Looking for authentic, 100% textbook-grounded NCERT Solutions for Class 8 Science Chapter 11: Keeping Time with the Skies (Curiosity, 2026-27 CBSE Curriculum)? Below is the complete Topper’s Handwritten Style solutions notebook covering every single exercise, numerical problem, and reflective inquiry question directly from the official NCERT textbook.
Chapter Overview: Keeping Time with the Skies
NCERT Class 8 Science Chapter 11 explores observational astronomy and celestial mechanics. Students investigate lunar phases (from New Moon to Full Moon), debunk misconceptions regarding Earth’s shadow, explore synchronous rotation, examine constellations, and trace the history of astronomical timekeeping through solar, lunar, and luni-solar calendars.
Subject: Class 8 Science (Curiosity)
Chapter: 11 — Keeping Time with the Skies
Target: 100/100 CBSE Exam Prep
Topper’s Handwritten Notebook Solutions (Revise, Reflect, Refine)
Question 1: State whether the following statements are True or False: (i) We can only see that part of the Moon which reflects sunlight towards us. (ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases. (iii) Calendars are based on astronomical cycles that repeat predictably. (iv) The Moon can only be seen at night.
Step 1: Detailed Statement Analysis
- (i) We see only the sunlit portion reflecting to us: TRUE. The Moon is a non-luminous rocky body; it shines entirely by reflecting sunlight.
- (ii) Earth’s shadow causes Moon phases: FALSE. This is a common misconception! Earth’s shadow falling on the Moon causes a Lunar Eclipse, NOT the daily phases. The phases of the Moon are caused by the varying angle from which we view the illuminated half of the Moon as it orbits Earth!
- (iii) Calendars are based on predictable astronomical cycles: TRUE. Day = Earth’s rotation (24 hrs); Month = Moon’s orbit (29.5 days); Year = Earth’s orbit around the Sun (365.25 days).
- (iv) The Moon can only be seen at night: FALSE. The Moon is frequently visible during daylight hours (e.g., in the afternoon during waxing phases or in the morning during waning phases) whenever it is above the horizon and sufficiently bright.
Answers: (i) True; (ii) False (phases are due to viewing geometry, NOT Earth’s shadow); (iii) True; (iv) False (Moon is visible in daytime sky too).
💡 Top Misconception: Never write that Earth’s shadow causes moon phases. Earth’s shadow causes eclipses only!
Question 2: Amol was born on 6th of May on a Full Moon day (Purnima). Does his birthday fall on a Full Moon day every year? Explain your answer with astronomical calculations.
Step 1: Solar Year vs Lunar Month Dynamics
Amol’s birthday (May 6th) is based on the Solar Gregorian Calendar, which has approximately 365.25 days per year.
The cycle of Moon phases (from one Full Moon to the next, called a Synodic Month) takes exactly 29.53 days.
The cycle of Moon phases (from one Full Moon to the next, called a Synodic Month) takes exactly 29.53 days.
Step 2: Calculating the Annual Phase Shift
A lunar year consisting of 12 full lunar months has:
Difference between solar year and 12 lunar cycles:
This means that each subsequent year, the Full Moon occurs about 11 days earlier in the solar calendar. A specific date (May 6th) will coincide with Full Moon only once every 19 years (the Metonic Cycle).
12 × 29.53 days = 354.36 days.Difference between solar year and 12 lunar cycles:
365.25 − 354.36 = ~10.9 days (approx. 11 days per year).This means that each subsequent year, the Full Moon occurs about 11 days earlier in the solar calendar. A specific date (May 6th) will coincide with Full Moon only once every 19 years (the Metonic Cycle).
Conclusion: No, his birthday will NOT fall on Full Moon every year. The solar year is ~11 days longer than 12 lunar months, causing the lunar phases to shift by 11 days each year.
💡 Traditional Luni-Solar Calendar: Indian calendars (Saka / Vikram Samvat) periodically insert a leap month (Adhik Maas) every 3 years to synchronize lunar months with the solar agricultural year!
Question 3: Why do we always see only one same face/side of the Moon from Earth? What is Synchronous Rotation?
Step 1: Period of Rotation and Revolution
The Moon rotates on its own axis and also revolves in an orbit around Earth. Remarkably, both periods are precisely equal:
1. Period of revolution around Earth = 27.3 days.
2. Period of rotation on its axis = 27.3 days.
1. Period of revolution around Earth = 27.3 days.
2. Period of rotation on its axis = 27.3 days.
Step 2: Optical Effect of Synchronous Rotation (Tidal Locking)
Because the Moon turns on its axis at the exact same rate as it circles Earth, it presents the same hemisphere towards Earth at all times. This phenomenon is called tidal locking or synchronous rotation. Earthlings could never view the ‘Far Side of the Moon’ until space probes (Luna 3 and Apollo missions) orbited behind it!
Scientific Reason: The Moon’s period of rotation on its axis equals its period of revolution around Earth (27.3 days), locking one face toward Earth permanently.
💡 Classroom Demo: Walk in a circle around a friend while keeping your face constantly turned toward them. You will notice you have rotated 360° once!
Question 4: Define Waxing and Waning phases of the Moon. Trace the sequence of phases starting from New Moon (Amavasya) to Full Moon (Purnima) and back.
Step 1: Waxing and Waning Definitions
Waxing (Shukla Paksha): The 15-day period during which the illuminated portion of the Moon visible from Earth progressively grows larger from New Moon to Full Moon.
Waning (Krishna Paksha): The 15-day period during which the illuminated visible portion progressively shrinks from Full Moon back to New Moon.
Waning (Krishna Paksha): The 15-day period during which the illuminated visible portion progressively shrinks from Full Moon back to New Moon.
Step 2: Complete Phase Sequence
- New Moon (Amavasya): Moon is between Earth and Sun; dark unlit face faces Earth (invisible).
- Waxing Crescent: Thin crescent sliver visible in evening sky.
- First Quarter (Half Moon): Right half of illuminated hemisphere visible.
- Waxing Gibbous: More than half illuminated, approaching full disc.
- Full Moon (Purnima): Entire sunlit face fully visible as a radiant circular disc.
- Waning Gibbous: Disc starts shrinking from right side.
- Third Quarter (Half Moon): Left half of illuminated hemisphere visible in morning sky.
- Waning Crescent: Thin sliver visible just before dawn.
Sequence: New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Third Quarter → Waning Crescent → New Moon.
💡 Mnemonics: ‘Waxing’ = Growing bigger (wax on); ‘Waning’ = Shrinking smaller (wane away).
📌 Master Exam Revision Checklist for Chapter 11
- Moon Phases Cause: Caused by changing viewing angle of the sunlit hemisphere as Moon orbits Earth — NOT Earth’s shadow!
- Synodic Month: New Moon to New Moon = 29.5 days; Sidereal period = 27.3 days.
- Synchronous Rotation: Rotation period = Revolution period (27.3 days). Hence, same side always faces Earth.