Sun-Earth-Moon System Models

Look up at the night sky for one whole month and the Moon will look different almost every night. Sometimes it is a thin curve. Sometimes it is a big bright circle. Sometimes you cannot see it at all. The Moon is not changing shape — we are simply seeing different parts of it as it travels around Earth, and as Earth travels around the Sun. This is the Sun-Earth-Moon system.

Three Bodies, Two Orbits

The Sun is a star. It is huge and it makes its own light. Earth is a planet. It does not make light, but it travels around the Sun once every year. The Moon is a natural satellite — a smaller body that travels around Earth about once every 27 days. So there are TWO orbits happening at the same time: Earth around Sun, and Moon around Earth.

• Sun: a star. It produces its own light and heat.
• Earth: a planet that orbits the Sun once every ~365 days (1 year).
• Moon: a natural satellite that orbits Earth once every ~27 days.
• Earth also spins on its axis once every ~24 hours, giving us day and night.

Why We See Phases

The Moon does not make any light of its own. It SHINES because half of it is always lit by the Sun, just like a ball outside on a sunny day. As the Moon orbits Earth, we look at its lit half from different angles. Sometimes we see all of the lit side, sometimes we see only a sliver, and sometimes the lit side is facing away from us so we see almost nothing. The shape we see is called a phase.

The Eight Phases in Order

1. New Moon — the lit half faces away from Earth, so the Moon looks dark.
2. Waxing Crescent — a thin sliver of light on the right side appears.
3. First Quarter — the right half of the Moon is lit (looks like a half-circle).
4. Waxing Gibbous — most of the Moon is lit, but not all of it yet.
5. Full Moon — the whole face of the Moon is lit and bright.
6. Waning Gibbous — most of the Moon is still lit, but starting to shrink.
7. Last Quarter — the LEFT half of the Moon is lit.
8. Waning Crescent — only a thin sliver on the left is left, then back to New.

A Simple Model

You can model the system at home. Use a lamp as the Sun. Stand in the middle as Earth. Hold a tennis ball as the Moon at arm’s length. Slowly turn in a full circle, keeping the ball facing you. The lit side of the ball will appear to change shape just like the Moon — even though the lamp is always lighting the same half. The phases come from the angle, not from the light changing.

Why It Matters

The Moon’s pull on Earth causes ocean tides. Its 29.5-day cycle is the basis for the word "month" in many languages. Sailors, farmers, and astronomers have used Moon phases for thousands of years. And by building accurate models of how Sun, Earth, and Moon move, scientists can predict eclipses years in advance.

• Solar eclipse: the Moon passes between Sun and Earth, blocking the Sun.
• Lunar eclipse: Earth passes between Sun and Moon, casting a shadow on the Moon.
• Tides: the Moon’s gravity pulls Earth’s oceans, making them rise and fall.