Why the Moon has phases at all
The Moon is always half lit by the Sun — half of it, permanently, on the side facing the Sun. What changes is how much of that lit half faces Earth.
At new Moon the Moon sits between Earth and the Sun, so the lit half points away from us and we see nothing illuminated. At full Moon it is on the far side of Earth, the lit half faces us, and the whole disc is visible. First and last quarter happen when the Moon is 90° from the Sun-Earth line, giving the half-lit appearance.
Two things are worth being clear about. First, the phases are caused by geometry, not by Earth's shadow — an eclipse is a different mechanism entirely and requires the Moon to cross the plane of Earth's orbit. Second, the "dark" part of the Moon is not dark; it receives sunlight and is visible to spacecraft, which is why lunar landers photograph equipment that is on the far side of the Moon's own day.
The 29.53-day cycle, and why it is not 27.3
The Moon orbits Earth every 27.3 days (sidereal month), but the phase cycle — the synodic month of 29.53 days — is longer because Earth has moved about 27° along its own orbit in that time. The Moon has to travel that extra angle to return to the same Sun-Earth-Moon alignment.
This has three practical consequences:
- Phase age is independent of the date. New Moon falls on different calendar dates each year, drifting by about 11 days.
- Different months differ in length by up to about 1.5 days, because the Moon's orbit is elliptical and the speed varies.
- You can only see one phase at a specific time of day. First Quarter is highest at sunset, full Moon rises at sunset, and last Quarter is highest at sunrise. A photographer who forgets this is shooting the wrong face of the Moon.
How to read the illumination figure
The percentage of the disc that appears lit is (1 − cos 2πt) ÷ 2 where t is the fraction of the cycle elapsed. Two things surprise people:
- It is not linear in age. At 7 days (first quarter) the disc is 50% lit, but at 3 days it is already about 13% and at 10 days about 74%. The lit area grows slowly, then fast, then slowly again — the opposite of intuition.
- Waxing and waning both hit 50%, and only the direction of the terminator distinguishes first quarter from last. This is where a mental picture helps more than a number: waxing means the lit side is on the right in the northern hemisphere, waning on the left.
Why the phase matters astronomically
For a beginner, the new Moon is the best night sky of the month: no lunar glare, faint galaxies and the Milky Way are all visible. For a farmer or an astronomer, the opposite holds — the full Moon rises at sunset, stays up all night, and the Moon is opposite the Sun all night, making it the worst time for faint deep-sky targets. Neither of these is superstition; it is entirely a question of how much sunlight is scattered off a nearby surface.