Why Does the Moon Have Phases?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The phases are not caused by Earth casting a shadow on the Moon, which is the most common wrong answer. The Moon is always half lit by the Sun. What changes is how much of that lit half we can see from Earth as the Moon travels around us.
Always half lit
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 cycle in order
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, waning and quarters
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.
Why phases are not eclipses
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.
The far side is not the dark side
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.
Earthshine and the old moon
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.
The takeaway
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.