Activity 11.3 and the Three Natural Units of Time (11.2) — "Keeping Time with the Skies"

1. 13 marks

   Describe the set-up and method of Activity 11.3, used to measure the length of a solar day.

   Hint. A vertical stick, a sunny patch of ground, and dots traced at fixed intervals.

   ✦ Fix a 1 m stick vertically in a flat, sunlit patch of ground. Starting at 11:00 a.m., mark a dot on the ground at the tip of the stick's shadow every minute, since that regular interval lets you continue until around 1:10 p.m. and later count back to the exact shortest-shadow moment. Identify when the shadow was shortest (by counting dots to find the time), record it, and repeat on consecutive days to find the duration of a solar day from the time difference between two days' shortest-shadow moments.

2. 22 marks

   Why does the shortest shadow mark the Sun's highest position in the sky, and why does that particular moment matter for defining a day?

   Hint. A shadow's length depends on how high overhead the light source is.

   ✦ A shadow is shortest exactly when the Sun is at its highest point in the sky for that day, because the more directly overhead the Sun is, the less an object's shadow stretches out sideways. The book defines the mean solar day as the average time the Sun takes to go from its highest position on one day to its highest position on the next — so identifying this exact highest-point moment, repeated on consecutive days, is precisely how Activity 11.3 measures a day's length directly from observation.

3. 32 marks

   State the book's definition of a mean solar day, and its approximate length.

   Hint. This is given directly in the paragraph introducing Activity 11.3.

   ✦ 'The average time that the Sun takes to go from its highest position in the sky on one day to the highest position in the sky the next day, is 24 hours, and is called the mean solar day.'

4. 43 marks

   What natural cycle defines a month, and roughly how long is it? How does this compare to the natural cycle defining a year?

   Hint. One cycle is the Moon's own phase cycle; the other is Earth's revolution and its associated cycle of seasons.

   ✦ A month is based on the Moon's phase cycle, since — the Moon takes about 29.5 days (nearly a month) to cycle through all its phases. A year is based on Earth's revolution around the Sun, taking nearly 365 and a quarter days, during which Earth also undergoes one complete cycle of seasons — a distinctly longer natural cycle than the lunar month, and the basis for the solar year.

5. 52 marks

   In a real run of Activity 11.3, Table 11.2 shows the duration between consecutive days as 24:00 one day and 23:59 the next, not exactly 24:00 every time. Is this a mistake in the experiment?

   Hint. The book itself defines the solar day using the word 'average' — what does that imply about any single day's measurement?

   ✦ Not necessarily a mistake — the book's own definition is of the mean (average) solar day, meaning individual day-to-day measurements can vary very slightly around 24 hours due to small real astronomical effects and the practical limits of marking shadow-dots by hand. That is why the chapter instructs repeating the activity across several days and averaging the results — doing so smooths out this kind of small variation and arrive at a duration nearly equal to 24 hours.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 11 (hecu111.pdf), Reprint 2026-27, pages 170-189. Questions are referenced from the NCERT textbook for identification.