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Keeping Time with the Skies – Class 8 Science notes, important questions and MCQs

NCERT Class 8 Science, chapter 11. Notes, key terms, important questions with answers and MCQs.

Chapter overview

People have measured time using the regular motions of the Sun, Moon and stars. Days, months and years come from Earth's rotation, the Moon's phases and Earth's revolution.

Natural time units

Day: one rotation of Earth on its axis (about 24 hours) — gives day and night.
Month: the Moon goes through its phases (new moon to new moon) in about 29.5 days.
Year: one revolution of Earth around the Sun, about 365¼ days — hence a leap year every 4 years.

Sky clocks and calendars

Sundials use shadows to tell time; the shadow is shortest at local noon.
Lunar calendars follow the Moon; solar calendars follow the Sun; Indian calendars often combine both (lunisolar).
Seasons are caused by the tilt of Earth's axis (about 23.5°), not by its distance from the Sun.

Key terms

Rotation: spinning of Earth on its axis
Revolution: Earth's motion around the Sun
Phases of the Moon: changing shapes of the lit part of the Moon we see

Formulas

1 year ≈ 365.25 days; leap year has 366 days

Important questions with answers

What is the difference between rotation and revolution of the Earth?2 marks

Rotation is spinning on its axis (one day). Revolution is moving around the Sun in its orbit (one year).

Why do we have a leap year?3 marks

The Earth takes about 365¼ days to go around the Sun. The extra ¼ day adds up to one day in four years, so every fourth year has 366 days (29 February).

Explain how the Sun and the Moon were used to keep time, and how a lunar month and a solar year differ. Why do lunisolar calendars add an extra month sometimes?5 marks

The Sun's daily rise and set gives the day; its yearly movement (seasons, changing sunrise point) gives the year; sundials used shadows to tell the time of day. The Moon's phases from new moon to full moon and back give a month of about 29.5 days. Twelve lunar months make about 354 days, about 11 days shorter than a solar year of about 365¼ days. To keep festivals in the right seasons, lunisolar calendars (like the Indian Panchang) add an extra month (adhik maas) roughly every 2½–3 years.

Quick revision

Rotation → day and night.
Revolution + tilt → seasons.
Shadow is shortest at noon.

Exam tips

Moon has no light of its own — it reflects sunlight.
Full moon to full moon ≈ 29.5 days.

Practice MCQs

Day and night are caused by Earth's
1. rotation
2. revolution
3. tilt only
4. distance from the Sun
Show answer
Answer: (a) rotation. Rotation.
A leap year has
1. 364 days
2. 365 days
3. 366 days
4. 360 days
Show answer
Answer: (c) 366 days. Extra day in February.
The time from one full moon to the next is about
1. 7 days
2. 15 days
3. 29.5 days
4. 365 days
Show answer
Answer: (c) 29.5 days. Lunar month.
A sundial's shadow is shortest at
1. sunrise
2. sunset
3. noon
4. midnight
Show answer
Answer: (c) noon. Sun is highest.
One revolution of the Moon around the Earth takes about
1. 27–29 days
2. 1 day
3. 365 days
4. 7 days
Show answer
Answer: (a) 27–29 days. Basis of a lunar month.
Seasons on Earth are mainly caused by
1. the tilt of Earth's axis
2. changing distance from the Sun
3. the Moon's phases
4. clouds
Show answer
Answer: (a) the tilt of Earth's axis. Tilt changes sunlight angle and day length.
Why is a day added every four years?
1. the Moon slows down
2. a year is about 365¼ days
3. Earth stops rotating
4. to match the month
Show answer
Answer: (b) a year is about 365¼ days. The extra quarter days add up.
Most Indian festivals like Diwali shift dates in the Gregorian calendar because they follow
1. the solar calendar only
2. the stock market
3. a lunisolar calendar based on Moon phases
4. school holidays
Show answer
Answer: (c) a lunisolar calendar based on Moon phases. Lunar months.

All chapters of Class 8 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.