We see phases of the Moon because we view the sunlit half of the Moon from different angles as it orbits Earth, so different fractions of its lit side are turned toward us over the month.

Quick Scoop

What actually causes the phases?

  • The Moon does not make its own light; it only reflects sunlight, and at any moment half of the Moon is lit by the Sun and half is in darkness, just like day and night on Earth.
  • As the Moon orbits Earth (about once every 29.5 days), the angle between the Sun, Earth, and Moon changes, so we see different portions of that lit half from our viewpoint.
  • Phases are not Earth’s shadow; Earth’s shadow on the Moon only happens during a lunar eclipse, which is rare compared with the regular monthly phase cycle.

Why do the phases follow a pattern?

  • The Moon’s orbit around Earth is regular, so the changing Sun–Earth–Moon angle repeats in a predictable cycle, giving a repeating sequence of phases roughly every month.
  • Because the Moon’s rotation is synchronized with its orbit (it turns once on its axis in the same time it orbits once), we always see the same face, but the lighting on that face slides around, making crescents, quarters, and gibbous shapes.

The main named phases

  • New Moon: Moon is between Earth and Sun; the sunlit side faces away from us, so the Moon is hard to see.
  • First Quarter and Third (Last) Quarter: We see half of the visible face lit and half dark, like a “half-Moon,” because we’re viewing the lit half from the side.
  • Full Moon: Earth is between the Sun and Moon, so the entire Earth-facing side of the Moon is lit and appears fully bright.

In short, we see phases of the Moon because we’re watching a half-lit sphere move around us, and our changing viewpoint reveals different slices of that lit half over time.

TL;DR: We see phases of the Moon because the Moon is always half lit by the Sun, and as it orbits Earth we see different portions of that lit half from our changing angle, not because Earth’s shadow is covering it.

Information gathered from public forums or data available on the internet and portrayed here.