Revision Notes for Class 8 Science Chapter 11: Keeping Time with the Skies

Condensed revision notes for Class 8 Science Chapter 11: Keeping Time with the Skies – Curiosity textbook. These Class 8 Science Chapter 11 notes are ideal for quick revision just before exams.

• Day is based on Earth’s rotation on its own axis (≈24 hours).
• Month is based on the Moon’s revolution around Earth — its phase cycle (synodic month) is about 29.5 days; its orbit relative to the stars (sidereal month) is about 27.3 days.
• Year is based on Earth’s revolution around the Sun (≈365.25 days, hence leap years).
• Moon phases are caused by how much of the Moon’s sunlit half faces Earth as it orbits — not by Earth’s shadow (that’s a separate, rarer event: a lunar eclipse).
• New Moon: Moon’s dark side faces Earth, invisible/nearly invisible.
• First Quarter: half-lit, waxing, overhead around sunset.
• Full Moon: fully lit, visible all night from sunset to sunrise.
• Last Quarter: half-lit, waning, overhead around sunrise.
• Waxing = growing fuller (New → Full); Waning = shrinking (Full → New).
• Tidal locking: the Moon’s rotation and orbital periods are equal, so the same face always points towards Earth.
• Luni-solar calendars periodically add an intercalary (extra) month to stay aligned with the solar year, since 12 lunar months (≈354 days) fall about 11 days short of a solar year.
• Leap year: an extra day added roughly every 4 years to correct for the solar year being slightly more than 365 days.
• Artificial satellites are used for communication, navigation, weather monitoring, scientific research, disaster management, and Earth observation.
• Geostationary satellite: orbits once every 24 hours, matching Earth’s rotation, so it stays fixed above the same point on the ground.

More on this chapter: Solutions | Extra Questions | Class 8 Science Book | Formulas/Concepts Handbook

Written by Satish

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