Science · Class 8 · Chapter 11

Keeping Time with the Skies

Aim: Build a clear, working understanding of this chapter's key ideas, connected to real NCERT examples, worked problems, and everyday situations.
  • Full Moon day (Purnima) — The Moon appears as a full bright circle.
  • New Moon day (Amavasya) — The Moon isn't visible at all.
  • Waxing / Waning — Bright portion growing / shrinking.
  • Lunar calendar — Based only on the Moon's phase cycle.
  • Solar calendar — Based on Earth's revolution around the Sun.
  • Luni-solar calendar — Combines both, with a correction month.
  • Leap year — A year with an extra day to stay synced with the seasons.
  • Artificial satellite — A human-made object launched to orbit Earth.
40 min lesson

Chapter 11 · Concept 1 of 6

Phases of the Moon

  • Purnima — Full Moon day — The Moon appears as a fully bright circle.
  • Amavasya — New Moon day — The Moon isn't visible at all. Waxing (growing) is Shukla Paksha; waning (shrinking) is Krishna Paksha — the full cycle takes about a month.

Chapter 11 · Concept 2 of 6

Why the Moon's Appearance Changes

  • The Moon doesn't emit its own light — it shines by reflecting sunlight, and only the half facing the Sun is lit.
  • Not the same as an eclipse — Moon phases are not caused by Earth's shadow — that's a common misconception. Earth's shadow only explains lunar eclipses, a separate and much rarer event.

Chapter 11 · Concept 3 of 6

How Calendars Came Into Existence

  • Day — Based on Earth's rotation — about 24 hours.
  • Month — Based on the Moon's cycle of phases — about 29.5 days.
  • Year — Based on Earth's revolution around the Sun — about 365¼ days.

Chapter 11 · Concept 4 of 6

Lunar, Solar, and Luni-solar Calendars

  • Calendar | Based on | Stays synced with seasons?
  • Lunar — The Moon's cycle only (12 months = 354 days) — No — drifts over time
  • Solar — Earth's orbit (365 days, with leap years) — Yes
  • Luni-solar — Both, with an extra month added periodically — Yes, by correction

Chapter 11 · Concept 5 of 6

Festivals Follow Different Calendars

  • Shifting dates — Diwali and Holi follow lunar or luni-solar calendars, so their Gregorian calendar dates change each year.
  • Nearly fixed dates — Makar Sankranti, Pongal, Bihu, and Vaisakhi follow a solar calendar, so they fall on nearly the same date every year.

Chapter 11 · Concept 6 of 6

Artificial Satellites

  • The Moon is Earth's natural satellite — but countries also launch artificial satellites that orbit Earth, most about 800 km up.
  • India's missions — ISRO's Chandrayaan (Moon), Aditya-L1 (Sun), and Mangalyaan (Mars) missions all rely on the same satellite technology that supports communication and weather monitoring.

Quick Recap

Check Your Understanding

  1. 1True or False: (i) We only see the part of the Moon reflecting sunlight toward us. (ii) Earth's shadow blocking sunlight causes the Moon's phases. (iii) Calendars are based on predictable astronomical cycles. (iv) The Moon can only be seen at night.
  2. 2Amol was born on 6th May on a full Moon day. Does his birthday fall on a full Moon day every year?
  3. 3Malini saw the Moon directly overhead at sunset. (i) Sketch the phase she saw. (ii) Is the Moon waxing or waning?