Why Does the Moon Have Phases?

Sunlight falls on the Moon from one direction, so one half is always in daylight and the other in night. That never changes. What changes is our viewing angle.

When the Moon is roughly between Earth and the Sun, the lit half faces away from us and we see a new moon. When Earth is between the Sun and the Moon, the lit half faces us and we see a full moon. Every phase in between is a partial view of that same lit hemisphere.

The full cycle takes about 29.5 days and runs through eight named stages:

• •New moon, effectively invisible
• •Waxing crescent, a thin sliver appearing after sunset
• •First quarter, half lit and visible in the afternoon and evening
• •Waxing gibbous, more than half lit
• •Full moon, rising as the Sun sets
• •Waning gibbous, then last quarter, then waning crescent

Waxing means the lit portion is growing, waning means it is shrinking. The terms are old but still standard, and they are easier to remember once you connect waxing with adding, as in a candle building up wax.

The quarter names confuse people because the Moon looks half lit. They refer to the Moon being a quarter of the way through its cycle, not to how much of the disc is illuminated.

If phases were caused by Earth's shadow, there would be a lunar eclipse every month. There is not, because the Moon's orbit is tilted about five degrees relative to Earth's orbit around the Sun.

Most months the Moon passes above or below the shadow. Only when the alignment is close enough does an eclipse happen, which is why eclipses are events rather than monthly occurrences. Understanding this makes both phases and eclipses much clearer.

One more common mix up. The Moon is tidally locked, so the same face always points towards Earth, and there is a far side we never see from the ground.

But that far side is not permanently dark. It receives just as much sunlight as the near side, simply on a different schedule. During a new moon, when the near side is dark to us, the far side is fully lit.

Look at a thin crescent moon shortly after sunset and you can often see the rest of the disc faintly lit in a dim grey. This is earthshine, sunlight reflected off Earth onto the lunar surface and back again.

Leonardo da Vinci worked out the explanation in the early sixteenth century, correctly reasoning that Earth reflects light onto the Moon just as the Moon reflects light onto Earth. The traditional description of the effect as the old moon in the new moon's arms predates the explanation by a long way, which is a nice example of an accurate observation waiting for its physics.

Phases and moonrise times are linked. The Moon travels around Earth in the same direction that Earth spins, covering about thirteen degrees of its orbit each day. Earth therefore has to turn a little further each day before the Moon comes back into view, so moonrise happens on average around fifty minutes later than the day before. Over a month, that delay carries the Moon all the way round the clock.

This gives you a way to predict when to look. A full moon sits opposite the Sun, so it rises around sunset and sets around sunrise. A first quarter moon rises around midday and is high in the sky at dusk. A last quarter moon rises around midnight and is still visible in the morning sky after sunrise. Seeing the Moon in daylight is completely normal, and the phase tells you roughly where to find it.

The Moon completes one orbit relative to the distant stars in about 27.3 days, yet the phases repeat every 29.5 days. The difference comes from Earth's own motion. During that month Earth travels roughly a twelfth of the way around the Sun, so the direction of sunlight shifts. After one full orbit, the Moon must travel a couple of extra days to line up with the Sun again in the same way. The 27.3 day period is called the sidereal month, and the 29.5 day phase cycle is the synodic month.

Where you stand on Earth also changes how the phases look. In the Northern Hemisphere a waxing moon is lit on its right side. Observers in Australia or South Africa face north to see the Moon, so the same waxing moon appears lit on the left, and memory tricks based on letter shapes work backwards there. Near the equator, a crescent can lie almost flat, like a smile. In every case one rule holds: the lit edge always points towards the Sun, which helps you tell waxing from waning anywhere.

The phase cycle gave many cultures their first unit of time longer than a day, and the word month itself is related to moon. The Islamic calendar still follows the Moon closely. Each month traditionally begins when the thin crescent is first sighted after sunset following the new moon, and twelve lunar months add up to about 354 days. That is roughly eleven days shorter than a solar year, so Ramadan arrives about eleven days earlier each year and drifts through every season over a cycle of around 33 years.

Other calendars try to keep lunar months in step with the seasons, which is awkward because a year does not contain a whole number of phase cycles. The Greek astronomer Meton noticed in the fifth century BCE that 19 solar years almost exactly equal 235 lunar months, differing by only a couple of hours. The Hebrew calendar uses this cycle, adding an extra month in seven years out of every nineteen so that festivals such as Passover stay in spring. The date of Easter, set as the first Sunday after a calculated spring full moon, relies on the same 19-year pattern.

The average fifty-minute daily delay in moonrise hides large seasonal swings. Around the September equinox, the full moon's path meets the eastern horizon at a shallow angle in the evening, so at mid-northern latitudes it rises only around twenty to thirty minutes later each night for several nights in a row. Bright moonlight therefore follows sunset evening after evening, which once helped farmers bring in crops after dark. The full moon nearest the autumn equinox is called the harvest moon for this reason. In spring the angle is steep, and the daily delay can stretch beyond an hour.

A blue moon is also a product of the 29.5 day cycle. Because most calendar months are longer than one phase cycle, a month that begins with a full moon can end with another, and that second full moon is popularly called a blue moon. It happens roughly every two to three years, and never in February. This definition came from a 1946 magazine article that misread an older almanac rule, which called a blue moon the third of four full moons in a season instead of the usual three. Neither kind looks blue. A genuinely bluish moon needs smoke or volcanic ash in the air.

The moon has phases because we see different amounts of its permanently sunlit half as it orbits Earth, and confusing this with Earth's shadow is what makes eclipses seem mysterious rather than the rarer alignments they actually are.