what causes the different phases of the moon?

The Moon’s phases are caused by its orbit around Earth and how sunlight hits it, not by Earth’s shadow.

The core idea in one line

Half of the Moon is always lit by the Sun, but as the Moon moves around Earth, we see different fractions of that lit half, which creates the phases.

How the geometry works

• The Moon orbits Earth roughly once every 29.5 days, a cycle called a synodic month.

• At any moment, one hemisphere of the Moon faces the Sun (its daytime side) and the other is in darkness (its nighttime side).

• From Earth, we see some combination of the lit side and the dark side, depending on the Moon’s position relative to the Sun and Earth.

• The changing angle between Sun–Moon–Earth as the Moon orbits is what changes how much of the lit half we can see.

A common misconception is that Earth’s shadow causes the phases; that only happens during a lunar eclipse, which is rare.

Main phases, step by step

Over one lunar month, the Moon goes through a predictable sequence:

1. New moon – Moon is between Earth and Sun. The lit half faces away from us, so we see little or nothing.

2. Waxing crescent – Moon moves a bit along its orbit; we see a thin sliver of the lit side growing (“waxing”).

3. First quarter – Sun, Earth, and Moon form a right angle. We see half of the lit side: the right half appears bright.

4. Waxing gibbous – More than half of the visible face is lit and still increasing toward full.

5. Full moon – Earth is between Sun and Moon. The lit side faces us directly, so the whole face looks bright.

6. Waning gibbous – After full, the lit area begins shrinking (“waning”), still more than half.

7. Last (third) quarter – Another right angle; again we see half of the lit side, but now the left half is bright.

8. Waning crescent – Only a shrinking thin sliver remains lit before returning to new moon.

Quick HTML table of phases

html

<table>
  <thead>
    <tr>
      <th>Phase</th>
      <th>Moon’s position</th>
      <th>What we see from Earth</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>New moon</td>
      <td>Moon between Earth and Sun [web:1][web:3]</td>
      <td>Lit side turned away; Moon mostly invisible [web:1][web:3]</td>
    </tr>
    <tr>
      <td>Waxing crescent</td>
      <td>Just past new; small angle from Sun [web:1]</td>
      <td>Thin growing crescent [web:1][web:3]</td>
    </tr>
    <tr>
      <td>First quarter</td>
      <td>Sun–Earth–Moon form a right angle [web:1][web:9]</td>
      <td>Right half lit [web:1][web:10]</td>
    </tr>
    <tr>
      <td>Waxing gibbous</td>
      <td>Between first quarter and full [web:1]</td>
      <td>More than half lit, growing [web:1][web:3]</td>
    </tr>
    <tr>
      <td>Full moon</td>
      <td>Earth between Sun and Moon [web:1][web:3]</td>
      <td>Entire face lit [web:1][web:3]</td>
    </tr>
    <tr>
      <td>Waning gibbous</td>
      <td>Just past full [web:1]</td>
      <td>More than half lit, shrinking [web:1][web:3]</td>
    </tr>
    <tr>
      <td>Third quarter</td>
      <td>Another right angle, later in orbit [web:1][web:9]</td>
      <td>Left half lit [web:1][web:10]</td>
    </tr>
    <tr>
      <td>Waning crescent</td>
      <td>Near end of cycle, approaching new [web:1]</td>
      <td>Thin shrinking crescent [web:1][web:3]</td>
    </tr>
  </tbody>
</table>

Timing: why ~29.5 days?

• The Moon takes about 27.3 days to orbit Earth once relative to the stars (a sidereal month).

• But Earth is moving around the Sun at the same time, so the Moon must travel a little farther to line up with the Sun–Earth direction again.

• That extra bit takes about 2.2 days, giving a 29.5‑day phase cycle (the synodic month) that we notice from one full moon to the next.

A simple way to visualize it

Imagine holding a small white ball (the Moon) at arm’s length in a dark room while a lamp (the Sun) sits in the center.

As you walk in a circle around the lamp and look at the ball, you always know one side of the ball faces the lamp, but from your viewpoint, sometimes you see mostly the lit side (like a full moon), sometimes mostly the dark side (new moon), and sometimes half and half (quarter moons).

In short: the phases are a changing view of a half‑lit sphere, caused by the Moon’s orbital dance with Earth and the Sun.

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