence is called ‘blue planet’.
(e) About 71 percent of the Earth's surface is water-covered, and the oceans hold about 96.5 percent of all Earth's water. The remaining water is found in the air as water vapor, in rivers and lakes, in icecaps and glaciers, in the ground as soil moisture, and in aquifers.
- Our earth has only one satellite that is the moon.
- A satellite is a celestial body which moves around the planets in the same way as the planets move around the sun.
- Moon's diameter is only one- fourth of the earth.
- It appears bigger because it is closer to earth than other celestial bodies.
- It is about 3, 84,400 km away from earth.
- The Moon does spin on its axis, completing a rotation once every 27.3 days. It takes the same amount of time to revolve around the earth.
- The light reflected from the surface of the Moon takes only 1.3 seconds to reach Earth.
- The moon does not have conditions favourable for life. It has mountains, plains, and depressions on its surface. These cast shadows on the moon’s surface.
Shadows on Moon
- In July 1969, NASA launched the Apollo 11 space flight which landed humans on the moon for the first time. Apollo 11 blasted off on July 16, 1969. Neil Armstrong, Edwin "Buzz" Aldrin, and Michael Collins were the astronauts on Apollo 11.
- On July 20, 1969, Neil Armstrong became the first man to set foot on the moon followed Buzz Aldrin. They landed on the moon in the Lunar Module. It was called the Eagle. Collins stayed in orbit around the moon. He did experiments and took pictures. They walked on the moon’s surface for around three hours.
- Apart from the stars, planets, and their satellites, there are numerous tiny bodies known as Asteroids.
- An asteroid is a celestial body - composed of rock, metal or, a mixture of both - that is orbiting the Sun.
- They are found between the orbits of Mars and Jupiter.
- Scientists are of the view that asteroids are parts of the planet which exploded many years back.
- Meteoroids are small pieces of rocks that move around the sun.
- Sometimes these meteoroids come near the earth and tend to drop on it.
- Meteoroids are similar to an asteroid but significantly smaller. They are mostly made debris of comets, sometimes debris of asteroids.
- When meteoroids enter Earth's atmosphere at high speed they catch fire due to friction with air and are called “shooting stars” are called meteors. When a meteor survives a trip through the atmosphere and hits the ground, it's called a meteorite.
- Our solar system is part of the Milky Way galaxy.
- A galaxy is a huge system of billions of stars and clouds of dust and gaseous. There are thousands of such galaxies that make the Universe.
- In ancient India, the Milky Way galaxy was called ‘Akash Ganga’.