Why is the lunar cycle 29.5 days rather than 30?

The Moon covers roughly 13.2° of its orbit per day, and returning to the same position relative to the Sun takes about 360°. Hence a synodic period of 29.53 days. Twelve such months fall short of 365, so every two or three years the calendar carries a 13th month.

Why do illumination and phase not match?

Phase describes geometry — the arrangement of the three bodies. Illumination describes the fraction of the visible disc that is lit. They diverge in the first and last quarters: the Moon is half lit there but appears as a crescent, because the lit half faces away from us.

What can be seen on the Moon without a telescope?

The dark patches are the maria: basaltic plains flooded with lava about three and a half billion years ago. The terminator is sharp because there is no atmosphere. The Moon has no rings, and it always shows us the same face.

Why do we always see the same face of the Moon?

Because of tidal locking: the Moon's rotation is synchronised with its orbit around Earth, as tides on the Moon slowed its spin over billions of years. The far side is not 'dark' — it gets the same sunlight, it is simply never visible from Earth.

Frequently asked questions about the Moon: phases, signs and observing

Q: Why is the lunar day the 30th and not the 1st?

There are two counting systems. In common astrological use the new moon is day 0 or day 30, and the first day follows it. In the lunar calendars of several traditions the first day is the new moon itself. Both are used; the confusion between them comes from different conventions, not from bad data.

We collected what gets asked most: how the phase cycle works, what a lunar day is, how illumination differs from phase, and what the naked eye can show.

Updated October 4, 20262 min read

Short answers to the questions that come up most. The long version of each topic lives in the linked articles.

How the cycle is built

The synodic month is 29.53 days. Not 30, and not 29.5 rounded: the Moon moves along its orbit unevenly, so the interval between two identical phases varies by up to about half a day.

There are eight principal phases: the four quarters (new moon, first quarter, full moon, last quarter) and the four in between (waxing crescent, waxing gibbous, waning gibbous, waning crescent).

Illumination is not phase

This is the most common mix-up. Illumination is the fraction of the visible disc that is lit, shown as a percentage. Phase is the geometric position. At the first and last quarters illumination is exactly 50%, yet the visible disc looks like a crescent: the lit half is turned sideways, away from us.

The lunar day

In astrological tradition the new moon is day 0 or day 30, and the first day of the lunar month follows it. In the lunar calendars of several traditions the first day is the new moon itself. Both counts are in use, and the confusion between them is a difference of convention rather than of data.

What the naked eye shows

The dark patches are the lunar maria, basaltic plains about 3.5 billion years old. They are darker because they reflect less light than the bright highlands. The terminator is sharp — the Moon has no atmosphere, so there is no scattered light in the shadowed part.

At 30% illumination and a bright sky you can make out craters. Under a very dark sky, almost all the maria are visible.

The far side

We always see the same face because the Moon's rotation is synchronised with its orbit: tidal forces on the Moon slowed its spin over billions of years until the two periods matched. The far side receives as much sunlight as the near side — it simply never comes into view from Earth.

What to check on a city page

A phase page for a given date lists rise and set, culmination, distance and visible hours. If you are planning an observation, the most useful figure there is not the phase but the altitude at culmination: it decides whether the Moon clears the trees and rooftops.

What Is the Moon Phase Cycle

The short answer: moon phases are how much of the Sun-lit half of the Moon we see from Earth. The full cycle (synodic month) lasts 29.53 days: from new moon through first quarter and full moon to last quarter.

3 min readUpdated Sep 19, 2026

Moon Illumination, Age and Distance: What the Numbers Mean

The short answer: illumination is the share of the visible disc lit by the Sun (0–100%); astronomical age is the time since new moon; distance is the kilometres to the Moon. These are different measures: they help describe the phase, position in the cycle and apparent size, but do not fully determine observing conditions.