US Trends

what causes moon phases

The Moon’s phases are caused by how sunlight hits the Moon and how we see that lit-up part as the Moon orbits Earth.

Quick Scoop

Core idea: why the Moon changes shape

  • The Moon does not make its own light; it only reflects sunlight.
  • As the Moon travels around Earth (about once every month), we see different amounts of its sunlit half.
  • The phase you see depends on the positions of the Sun, Earth, and Moon at that moment.
  • Phases are not caused by Earth’s shadow; Earth’s shadow on the Moon only happens during a lunar eclipse, which is rare.

Think of the Moon as a half-lit ball moving around you under a lamp: the lit side is always facing the lamp (the Sun), but the part of that lit side you can see changes as it moves.

Main Moon phases in order

As the Moon orbits Earth, the cycle goes roughly like this:

  1. New Moon – Moon is between Earth and Sun, the sunlit side faces away from us, so it looks dark.
  2. Waxing Crescent – A thin lit “crescent” appears on one side and slowly grows.
  3. First Quarter – We see half the Moon lit (like a “D” shape).
  4. Waxing Gibbous – More than half is lit and still growing toward full.
  5. Full Moon – Earth is between Sun and Moon, and we see the whole sunlit face.
  6. Waning Gibbous – Still mostly lit, but the bright area starts shrinking.
  7. Last (Third) Quarter – Again half lit, but the opposite side compared to First Quarter.
  8. Waning Crescent – A thinning crescent remains until it disappears back to New Moon.

What really causes moon phases (step‑by‑step)

  • The Sun always illuminates half of the Moon’s surface.
  • Because the Moon’s orbit carries it around Earth, the angle between Sun–Moon–Earth keeps changing.
  • From our viewpoint on Earth, that changing geometry lets us see sometimes a big slice of the lit half (full moon), sometimes a small slice (crescent), and sometimes exactly half (quarter).
  • The Moon keeps the same side facing Earth (synchronous rotation), so what changes is not which side we see, but how much of that side is sunlit.

Quick HTML table of the phases

Below is an HTML table summarizing the key phases and what causes how they look:

html

<table>
  <thead>
    <tr>
      <th>Phase</th>
      <th>What you see from Earth</th>
      <th>Sun–Earth–Moon geometry (cause)</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>New Moon</td>
      <td>Moon appears dark or invisible.[web:1][web:5]</td>
      <td>Moon is between Sun and Earth; sunlit side faces away from us.[web:1][web:5][web:9]</td>
    </tr>
    <tr>
      <td>Waxing Crescent</td>
      <td>Thin crescent of light grows each night.[web:1][web:5][web:10]</td>
      <td>Moon has moved slightly in its orbit, and a small part of the sunlit half is now visible.[web:3][web:5][web:10]</td>
    </tr>
    <tr>
      <td>First Quarter</td>
      <td>Right (or left, depending on hemisphere) half is lit.[web:1][web:3][web:10]</td>
      <td>Sun–Earth–Moon form a right angle; we see half of the sunlit side.[web:3][web:7][web:9]</td>
    </tr>
    <tr>
      <td>Waxing Gibbous</td>
      <td>More than half lit, approaching full.[web:1][web:5][web:10]</td>
      <td>Moon is moving toward the opposite side of Earth from the Sun; most of its sunlit half is visible.[web:3][web:5][web:7]</td>
    </tr>
    <tr>
      <td>Full Moon</td>
      <td>Entire face looks bright.[web:1][web:3][web:5]</td>
      <td>Earth is between Sun and Moon; we see the full sunlit half.[web:1][web:3][web:5][web:9]</td>
    </tr>
    <tr>
      <td>Waning Gibbous</td>
      <td>Still mostly lit, but the bright area shrinks night by night.[web:1][web:5][web:10]</td>
      <td>Moon has passed full; angle shifts so we see less of the sunlit side.[web:3][web:5][web:7]</td>
    </tr>
    <tr>
      <td>Last (Third) Quarter</td>
      <td>Again, half of the disk is lit, opposite half compared to First Quarter.[web:1][web:3][web:10]</td>
      <td>Sun–Earth–Moon form another right angle, but on the far side of the orbit.[web:3][web:7][web:9]</td>
    </tr>
    <tr>
      <td>Waning Crescent</td>
      <td>Thin shrinking crescent just before sunrise.[web:1][web:5][web:10]</td>
      <td>Moon nears the line between Earth and Sun again; only a small part of the sunlit half remains in view.[web:3][web:5][web:9]</td>
    </tr>
  </tbody>
</table>

Common myth vs reality

  • Myth: “Earth’s shadow makes the Moon’s phases.”
  • Reality: Phases come from the Moon’s own day–night line (its shadow on itself) as it orbits Earth; Earth’s shadow only matters during eclipses.

Tiny time note

  • The full cycle of phases, from one New Moon to the next, takes about 29.5 days, so the pattern you see today slowly shifts over the month.

TL;DR: Moon phases are simply the changing view of the Moon’s sunlit half as it orbits Earth, not Earth’s shadow playing tricks.

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