The Moon Phases in Order

The Moon runs through eight phases and the order never varies: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent, then a new moon again. One complete loop takes roughly 29.5 days, and that stretch of time is where the idea of a month came from.

Why phases happen at all

The Moon produces no light of its own. It is visible because sunlight falls on it, and sunlight falls on exactly half of it at any given moment, in the same way it falls on half of the Earth. That lit half is always there. What keeps changing is how much of it happens to point in our direction.

As the Moon travels around us, our angle on that bright half swings from behind it to beside it to in front of it. When the Moon lies roughly between the Earth and the Sun, the lit face is turned away and we see nothing at all. When the Earth lies between the other two, the lit face is turned straight at us and we get a full disc. Every shape in between is a partial view of the same permanently half-lit ball of rock.

What the phases are not is the shadow of the Earth falling across the Moon. That does occur, but only occasionally and only at full moon, and it goes by the name of a lunar eclipse. The ordinary monthly shapes are a matter of viewing angle and nothing else.

The eight phases in order

Day counts below are approximate, since the cycle is a smooth change rather than eight separate events. Only four of the eight have an exact moment: the new moon, the two quarters and the full moon. The other four are the stretches in between.

• New moon, day 0. The bright side faces away from us. The Moon is in the sky during daylight, close to the Sun, and effectively invisible.
• Waxing crescent, days 1 to 6. A thin sliver appears low in the west shortly after sunset and sets soon afterwards. It thickens night by night.
• First quarter, about day 7. Half the disc is lit. It is already high in the sky at sunset and sets around the middle of the night.
• Waxing gibbous, days 8 to 14. More than half lit, not yet complete, and bright enough to read a road sign by. It rises in the afternoon.
• Full moon, about day 15. The whole visible face is lit. It comes up as the Sun goes down, stays up all night, and sets around dawn.
• Waning gibbous, days 16 to 21. Shrinking from the far side now. It rises later every evening and is at its highest well after midnight.
• Last quarter, about day 22. Half lit again, but the opposite half from the first quarter. It rises near midnight and hangs in the morning sky after sunrise.
• Waning crescent, days 23 to 29. A shrinking sliver visible low in the east before dawn, until it is lost in the glare and the cycle restarts.

What the names actually mean

Waxing means growing and waning means shrinking. Both are old English words that have survived almost nowhere else, which is why they sound more mysterious than they are. Everything from the new moon up to the full moon is waxing; everything from the full moon back down is waning.

Crescent describes any shape less than half lit. Gibbous, from a Latin word meaning humped, describes any shape more than half lit but short of complete. Quarter is the one that trips people up, because a quarter moon looks half lit. The word refers to the Moon being a quarter of the way through its cycle, not to a quarter of the disc being bright.

There is also a rhythm behind the rising times listed above. The Moon comes up around fifty minutes later each day, which is why the crescent that was an early evening sight becomes a pre-dawn one a fortnight later. Nothing about the Moon has changed. It has simply drifted out of step with our clocks.

The shape flips depending where you stand

From the northern hemisphere a waxing crescent is lit along its right edge and a waning one along its left. From the southern hemisphere it is the other way round, because an observer there is effectively upside down relative to one in the north. Near the equator the crescent tends to sit along the bottom like a bowl.

This is worth knowing because the memory trick taught in northern schools, that a shape like the letter D is growing and a shape like the letter C is going, is reversed in Australia, Chile or South Africa and means very little at all in Kenya or Indonesia. A photograph of the Moon tells you which hemisphere it was taken in.

Why the dates never sit still on a calendar

Calendar months run 28 to 31 days and the phase cycle runs about 29.5, so the two slide against each other permanently. A full moon falling on the 20th of one month arrives around the 18th or 19th of the next. A long month can occasionally hold two full moons, which is where the popular idea of a blue moon comes from.

Time zones add a second wobble. The exact instant of a full moon is one instant for the entire planet, but local clocks disagree about what to call it. A full moon at 23:40 in London is 18:40 the same evening in New York and 08:40 the following morning in Sydney, so two perfectly correct calendars can print dates a day apart.

There is a related oddity in the length itself. The Moon completes one circuit of the Earth against the background stars in about 27.3 days, yet it needs about 29.5 to return to the same phase. In the meantime the Earth has moved along its own orbit around the Sun, so the Moon has a little further to travel before the same angle to the sunlight lines up again.

What the cycle is taken to mean

Astrological tradition reads the eight phases as one continuous arc rather than eight unrelated events. The waxing half, from the first sliver to the full disc, is associated with building, adding and putting things in motion. The waning half is associated with finishing, editing, clearing out and letting go of what has served its purpose.

In practice most people use the sequence as a calendar rather than an oracle: begin something around the new moon, look honestly at it at the full moon, tidy up as the light fades. Nothing in the physics of a viewing angle says a plan started under a crescent will fare better than one started under a gibbous moon. What the cycle does offer is a visible, unmissable, repeating fortnightly checkpoint that nobody has to remember to set.