Activity 11.2 — Modelling the Moon's Phases — "Keeping Time with the Skies"

1. 12 marks

   In Activity 11.2's model, what do the stick-mounted ball, the torch or lamp, and your own head each represent?

   Hint. Match each prop to the real Sun–Earth–Moon body it stands in for.

   ✦ The stick-mounted ball represents the Moon; the torch (or a nearby electric lamp) represents the Sun; and your own head represents the Earth, with the ball held at arm's length, slightly above head height, since that positioning is what lets you compare its lit side to the lamp's direction as you turn around it.

2. 23 marks

   When the ball is held at position A (opposite the lamp), what do you see, and which real Moon phase does this match?

   Hint. At A, you are standing between the lamp and the ball — which face of the ball does that put in front of you?

   ✦ At position A you face the entire illuminated portion of the ball, since the ball is on the far side of you from the lamp and its whole Earth-facing side catches the light — this matches the full Moon, seeing the Moon's complete illuminated face.

3. 33 marks

   When the ball is held at position E (towards the lamp), what do you see, and which real Moon phase does this match?

   Hint. At E, the ball sits between you and the lamp — which face of the ball is now turned towards you?

   ✦ At position E you face the ball's non-illuminated side and cannot see any of its illuminated portion at all, because the lit side is facing away from you towards the lamp — this matches the new Moon, when the Moon's dark side faces Earth.

4. 42 marks

   At every position other than A and E, what shape is the line separating the illuminated and non-illuminated portions of the ball, and why does that detail matter?

   Hint. The chapter deliberately points this out as matching a real feature of how the Moon looks in the sky.

   ✦ The dividing line appears curved at every position besides the two extremes (A and E) — this matters because it matches the curved edge seen on a real crescent or gibbous Moon, confirming that the ball-and-lamp model is genuinely reproducing the geometry behind the Moon's phases, not just a coincidence of the activity's set-up.

5. 53 marks

   Using Fig. 11.5's labelling (positions A–H), explain the difference between the gibbous and crescent phases in terms of how much of the illuminated portion is visible.

   Hint. Positions B and H sit near the full-Moon end of the cycle; positions D and F sit near the new-Moon end.

   ✦ At positions B and H, more than half of the Moon's illuminated portion can be seen from Earth — this is the gibbous phase. At positions D and F, less than half of the illuminated portion can be seen — this is the crescent phase. This is why both are intermediate stages: gibbous phases sit between half Moon and full Moon, crescent phases sit between half Moon and new Moon.

6. 63 marks

   According to Fig. 11.5, on which day is the Moon closest to the Sun in the sky, and on which day is it farthest? Does this match what you would have found in Activity 11.1?

   Hint. New Moon sits directly between the Sun and Earth in the model; full Moon sits on the opposite side of Earth from the Sun.

   ✦ The Moon appears closest to the Sun in the sky on the new Moon day (position E, between the Sun and Earth) and farthest from the Sun on the full Moon day (position A, opposite the Sun). This is why Activity 11.1's own log should have shown, in its 'Moon and Sun separation' column, — the separation shrinking through the waning fortnight and growing back through the waxing fortnight.

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.