About this experiment
The Sun always lights exactly half of the Moon. What changes during the month is how much of that lit half faces Earth. Move the Moon around its orbit in the top view and watch its shape in the sky change from new moon to full moon and back in 29.5 days. The orange arc marks the half that faces Earth, which makes it easy to see why we get crescents and gibbous moons, and why a first quarter moon is high in the sky at sunset.
Learning goals
- Explain that moon phases come from the changing angle between the Sun, Earth and Moon, not from Earth’s shadow
- Name the eight phases in order and tell waxing from waning
- Relate the phase to the time the Moon rises, is highest and sets
- Explain why the same phase looks reversed from the Southern Hemisphere
How to use it
- 1Start at new moon and drag the Moon slowly around its orbit. Watch the orange arc and the view from Earth.
- 2Stop at each of the eight phases and describe how much of the half facing Earth is lit.
- 3Use the times of day around Earth to find when a full moon rises and when a first quarter moon is highest.
- 4Switch to the Southern Hemisphere and compare which side of the waxing Moon is lit.
The model
Assumptions
- The orbit is a circle in the same plane as Earth’s orbit, so eclipses are ignored. The real orbit is tilted by about 5°, which is why eclipses do not happen every month.
- The top view is not to scale. The real Moon is about 30 Earth diameters away.
- Rise and set times are for an observer near the equator, rounded to the nearest minute. At high latitudes they can differ by hours.
Try this in class
- Q1At full moon the Moon is on the opposite side of Earth from the Sun. Why is it not in Earth’s shadow every month?
- Q2A crescent Moon is low in the west just after sunset. Is it waxing or waning?
- Q3Why can you sometimes see the Moon in the daytime? Which phases are best for this?
- Q4A student in Australia and a student in Canada both photograph a first quarter Moon. How do their photos differ?
Frequently asked questions
Are the phases caused by Earth’s shadow?
No. Earth’s shadow only falls on the Moon during a lunar eclipse. Phases happen because we see different amounts of the half of the Moon that the Sun is lighting.
Why does the cycle take 29.5 days when the Moon orbits in 27.3?
While the Moon goes around Earth, Earth also moves along its orbit around the Sun. The Moon needs about two more days to return to the same position relative to the Sun.
Why do we always see the same face of the Moon?
The Moon spins once on its axis in exactly the time it takes to orbit Earth, so the same side always faces us. This is called tidal locking.