NCERT Curiosity · Class 8 · Science
Keeping Time with the Skies
At the Kite Festival, Meera notices the Moon shining in the daytime — not as a full circle at all. Why does the Moon's shape and visibility keep changing?
Key idea
Phases of the Moon
1
Purnima
Full Moon day — The Moon appears as a fully bright circle.
→
2
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.
Key idea
Why the Moon's Appearance Changes
The Moon doesn't emit its own lightit shines by reflecting sunlight, and only the half facing the Sun is lit.
Not the same as an eclipseMoon 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.
Key idea
How Calendars Came Into Existence
1
Day
Based on Earth's rotation — about 24 hours.
→
2
Month
Based on the Moon's cycle of phases — about 29.5 days.
→
3
Year
Based on Earth's revolution around the Sun — about 365¼ days.
Key idea
Lunar, Solar, and Luni-solar Calendars
Calendar | Based on | Stays synced with seasons?
LunarThe Moon's cycle only (12 months = 354 days) — No — drifts over time
SolarEarth's orbit (365 days, with leap years) — Yes
Luni-solarBoth, with an extra month added periodically — Yes, by correction
Key idea
Festivals Follow Different Calendars
1
Shifting dates
Diwali and Holi follow lunar or luni-solar calendars, so their Gregorian calendar dates change each year.
→
2
Nearly fixed dates
Makar Sankranti, Pongal, Bihu, and Vaisakhi follow a solar calendar, so they fall on nearly the same date every year.
Key idea
Artificial Satellites
The Moon is Earth's natural satellitebut countries also launch artificial satellites that orbit Earth, most about 800 km up.
India's missionsISRO's Chandrayaan (Moon), Aditya-L1 (Sun), and Mangalyaan (Mars) missions all rely on the same satellite technology that supports communication and weather monitoring.
Chapter · Key terms to remember
Key Terms
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.
Chapter · Key terms to remember
More Key Terms
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.
Chapter · Key terms to remember
More Key Terms
Leap year
A year with an extra day to stay synced with the seasons.
Artificial satellite
A human-made object launched to orbit Earth.
Questions for your notebook
Write these down, then discuss
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.
2Amol was born on 6th May on a full Moon day. Does his birthday fall on a full Moon day every year?
3Malini saw the Moon directly overhead at sunset. (i) Sketch the phase she saw. (ii) Is the Moon waxing or waning?
Project as-is — students copy the questions, then the class discusses answers together.