What this chapter is about
This chapter takes you on a journey beyond our planet Earth. You will learn about the sky we see at night, the Moon, the Sun, stars, and planets. You will discover what makes up our solar system and how Earth is just one small part of a much bigger universe.
At Class 6, you are ready to think about what lies beyond the world you can touch and walk on. You already know about day and night, seasons, and the phases of the Moon from everyday life. Now you will understand why these things happen and learn about the objects in space that affect our Earth.
After studying this chapter, you should be able to name the planets in our solar system, explain the difference between stars and planets, describe the Moon and its phases, and understand what keeps all these objects moving in space.
Key ideas
- Stars are huge balls of hot, glowing gases that produce their own light and heat. The Sun is the star closest to Earth.
- Planets are large objects that move around a star. They do not produce their own light but reflect the light of their star.
- The Solar System is the Sun and all the objects that move around it, including eight planets, moons, asteroids, and comets.
- The Moon is Earth's natural satellite. A satellite is an object that moves around a planet. The Moon takes about 27 days to go around Earth once.
- The Moon appears to change shape over a month. These shapes are called phases of the Moon. We see different phases because sunlight falls on different parts of the Moon as it moves around Earth.
- Earth rotates (spins) on its axis once every 24 hours. This rotation causes day and night.
- Earth revolves (moves in a path) around the Sun once every 365 days, which we call one year.
- Constellations are groups of stars that appear to form patterns in the night sky. Saptarishi (Ursa Major) looks like a big ladle or question mark.
Formulas and facts to remember
- The eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Memory trick for planets: My Very Eager Mother Just Served Us Noodles (first letter of each word matches the first letter of each planet).
- The Sun is about 150 million kilometres away from Earth.
- Light from the Sun takes about 8 minutes to reach Earth.
- The Moon is about 3,84,000 kilometres from Earth.
- Jupiter is the largest planet; Mercury is the smallest planet in our solar system.
- A full moon and a new moon each appear once in about 29 days.
- Earth is the only planet in our solar system known to have liquid water and life.
Worked examples
Example 1: Identifying stars and planets in the night sky
Riya sees two bright objects in the evening sky. One twinkles, and the other shines steadily without twinkling. Which one is likely a star and which is a planet?
Solution: Stars are very far away, so their light bends as it passes through Earth's moving air. This bending makes them appear to twinkle. Planets are closer to Earth and appear as tiny discs, so their light does not twinkle as much. The twinkling object is a star. The steady, non-twinkling object is a planet.
Example 2: Understanding Moon phases
Arjun notices that the Moon looks like a bright full circle on Monday. His grandmother says the next new moon will come in about two weeks. Why does the Moon change shape?
Solution: The Moon does not produce its own light. It reflects sunlight. As the Moon moves around Earth, we see different portions of its sunlit side. When the Moon is between the Sun and Earth, sunlight falls on the side we cannot see. This is the new moon. When Earth is between the Sun and Moon, we see the full sunlit side. This is the full moon. From full moon to new moon takes about 14 to 15 days.
Example 3: Day and night
Meena in Delhi is eating lunch at noon. Her cousin Sam lives in New York, which is on the opposite side of Earth. What time is it likely for Sam?
Solution: Earth rotates on its axis. When one side faces the Sun, it has daytime. The opposite side has nighttime. Delhi faces the Sun at noon, so places on the far side of Earth, like New York, are turned away from the Sun. It is likely night-time or very late evening for Sam in New York.
Common mistakes
Thinking the Moon produces its own light → The Moon only reflects sunlight; it has no light of its own.
Believing stars are smaller than planets because they look tiny → Stars are actually much larger than planets, but they look small because they are extremely far away.
Confusing rotation and revolution → Rotation is spinning in place (causes day and night); revolution is moving around another object (causes a year).
Thinking Pluto is still a planet → Scientists reclassified Pluto as a dwarf planet in 2006 because it has not cleared its orbit of other objects.
Believing the Sun moves around Earth → Earth and other planets revolve around the Sun, not the other way around.
Quick revision
- The Sun is a star; planets move around it and reflect its light.
- Eight planets orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Earth's rotation causes day and night; its revolution causes a year.
- The Moon orbits Earth and shows phases because we see different sunlit parts.
- Stars twinkle; planets usually do not.
- Constellations are patterns of stars, like Saptarishi.