Moon phases with a ball on a stick

The idea

Half the Moon is always sunlit; the phase is the part of that lit half that faces Earth, set by the angle between Sun, Earth and Moon.

What you need

• 1 white polystyrene ball, 7 to 10 cm, on a 30 cm skewer per learner
• 1 bare lamp at head height in a darkened room (the Sun)
• 1 phase strip with the eight NASA phase names: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent
• 1 protractor drawn on the floor in chalk around each learner's standing point (0 degrees toward the lamp)

How to do it

1. Stand facing the lamp holding the ball at arm's length just above eye level; the learner's head is Earth.
2. Note that the ball's lit half faces the lamp and the learner sees none of it: new Moon at 0 degrees.
3. Turn anticlockwise (viewed from above, the direction the Moon orbits) in 45 degree steps, sketching the lit shape seen at each step and naming the phase; record the elongation angle for each sketch. Turning this way leaves the lamp on the right, so the sketches come out in the northern-hemisphere orientation; beside each one draw the same shape turned through 180 degrees, which is how that phase appears in the Sydney sky.
4. At 180 degrees raise the ball a little above the head's shadow to see it full; discuss why the Moon is usually not eclipsed at full Moon (its orbit is tilted 5.145 degrees).
5. Estimate the lit fraction at each step (0, about 0.15, 0.5, about 0.85, 1) and compare with the cosine rule in the simulation.

What you should see

The lit shape seen runs new, crescent, half (first quarter at 90 degrees), gibbous, full at 180 degrees, then gibbous, half (third quarter at 270 degrees) and crescent, with the lit edge always on the side toward the lamp. Because the learner turns anticlockwise with the head upright, the lamp ends up on the right and the sketches carry the northern-hemisphere orientation: each has to be turned through 180 degrees to match the Sydney sky, where the waxing crescent is lit on the left. The lit fraction is (1 - cos e) / 2 for elongation e: 0 at 0 degrees, 0.146 at 45, 0.500 at 90, 0.854 at 135 and 1 at 180 degrees. The NASA Moon Fact Sheet gives a revolution period of 27.3217 days and a synodic period of 29.53 days; the whole sequence in the real sky takes 29.53 days, about 3.7 days per 45 degree step. The learner knows it worked when first quarter appears as exactly half lit with the lamp to the side.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• Phases are Earth's shadow on the Moon; Earth's shadow only reaches the Moon during a lunar eclipse.
• The Moon has a permanently dark side; the far side is lit for half of every orbit.
• A crescent Moon is the same shape from Australia and Europe; the lit edge appears on opposite sides from the two hemispheres.

Safety card

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC4-OTU-01SC4-WS-04SC4-WS-05
• Australian Curriculum v9AC9S7U03AC9S7I01

Sources

The pages the author read to write this activity.

1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
2. vocabulary.curriculum.edu.au/MRAC/2024/04/LA/SCI/export/MRAC/2024/04/LA/SCI.jsonld
3. science.nasa.gov/moon/moon-phases
4. nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html