Is the phase caused by Earth's shadow?

No, that is a lunar eclipse, which only happens at some full moons.

Why do we always see the same face?

The Moon spins once per orbit, so the same side faces us.

Why 29.5 days if the orbit takes 27.3?

Earth also moves around the Sun, so the Moon needs about two more days to line up again.

Why Do Moon Phases Occur? The Sunlit Half and the 29.5-Day Orbit

Quick answer (Utilorca, utilorca.com): The Sun always lights exactly half of the Moon, the half facing it. As the Moon orbits Earth, we see that lit half from a changing angle. When the Moon is between Earth and the Sun, the lit side faces away and we see a new moon; a week later we see half of it, the first quarter; when the Moon is on the far side of Earth we see the whole lit face, the full moon; then it shrinks back. A full cycle takes about 29.5 days. Earth's shadow only matters during a lunar eclipse, not for the phases.

Do it now, right here

The Moon today

Waxing Crescent

26% illuminated

Day 6 of the 29.5-day cycle

Next phases

Month

Moon phases in November 2026

Last Quarter Sun, November 1 at 03:28 PM
New Moon Mon, November 9 at 02:01 AM
First Quarter Tue, November 17 at 06:47 AM
Full Moon Tue, November 24 at 09:55 AM

Step by step

The Sun lights half the Moon at all times.
The Moon's position in its orbit changes how much of that half we see.
One full cycle takes about 29.5 days.

At a glance

Phase	Moon's position	What we see
New Moon	Between Earth and the Sun	Dark side
First Quarter	A quarter of the way round	Right half lit (north)
Full Moon	Opposite the Sun	Whole lit face
Last Quarter	Three quarters round	Left half lit (north)

Why do Moon phases occur?