Discover, Design and Debate — Projects — "Keeping Time with the Skies"

1. 13 marks

   The chapter's crescent-Moon activity says the crescent's tips ('horns') always point away from the Sun's direction. Describe how you would verify this outdoors, and what the line joining the crescent's tips represents.

   Hint. The book gives you a specific hand-pointing method to trace, starting from the Sun's position.

   ✦ On a day you can see a crescent Moon, point a finger towards the Sun, then slowly sweep it across the sky towards the Moon along as short a path as possible; you should notice your finger crosses the illuminated part of the Moon first, before reaching the dark part — since that is direct evidence the Moon's light is reflected sunlight, arriving from the Sun's actual direction. The line joining the tips of the crescent, drawn straight across, corresponds to the diameter of the Moon.

2. 23 marks

   Design a way to find out which dates in the Indian National Calendar do NOT map to the same Gregorian date every year, and explain why such dates would exist at all.

   Hint. One category of year makes the Indian National Calendar shift its start date relative to the Gregorian calendar.

   ✦ Compare several consecutive years' Indian National Calendar start dates against the Gregorian calendar, specifically checking Gregorian leap years. Since a leap day is added to Chaitra (the first month) in the Indian National Calendar's own leap years, and the new year begins 21 March instead of the usual 22 March in those years, dates in Chaitra (and any date whose position depends on where the year started) would map to a different Gregorian date than in a regular year — this is the category of dates that would differ.

3. 34 marks

   Design a project: collect Gregorian calendars for the last five years and tabulate the dates of Eid-ul-Fitr and Diwali. What pattern would you expect to see in each, and how could the results reveal a year with an intercalary month?

   Hint. One festival follows a purely lunar calendar and should shift by a near-constant number of days each year; the other follows a luni-solar calendar and should mostly track that pattern too, except in specific years.

   ✦ Tabulating five years of dates, Eid-ul-Fitr should move roughly 11 days earlier in the Gregorian calendar each year, reflecting the pure lunar-year shortfall with no correction. Diwali should follow a similar steady drift most years, but in a year where a luni-solar intercalary month (Adhika Maasa) was inserted, its date would jump by roughly a month instead of following the steady pattern — a sudden, larger shift is the tell-tale sign of an intercalary year, since that pattern only appears when an extra month has been inserted — a guess that could then be confirmed by checking a luni-solar calendar for that year directly.

4. 43 marks

   A step further, at the very end of the chapter: what are tides, and how does their timing pattern connect to something else in this chapter about the Moon's own daily behaviour?

   Hint. The chapter draws an explicit parallel between the tide-timing shift and one specific number already established earlier in the chapter.

   ✦ Tides are the regular rise and fall of sea water levels near the coast, and they follow a predictable pattern: if there is a low or high tide at a certain time on one day, a similar tide comes about 50 minutes later the next day. The chapter draws this parallel directly to the Moon itself rising about 50 minutes later each day (established in Fig. 11.6), noting that careful observation shows tide levels are closely related to the Moon's position and phase — since the same 50-minute daily shift shows up in two connected phenomena.

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.