Why does the Moon change shape? A one-lamp activity for children
W4C Science · Astronomy · 4 min read · Evidence: established science; activity is classroom practice
Many adults believe Earth's shadow causes the Moon's phases. It does not. A lamp and a ball on a stick let children see the real reason in ten minutes.
The Moon makes no light of its own. As NASA explains, the Sun always lights half of the Moon, while the other half is dark. As the Moon travels around Earth, we see different amounts of that lit half. The full cycle of phases repeats about every 29.5 days. Earth's shadow only falls on the Moon during a lunar eclipse, which is rare.
The activity
Darken the room. Put one bright lamp without a shade in the middle of the room at head height. Only the adult touches the lamp.
Each child holds a ball on a stick at arm's length, slightly above their head. The child's head is Earth, the ball is the Moon, the lamp is the Sun.
Children turn slowly on the spot and watch the lit part of the ball grow and shrink.
In pairs, one child stops at eight positions while the other draws the lit shape each time. Then they swap and compare with photos of real phases.
Ask: when the Moon is full, where is it compared with the Sun? Children usually discover that a full Moon is on the opposite side of Earth from the Sun, which is why it rises around sunset.
Keep a Moon diary
Give children a sheet with 29 boxes. Every clear evening they draw the Moon and write the time and direction. After a month they have collected real data and can check their predictions.
One safety rule
Never look at the Sun directly, through binoculars, or with ordinary sunglasses. NASA's advice is to use certified solar viewers or an indirect method such as a pinhole projector, where you stand with your back to the Sun and look at the image on a card.
Try this tonight. Look for the Moon and draw it. Is the lit side on the left or the right? Check again in three days.
Related resource: W4C-120 Young Scientists Lab includes this session, a Moon diary sheet, shadow tracking and a pinhole viewer build. Coming soon to EduStore; subscribe to the newsletter to hear when it opens.
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