Moon phases simulator

What this simulator shows

The left picture looks down on Earth from above the North Pole, with sunlight coming from the left. The Moon travels around Earth on a dashed orbit, and you can drag it anywhere on that path or step through the eight main phases. The right picture shows the same moment as you would see it from Earth. The readout names the phase, says roughly which day of the cycle it is, and gives the percentage of the Moon's face that is lit.

• Half of the Moon is always lit by the Sun, just as half of Earth is always in daytime.
• The phase depends on how much of that lit half is turned toward Earth.
• A full cycle, from one new moon to the next, takes about 29.5 days.
• Waxing means the lit part is growing each night; waning means it is shrinking.

Why a model helps

Phases are hard to picture because we only ever see the view from Earth, one night at a time. Seeing the top view and the Earth view side by side lets a child connect the Moon's position with its shape, which is the whole explanation. The blue edge on the Moon in the top view marks the half that faces Earth, so children can see which part of the lit side we get to look at.

Common misconceptions

The most common wrong idea is that Earth's shadow causes the phases. It does not: Earth's shadow only falls on the Moon during a lunar eclipse, which happens at full moon on just a few nights a year. Try it in the simulator: at a crescent, the Moon is nowhere near Earth's shadow. Other mix-ups are that the Moon makes its own light (it reflects sunlight) and that the far side of the Moon is always dark. The far side gets as much sunlight as the near side; at new moon, it is fully lit.

The pictures are not to scale. The real Moon is about 30 Earth widths away, which would not fit on the screen.

A 10-minute moon activity

• Press Start again so the Moon is at new moon. Ask: "Is the Moon lit right now? Which side?"
• Step forward one phase at a time. Before each step, ask the child to predict the shape.
• Stop at full moon and ask where the Sun, Earth and Moon are compared with each other.
• Ask: "If we see a first quarter moon tonight, what will we see in about a week?"
• Finish with one round of Quiz me.

Where this fits in school

• US, grade 1: NGSS 1-ESS1-1 asks students to use observations of the Sun, Moon and stars to describe patterns that can be predicted.
• US, middle school: NGSS MS-ESS1-1 asks students to develop and use a model of the Earth-Sun-Moon system to describe lunar phases, eclipses and seasons.
• England, Year 5: pupils describe the movement of the Moon relative to Earth and the Sun, Earth and Moon as roughly spherical bodies.

Learning more

Earth and space science is part of Eurekami's skill map, with lessons that draw diagrams live and talk them through step by step. Daily practice brings back a few questions on topics like this one, so the phase names stick.