Chapter 10: Our Sky
Look up on a clear night — you see countless stars, a glowing moon, and sometimes planets! During the day the sky belongs to the Sun. In this chapter we learn about the Sun, Moon, stars, planets, and how humans have explored space.
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Key Concepts
- The Sun
- The Sun is a star — a huge ball of hot glowing gas.
- It is the closest star to Earth.
- The Sun provides heat and light to all life on Earth.
- The Sun is so large that about 1.3 million Earths could fit inside it.
- Never look directly at the Sun — it can damage your eyes.
- The Moon
- The Moon is Earth's only natural satellite — it goes around (orbits) Earth.
- The Moon does NOT produce its own light; it reflects sunlight.
- The Moon takes about 29-30 days to orbit Earth — this gives us one lunar month.
- Phases of the Moon: As the Moon orbits Earth, we see different amounts of its lit side — new moon (not visible), crescent, half moon, full moon, and back again.
- Stars
- Stars are huge balls of hot, glowing gas that produce their own light.
- Stars look small because they are very far away.
- The North Star (Pole Star / Dhruv Tara) always appears in the north and was used by sailors for navigation.
- Stars appear to form patterns called constellations (e.g., Orion the hunter, Ursa Major / Saptarishi).
- Planets
- Planets are large spherical bodies that orbit (go around) the Sun.
- They do NOT produce their own light — they reflect sunlight.
- Our solar system has 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Earth is the third planet from the Sun and the only known planet with life.
- Saturn is famous for its rings made of ice and rock.
- Day and Night
- Earth rotates (spins) on its own axis once every 24 hours.
- The side of Earth facing the Sun experiences day; the opposite side experiences night.
- Seasons
- Earth revolves (goes around) the Sun once in about 365 days (one year).
- Earth's axis is tilted, causing different parts to receive more or less sunlight at different times — giving us seasons (summer, winter, autumn, spring).
- Space Exploration
- Sputnik 1 (1957) — first artificial satellite, launched by the Soviet Union.
- Yuri Gagarin (1961) — first human in space (Soviet cosmonaut).
- Neil Armstrong (1969) — first human to walk on the Moon (Apollo 11, USA).
- ISRO (Indian Space Research Organisation) — India's space agency; launched Chandrayaan (Moon mission) and Mangalyaan (Mars mission).
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Worked Examples
Why can we see the Moon at night but not stars during the day?
- Stars ARE present during the day, but the Sun's bright light scatters in Earth's atmosphere, making the sky so bright that stars are drowned out.
- At night, without sunlight, the sky is dark and stars become visible.
Why does the Moon appear to change shape each night?
- The Moon itself does not change. As the Moon orbits Earth, we see different portions of its sunlit side.
- Full moon: we see the entire sunlit half. New moon: the sunlit side faces away from us.
- The changing appearance is called the phases of the Moon.
How many planets are in our solar system? Name them in order from the Sun.
- There are 8 planets.
- Order from Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Memory trick: "My Very Educated Mother Just Served Us Nachos."
Earth takes 24 hours to rotate and 365 days to revolve. What does each motion cause?
- Rotation (24 hours): Causes day and night — the rotating Earth alternately faces and turns away from the Sun.
- Revolution (365 days): Combined with Earth's tilted axis, causes the four seasons.
Why is the North Star (Dhruv Tara) useful for navigation?
- The North Star sits almost directly above Earth's North Pole.
- As Earth rotates, all other stars appear to move across the sky, but the North Star stays in almost the same position.
- Sailors and travellers used it to find north without a compass.
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Common mistakes
- The Moon does NOT produce its own light — it only reflects sunlight.
- Planets are NOT stars — stars produce their own light; planets do not.
- The Sun is a star, not a planet.
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Summary
Our sky contains the Sun (nearest star), Moon (Earth's satellite), planets (including Earth), and countless stars. Earth's rotation causes day and night; its revolution around the Sun causes seasons. Space exploration by ISRO and other agencies has helped us learn more about our solar system.