Moon Phases Explained

Time & Astronomy

The Moon changes shape on a fixed 29.5-day schedule, and almost every part of that statement hides something counter-intuitive. The Moon does not actually change shape, the Earth casts no shadow on it, the lunar month is not the time it takes to orbit us, and the date of a full moon depends on which country you are standing in.

Key takeaways

Half the Moon is always lit. Phases are about how much of the lit half faces us, not about any shadow of the Earth.
The cycle from one new moon to the next takes 29.53 days, while a single orbit takes only 27.32.
The Moon rises about 50 minutes later each day, which is why each phase belongs to its own part of the night.
A full moon rises at sunset and sets at sunrise; a new moon travels with the Sun and cannot be seen at all.
The crescent is lit on the right in the northern hemisphere and on the left in the southern.
Phase times are published in UTC, so the same full moon can carry two different calendar dates.

Half the Moon Is Always Lit

The Sun illuminates one hemisphere of the Moon at all times, exactly as it illuminates one hemisphere of the Earth. Nothing about that changes through the month. What changes is our viewing angle: as the Moon travels around us we see the lit hemisphere from the side, from behind, or head on, and the fraction of it visible from here is what we call a phase.

The most common misconception is that phases are the shadow of the Earth falling on the Moon. They are not, and the giveaway is the arithmetic: the shadow of the Earth reaches the Moon only two or three times a year, whereas phases run continuously. When the shadow does land on it, the result is a lunar eclipse, which looks nothing like a crescent — the edge is blurred rather than sharp, the colour is red rather than black, and the whole thing is over in hours rather than weeks.

There is also no permanent dark side. Every part of the Moon receives about two weeks of sunlight followed by two weeks of night. The phrase people reach for is far side: the hemisphere permanently turned away from us, which is a fact about the rotation of the Moon rather than about sunlight.

The Eight Phases, and When to Look

Because each phase corresponds to a particular angle between the Sun and the Moon, it also corresponds to a particular time of day. This table is the single most useful thing to know about the Moon:

Phase	Rises	Sets	Best seen
New moon	About sunrise	About sunset	Not at all: it is lost in the glare of the Sun
Waxing crescent	Mid-morning	After sunset	Low in the west in the evening twilight
First quarter	About noon	About midnight	High in the south at sunset; visible in daylight
Waxing gibbous	Mid-afternoon	Small hours	Most of the evening
Full moon	About sunset	About sunrise	All night, opposite the Sun
Waning gibbous	Mid-evening	Mid-morning	Late night and early morning
Last quarter	About midnight	About noon	The morning sky; still up after breakfast
Waning crescent	Small hours	Mid-afternoon	Low in the east before dawn

Two practical consequences follow. First, anyone who says they never see the Moon in daylight is simply looking at the wrong time: a first-quarter Moon is up all afternoon and is perfectly visible against a blue sky. Second, if you want a dark sky for stars, the week around new moon is the one to choose, while the week around full moon floods the whole night.

The Moon also rises about 50 minutes later each day, because it moves roughly 13 degrees eastward against the stars in that time and the Earth has to turn a little further to catch up. The exact delay varies with the season and latitude, which is what produces the harvest moon effect: around the September equinox in the northern hemisphere the delay shrinks to as little as half an hour, so several evenings in a row get moonrise close to sunset.

Why the Month Is 29.5 Days and the Orbit Is 27.3

The Moon completes one orbit of the Earth in 27.32 days measured against the background stars — the sidereal month. But by the time it gets back to the same position among the stars, the Earth has moved about 27 degrees along its own orbit, so the Sun is no longer in the same direction. The Moon needs a further two days or so to catch up and line up with the Sun again.

That catch-up gives the synodic month of 29.53 days, the cycle of phases, and it is the month every lunar calendar in history has been built on. Twelve of them come to 354 days, eleven short of a solar year, which is why a purely lunar calendar such as the Islamic one drifts steadily backwards through the seasons, and why calendars that try to track both, such as the Hebrew and Chinese, have to insert an extra month periodically. The tension between astronomical periods that refuse to divide evenly runs right through the history of timekeeping.

The Moon also keeps one face turned towards us throughout, because its rotation period and its orbital period are locked together by tides. We do get a little more than half of it: the orbit is elliptical and slightly tilted, so the Moon appears to rock gently back and forth, an effect called libration that reveals about 59 per cent of the surface over time.

Which Way Round Is the Crescent?

The orientation of the lit edge depends on where you are standing on the planet, which catches out anyone using a diagram drawn for the wrong hemisphere:

In the northern hemisphere — London, New York, Tokyo — a waxing crescent is lit on the right, and a waning one on the left.
In the southern hemisphere — Sydney, Cape Town — it is the other way round, because you are looking at the same object while standing upside down relative to your northern counterpart.
Near the equator, in Quito or Nairobi, the crescent often sits with its lit edge along the bottom, giving the smiling moon or the boat that appears on so many flags of tropical countries.

The old northern mnemonic still works locally: if the crescent looks like the start of the letter D it is developing, and if it looks like a C it is closing. Anyone taking that rule south of the equator will get every prediction backwards.

Earthshine, Supermoons and Blue Moons

Earthshine. A thin crescent is often accompanied by a faint grey glow filling the rest of the disc — the old moon in the arms of the new. It is sunlight reflected off the Earth onto the lunar night side, and it is brightest when a nearly full Earth hangs in the lunar sky. Leonardo da Vinci worked out the explanation around 1510.
Supermoons. The distance to the Moon varies from about 356,500 km at perigee to 406,700 km at apogee. A full moon at the near end looks around 14 per cent wider and 30 per cent brighter than one at the far end. The difference is real but hard to notice without a comparison photograph, and it has nothing to do with the moon illusion, which makes a low Moon look enormous and is a trick of perception rather than optics.
Blue moons. The popular definition is the second full moon in a calendar month; the older seasonal one is the third full moon in a season containing four. Neither involves any change of colour, and the first has a peculiar property covered below.

A Full Moon Is an Instant, Not a Night

Full moon is a single moment: the instant the Moon reaches the point opposite the Sun. Almanacs publish it in UTC, and converting it can move it onto a different calendar date, exactly as with a solstice or a lunar eclipse.

This makes some familiar claims local rather than universal. A full moon at 00:40 UTC on 1 September is a 31 August full moon in New York and Los Angeles — so a month that holds a blue moon in the Americas may hold none in Europe, and the reverse happens just as often. There is no fact of the matter to settle; the calendar date is a property of your time zone, which is the same effect that gives travellers two Mondays or none at the International Date Line.

Calendars that are anchored to the Moon inherit the problem in a stronger form. The Islamic month traditionally begins with the first naked-eye sighting of the crescent, which depends on longitude, latitude and weather, so Ramadan can start on different days in Jeddah, Istanbul and Jakarta. The Chinese calendar sidesteps observation by defining its new moons astronomically — but at Beijing time, which is a legal choice as much as an astronomical one. How to do these conversions carefully is set out in How Astronomical Events Are Converted to Local Time.

Moonrise and moonset, by contrast, are local events like sunrise and sunset: they happen at a different instant for every point on Earth and are always published in local time rather than converted from UTC.

Phases and Eclipses

Eclipses are simply the two phases taken to their extreme. A solar eclipse can only happen at new moon, when the Moon is between us and the Sun, and a lunar eclipse only at full moon, when it is directly opposite. If you ever see a diagram showing an eclipse at first quarter, the diagram is wrong.

Which raises the obvious question, since new and full moons come round every month: why is there not an eclipse every fortnight? The answer is the five-degree tilt of the lunar orbit, and it is worked through in Why Don’t Eclipses Happen Every Month? and What Is an Eclipse?

Frequently Asked Questions

What causes the phases of the Moon?

The changing angle from which we see the sunlit half of the Moon as it orbits the Earth. Half of the Moon is lit at all times; the phase is the fraction of that lit half turned towards us. It has nothing to do with the shadow of the Earth.

Why is the lunar month 29.5 days when the orbit takes 27.3?

Because the Earth moves along its own orbit while the Moon goes round. After one full orbit the Moon has to travel for a further two days or so to line up with the Sun again, and it is that alignment, not the orbit, that defines the cycle of phases.

When does a full moon rise?

At about sunset, and it sets at about sunrise, because it sits opposite the Sun in the sky. A first-quarter Moon rises around noon and a last-quarter Moon around midnight, which is why each phase belongs to a different part of the day.

Why does the crescent point the other way in Australia?

Because an observer in the southern hemisphere is oriented upside down relative to one in the north, so the same lit edge appears on the opposite side. Near the equator the crescent frequently appears lit from below instead.

Can a full moon fall on two different dates?

Yes. It is a single instant published in UTC, so converting it to local time can place it on the previous or the following day. This is why a blue moon can occur in one country and not in another during the same month.