NCERT Solutions for Class 8th Science Chapter 11 Keeping Time with the Skies
Updated on 2026-09-19
About this chapter
The Moon does not shine by itself. Sunlight always lights exactly one half of it. The half turned towards the Earth is not always the lit half, so on different days we see different fractions of the lit half — these are the phases of the Moon . In the waning period (Krishna Paksha) the bright part shrinks from a full circle to nothing in about two weeks; in the waxing period (Shukla Paksha) it grows back. One full Moon to the next takes about 29.5 days . Phase and position go together. On full Moon day the Moon is opposite the Sun (it rises at sunset); on new Moon day it is in almost the same direction as the Sun. Because the Moon moves about 12° eastward each day, it rises about 50 minutes later every day. Three natural periodic events give three units of time — Earth's rotation gives the
- Chapter opener
- .1 How Does the Moon’s Appearance Change and Why?
- .1.2 Locating the Moon
- .1.3 Making sense of our observations
- .2 How Did Calendars Come into Existence?
- .3 Are Festivals Related to Astronomical Phenomena?
- .4 Why Do We Launch Artificial Satellites in Space?
- End-of-chapter questions
- Science • Society • Interdisciplinary Projects
Quick revision
| Term | What it means | How you recognise or measure it | Where it appears in the chapter |
|---|---|---|---|
| Phases of the Moon | The changing shapes of the bright part of the Moon as seen from the Earth from one day to the next | Caused by the change in the Sun–Moon angle in the sky as the Moon revolves round the Earth — not by Earth's shadow | Fig. 11.2 and Fig. 11.5; Activity 11.1 and Activity 11.2 |
| Waxing (Shukla Paksha) and waning (Krishna Paksha) periods | The fortnight in which the bright part grows, and the fortnight in which it shrinks | A waxing Moon is easiest to spot at sunset, a waning Moon at sunrise | Section 11.1.1, page 172–173 |
| Full Moon (Purnima) | The Moon is nearly opposite the Sun, so the entire lit half faces us | Rises about when the Sun sets and sets about when the Sun rises | Position A, Day 1 in Fig. 11.5 |
| New Moon (Amavasya) | The Moon is in nearly the same direction as the Sun, so only its non-illuminated half faces us | Not visible at all; it rises and sets nearly with the Sun | Position E, Day 15 in Fig. 11.5 |
| Gibbous phase / Crescent phase | More than half of the lit portion is seen / less than half is seen | Gibbous at positions B and H; crescent at positions D and F | Page 176 |
| Mean solar day | The average time the Sun takes from its highest point in the sky one day to its highest point the next day | Mark the tip of a vertical stick's shadow; the shadow is shortest when the Sun is highest | Activity 11.3 and Table 11.2 — 24 hours |
| Lunar month | One complete cycle of the phases of the Moon | About 29.5 days; 12 of them make a lunar year of 354 days | Section 11.2.1, page 179 |
| Solar year and leap year | One revolution of the Earth round the Sun, that is, one cycle of seasons; the extra day that keeps the calendar with the seasons | About 365¼ days. A year divisible by 4 is a leap year; 1700, 1800, 1900 were skipped, 1600 and 2000 were kept | Section 11.2.2, page 180 |
| Lunar, solar and luni-solar calendars | Calendars that follow the Moon's phases, the cycle of seasons, or both together | 354 days / 365 days / months by the Moon with a correction for the seasons | Sections 11.2.1 to 11.2.3 |
| Adhika Maasa (intercalary month) | An extra month added to a luni-solar year every 2–3 years | It makes up the 11-day shortfall of the lunar year, so festivals stay in their seasons | Page 181–182 |
| Tropical year and sidereal year | Spring equinox to spring equinox; and the time for the same stars to rise again at sunset | The sidereal year is longer than the tropical year by just 20 minutes | 'A step further', page 180 |
| Indian National Calendar | India's official solar calendar of the Shaka Era, used with the Gregorian calendar | 365 days; the year begins on 22 March; months of 30 or 31 days, the 2nd to 6th having 31 | Fig. 11.9; adopted 21 March 1956 CE = 1 Chaitra 1878 Saka |
| Artificial satellite | A human-made object launched from the Earth to orbit it | Most orbit about 800 km up and take about 100 minutes for one orbit; seen as a steady point of light moving fast | Section 11.4; Cartosat, AstroSat, Chandrayaan, Aditya L1, Mangalyaan |
Exercises
- Chapter opener — Probe and ponder Page 170
- .1 How Does the Moon’s Appearance Change and Why? — Activity 11.1: Let us explore Page 17211
- .1.2 Locating the Moon — In-text Questions Page 17311
- .1.2 Locating the Moon — A step further Page 17311
- .1.3 Making sense of our observations — In-text Questions Page 17411
- .1.3 Making sense of our observations — Activity 11.2: Let us explore Page 17411
- .1.3 Making sense of our observations — In-text Questions Page 17611
- .2 How Did Calendars Come into Existence? — In-text Questions Page 17811
- .2 How Did Calendars Come into Existence? — Activity 11.3: Let us measure a day! Page 17811
- .3 Are Festivals Related to Astronomical Phenomena? — In-text Questions Page 18311
- .4 Why Do We Launch Artificial Satellites in Space? — In-text Questions Page 18511
- End-of-chapter questions — Keep the curiosity alive Page 187
- Science • Society • Interdisciplinary Projects — Discover, design, and debate Page 188