What Is the Moon Phase Cycle

The Moon does not shine by itself — it reflects sunlight. Phases are simply how much of the Sun-lit half of the Moon we see from Earth on a given day. Let us walk through the whole cycle.

Why phases exist at all

Picture three balls: the Sun shines, Earth watches, the Moon orbits Earth. The Sun always lights exactly one half of the lunar globe. But the angle from which we look at that lit half changes every day:

• The Moon is in conjunction with the Sun in ecliptic longitude → the lit half faces away → new moon (~0%).
• The Moon is in opposition to the Sun in ecliptic longitude → the lit half faces us → full moon (~100%).
• In-between positions → crescents, quarters, gibbous phases.

The Moon itself never changes — only the viewing angle does.

The 8 phases of the cycle

The full cycle is divided into 8 phases. The table shows the order starting from new moon:

Quarters are called so because the Moon has covered about one quarter (first) or three quarters (last) of the synodic cycle; its ecliptic longitude differs from the Sun's by about 90° — east at first quarter and west at last quarter.

Synodic month: 29.53 days

The time from one new moon to the next — the synodic month — averages 29.53 days. It is longer than the sidereal month (27.32 days, a revolution relative to the stars) because Earth itself moves along its orbit around the Sun, and the Moon needs to "catch up" with the new-moon geometry.

That is why:

• full moon dates differ every year;
• sometimes two full moons fall in one calendar month (the second is called a "blue moon");
• the lunar calendar drifts from the solar one by about 11 days per year.

Waxing and waning Moon

• Waxing — from new to full moon. Each evening the crescent is thicker. In the Northern Hemisphere the right side is lit.
• Waning — from full to new moon. Each morning the disc is thinner. The left side is lit.

In the Southern Hemisphere the picture is mirrored — a frequent source of confusion in mnemonic rules.

Moon age

Moon age is the number of days since the last new moon (0–29.5). New moon is age ~0, first quarter ~7.4, full moon ~14.8, last quarter ~22.1 days. On lunar.day the day card shows the lunar-day number; astronomical age is a separate value.

How to follow phases on lunar.day

On your city page you see the current phase, illumination percentage and lunar-day number. The moon/phase/[date] page gives values for any day and helps you check the date of each main phase.

Summing up

Phases are the view of the lit half of the Moon from Earth. The 8-phase cycle lasts 29.53 days: new moon → growth to full moon → waning to the next new moon. Knowing the phase, illumination and lunar-day number lets you read any lunar calendar date.