How do the phases of the Moon form?
On some nights the Moon appears as a thin crescent, on others as a bright disc. The reason for this change is not that the Moon changes shape, but that the geometry of the Sun, Earth and Moon is constantly shifting.
Short answer
The Moon produces no light of its own; it reflects the Sun's light. As it travels around the Earth its position relative to the Sun changes, and so does how much of its lit half we can see. The time from one new moon to the next is about 29.5 days, and this period is called the synodic month.
Moonlight is really reflected sunlight
At any moment one half of the Moon is lit by the Sun, just as one half of the Earth is always in daylight. What we call a phase is how much of that lit half can be seen from Earth. As the Moon completes its circuit of the Earth, its position relative to the Sun changes constantly, and so we sometimes see the whole of the lit face, sometimes only a thin sliver, and sometimes none of it at all.
The order of the phases: new moon to full, full back to new
The cycle begins at new moon: the Moon lies between the Sun and the Earth, its lit face is turned away from us, and it goes unnoticed in the sky. Over the following days the phases appear in this order:
- Waxing crescent: a thin sliver appears above the western horizon in the evening.
- First quarter: half of the lit face is visible, so we see a half disc rather than a quarter of one.
- Waxing gibbous: the stretch between half disc and full disc.
- Full moon: the Moon stands opposite the Sun in the sky and the whole lit face is visible.
- Waning gibbous, last quarter and waning crescent: the same appearances run through in reverse, and the cycle closes at new moon.
Why is the synodic month 29.5 days?
Relative to the stars, the Moon completes one orbit in about 27.3 days; this is called the sidereal month. But in that time the Earth has also moved along its own orbit around the Sun. For the Moon to return to the same phase, from one new moon to the next, it has to make up that extra distance. This is why the phase cycle, the synodic month, lasts about 29.5 days. The calendar idea of a "month" has its origin in this cycle.
Why do we always see the same face?
The Moon completes one rotation on its own axis in exactly the time it takes to travel once around the Earth. Because of this condition, known as tidal locking, the same face is always turned towards us. A common misconception grows out of it: the phrase "the dark side of the Moon". The far side we never see also receives sunlight regularly. During a new moon, for instance, the side that is genuinely lit is precisely the far side we cannot see.
The phase cycle and calendars
The Moon's phase cycle is one of humanity's oldest measures of time; it is no accident that in many languages one word names both the body in the sky and the calendar unit. Twelve synodic months come to about 354 days, which is why calendars based entirely on the Moon are roughly 11 days shorter than the solar year. The Hijri calendar, the lunar calendar of the Islamic world, works in this way, and that is why Ramadan begins about 11 days earlier each year than the year before. The phase cycle also finds echoes in nature: marine biologists have long documented that the mass spawning of some coral species is synchronised with the full moon period. Real and measurable links of this kind should not be confused with the countless unfounded beliefs about the Moon; what separates them is evidence. The coral observations rest on repeatable records, whereas the belief that "things go wrong at full moon" does not.
Reading the phase in the sky
To work out the phase, look at which way the Moon's lit side is facing. If the bright edge points towards the Sun in the west, the Moon is waxing; if it points east before dawn, it is waning. The waxing crescent is visible in the evening and the waning crescent before sunrise, while the full moon rises as the Sun sets and stays in the sky all night. Looking up at the same hour every night for a month is the most enjoyable way to follow this cycle with your own eyes. No special equipment is needed, but binoculars, especially at the quarter phases, bring out the craters along the boundary between light and dark: shadows are longest at that line, so the surface gains the most depth there. The full moon, surprisingly, is the least rewarding time to observe; the light falls straight down, shadows disappear and the lunar surface looks flattened.
Checklist
- Get hold of a monthly phase calendar, or draw one yourself
- For a week, note the Moon's position and shape at the same hour each evening
- Record which way the lit edge is facing
- At first quarter, use binoculars to look at the craters along the shadow line
- Around new moon, try getting away from city lights for some stargazing
Frequently asked questions
Is the dark side of the Moon always dark?
No. The far side we cannot see from Earth also receives sunlight regularly. The phrase "dark side" is a settled misconception; what is meant is the side we do not see.
Why does the full moon sometimes look orange or very large?
Near the horizon the light passes through a thicker layer of atmosphere, so the Moon can look reddish-orange. The impression of size is mostly a perceptual illusion; the Moon's angular size does not measurably grow at the horizon.
What does supermoon mean?
The Moon's orbit is not a perfect circle. If a full moon coincides with the period when the Moon is closest to the Earth, it looks somewhat larger and brighter than average; this is popularly called a supermoon.
Do the phases of the Moon affect human behaviour?
This has been studied a great deal; analyses of large data sets have not shown a consistent, strong effect in any area other than sleep. We take up the subject in our guide to correlation and causation.
This guide draws on NASA's publicly available astronomy education materials and on the content of standard astronomy textbooks.