As the Moon continues to move in its orbit, it moves into a position in which it makes a right angle with the earth-sun line. This causes half of the Moon to be lit.
When my old friend Kim Long revised his classic, The Moon Book, I helped give it the once-over. As part of my process, the question came up for me about the names of moon phases.
New moon, of course, refers to the fact that this phase the moon is considered by astronomers to have entered a new orbital cycle (called a lunation by astronomers). Full moon signifies that the full visible surface of the moon is illuminated. First quarter moon means the moon is one quarter of the way through the current orbital cycle. Third or last quarter moon means the moon is three-quarters of the way through the cycle, as measured from one new moon to the next.
In between we have crescent moons, clearly named for their shape, which can be waxing (growing) or waning (diminishing). And we also have gibbous moons, indicating a shape that is unequally curved outward on both sides, but not full. Gibbous moons also wax and wane depending on its time in the cycle. Learn much more about the various phases of the moon at EarthSky’s article Four keys to understanding moon phases.
But just realize this. Astronomers recognize four primary moon phases (new, first quarter, full, last quarter) and four interstitial phases (waxing crescent, waxing gibbous, waning gibbous waning crescent). Notice … no half moon.
People frequently speak of half moons. There are geographical locations (e.g., Half Moon Bay in California), as well as restaurants, resorts and various other attractions and businesses with this moniker.
People also sometimes look up and say:
Is that a ‘half’ moon in the sky?
Astronomically, the answer to that question is always no. There are no half moons.
The term simply is not in the vocabulary of astronomers, at least not in any official way. Invariably, when referring to the half moon, observers really are looking at one of the quarter moon phases. The illuminated portion you see really is a quarter of the whole moon.
So it seems reasonable. And, meanwhile, it’s astronomically incorrect to a refer to a half moon.
The process of naming things in astronomy isn’t always logical, except perhaps in a very narrow sense. You may think that heavy metal refers to gold or lead, or your favorite band. But, in astronomy, a metal is means any element – even some that normally are gases on Earth – that’s anything other than the two lightest elements, hydrogen and helium. Oxygen and carbon, for example, are metals in the language of astronomers. Seriously.
Or consider the ad hoc definition of a dwarf planet adopted by the International Astronomical Union, and used to dump Pluto, unceremoniously, from full planethood. Astronomers are scientists, and, like all scientists, they use a jargon all their own.
In a couple of senses, though, every time you view the moon, you’re observing a half moon. First, just half the moon always faces us. You can’t see the back side since it’s always turned away from us.
Second, whether you can see it all or not, one half of the entire moon is always illuminated by the sun. In other words, the moon has a dayside, just as Earth does. We see this illuminated half of the moon – the entire dayside of the moon – only at full moon.