Earth in the Solar System

Key Points:

• Earth is a planet in the solar system.
• It spins on its axis and revolves around the Sun.
• Earth receives light and heat from the Sun.
• It is made up of land and water, surrounded by a layer of gases called the atmosphere.
• Earth is the only known planet with life.

Heavenly Bodies (Celestial Bodies):

• These include the Sun, stars, planets, and the Moon.
• Seen in the sky from Earth.

Conditions that Support Life on Earth:

• Water is essential for all life forms.
• Earth lies in the habitable zone – the right distance from the Sun where liquid water can exist.
• If Earth were:
  • Closer to the Sun: Water would evaporate.
  • Farther from the Sun: Water would freeze.
• Earth has an average temperature of 14°C, ideal for life.

Fun Fact:

• About two-thirds of the Earth is covered with water.
• Earth is also called the Blue Planet.

The Shape of the Earth

• Ancient people believed Earth was flat.
• Now it’s proven: Earth is shaped like a sphere.
• Evidence:
  • Ships return to starting point if they travel in one direction.
  • Shadow lengths at noon change with distance from the Equator.
  • When a ship arrives, we see its top first, not the whole ship.
  • Photos from space show Earth is round.
• Earth is not a perfect sphere – it’s flattened at poles and bulging at the equator.
• This shape is called a spheroid.

The Moon

• Moon is Earth’s natural satellite.
• Reflects sunlight, does not produce its own light.
• Takes 27 days and 8 hours to orbit Earth.
• We always see the same side of the Moon.
• No air on the Moon, but traces of water have been found.
• Surface is rocky and uneven, with many craters.
• Moon changes shape/position as seen from Earth – these are phases of the Moon:
  • Full Moon: fully visible.
  • Then it wanes (gets smaller) daily.

Phases of the Moon

• New Moon: Moon is not visible from Earth.
• The Moon completes its phases in about 29.5 days (one lunar month).
• The cycle goes from New Moon to Full Moon and back to New Moon.

The Sun and the Solar System

The Solar System:

• Made up of the Sun and various objects that revolve around it:
  • Planets
  • Moons (satellites)
  • Asteroids
  • Comets
  • Meteors

The Sun:

• The Sun is a star at the center of the solar system.
• All planets and celestial bodies revolve around it.
• About 4.6 billion years old.
• Its gravitational field holds the solar system together.
• Made of hot gases – mainly hydrogen and helium.
• Surface temperature: around 5,500°C.
• Diameter: 1,392,000 km (about 109 times bigger than Earth).
• Distance between the Earth and Moon is less than half the Sun’s diameter.

The Planets

• A planet is a celestial body that orbits the Sun.
• There are eight major planets in the Solar System (in order from the Sun):
  Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• Common features of planets:
  • All are spherical in shape.
  • They do not emit their own light – they reflect sunlight.
  • Each planet rotates on its axis (one rotation = a day).
  • Each revolves around the Sun in an orbit (one revolution = a year).

Types of Planets:

• Outer planets (Jovian planets): Jupiter, Saturn, Uranus, Neptune
  • Made of gases
  • Located far from the Sun
  • Cold in temperature
• Inner planets (Terrestrial planets): Mercury, Venus, Earth, Mars
  • Made of rock and metals
  • Closest to the Sun
  • Earth is the largest among them
• Most planets have moons (natural satellites)
  • Mercury and Venus have none
  • Earth has one moon
  • Saturn has many (about 145 moons)

Asteroids

• Small celestial bodies between Mars and Jupiter.
• Believed to be leftover debris from early Solar System formation or broken pieces of planets.
• Also called planetoids.

Meteors

• Also called meteoroids.
• Chunks of rock moving at high speed around the Sun.
• When they come near Earth, gravity pulls them in.

• When meteoroids enter Earth’s atmosphere, they burn due to friction and become meteors or shooting stars.
• If a meteor reaches the Earth’s surface, it leaves a crater.
• Craters caused by meteor impacts can be seen in:
  • Ramagiri, Rajasthan
  • Lonar, Maharashtra
  • Arizona, USA

Stars, Constellations, and the Universe

Stars:

• Huge balls of hot gases (mainly hydrogen and helium).
• They are self-luminous – emit their own light and heat.
• Formed from clouds of dust and gas that collapse under gravity and heat up.
• Found in massive clusters called galaxies.
• Our galaxy is called the Milky Way (Aakaash Ganga in Hindi).
• The Sun is a star, closest to Earth.

Galaxies:

• Large groups of stars, dust, and gas.
• Millions of stars in one galaxy.
• Billions of galaxies exist in the universe.

Comparison: Stars vs Planets

Constellations

• Recognisable star patterns seen from Earth.
• Often named after animals, people, or objects.

Constellations

• Constellations are star patterns that resemble animals, people, or objects.
• Common constellations:
  • Ursa Major (The Great Bear)
  • Orion (The Hunter)
  • Others: Canis Major, Leo, Taurus
• Ursa Major is also called the Big Dipper.
  • Shaped like a large ladle (3 stars in handle, 4 in bowl).
  • Drawing a line from the last two stars of the bowl leads to the Pole Star (North Star or Dhruva Nakshatra).

Pole Star

• Appears fixed in the night sky, above Earth’s rotational axis.
• Used for navigation in ancient times.

Universe

• Composed of:
  • Planets, stars, galaxies, dust clouds, dark matter, and energy.
• Earth is a tiny part of the vast universe.

Words to Remember:

• Gravity – Force pulling objects toward a planet/star.
• Gravitational field – Region where gravity acts strongly.
• Asteroid – Rocky object between Mars and Jupiter.
• Meteor – Light streak from a meteoroid burning in atmosphere.
• Meteoroid – Rock moving through space.
• Star – Hot, glowing gas ball (e.g. Sun).
• Galaxy – Massive star cluster (e.g. Milky Way).
• Constellation – Star pattern resembling a figure.
• Universe – All space, stars, planets, energy, and matter.

1. Give one word for the following:
2. The blanket of air surrounding the Earth: Atmosphere
3. One complete orbit of a planet around the Sun: Planet year (or Revolution)
4. The other name for asteroids: Planetoids
5. Very large clusters of stars: Galaxy
6. The Hindi name for the Milky Way Galaxy: Aakaash Ganga

1. Differentiate between the following terms:
2. Inner planets vs Outer planets
   • Inner planets: Mercury, Venus, Earth, Mars; rocky and small; closer to the Sun.
   • Outer planets: Jupiter, Saturn, Uranus, Neptune; made of gas; larger; farther from the Sun.
3. Meteoroid vs Asteroid
   • Meteoroid: A small rocky body moving in space.
   • Asteroid: A larger rocky object orbiting the Sun, mainly between Mars and Jupiter.
4. Meteors vs Meteorites
   • Meteors: Meteoroids that burn up in Earth’s atmosphere (shooting stars).
   • Meteorites: Meteors that survive the atmosphere and land on Earth.
5. Star vs Planet
   • Star: Self-luminous body that emits its own light and heat (e.g., Sun).
   • Planet: Non-luminous body that orbits a star and reflects its light (e.g., Earth).