phases of the Moon

The phases of the Moon are the changing shapes of its bright, illuminated portion as seen from Earth, cycling from new Moon to full Moon and back over about a month.

• The Moon doesn't emit its own light — it only reflects sunlight, so exactly half of it is always lit at any moment, and phases happen because we see a changing fraction of that lit half from Earth as the Moon orbits.
• A ball held at arm's length under a lamp recreates the whole cycle: held opposite the lamp it looks fully lit (full Moon), held toward the lamp it looks fully dark (new Moon), and every position between shows a curved boundary between light and dark, just like the real Moon.
• A common misconception is that Earth's shadow causes phases — it doesn't. Earth's shadow falling on the Moon causes a lunar eclipse, a separate, much rarer event that can only happen on a full Moon day.
• Because the Moon's phase cycle repeats reliably about every 29.5 days, it became one of humanity's earliest and most dependable tools for measuring time, giving rise to the very idea of a month.