Moon Phases: Crash Course Astronomy #4

The Moon is basically a giant ball of rock 3500 kilometers across hanging in space. Its surface is actually pretty dark, with about the same reflectivity as a chalkboard or asphalt.

However, it looks bright to us because it's sitting in full sunlight; the Sun illuminates it, and it reflects that light down to us here on Earth.

Because the Moon is a sphere, and orbiting the Earth, the way we see it lit by the Sun changes with time. That's what causes its phases: geometry.

Half of the Moon is always illuminated by the Sun, and half faces away. We call the part facing the Sun the daylight or bright side, and the half facing away the night or dark side.

The phase of the Moon refers to what shape the Moon appears to us; how much of it we see illuminated from the Earth. The key to all this is this line, dividing the lit day side from the unlit night side. We call that line the terminator.

The Moon orbits the Earth roughly once per month. In fact, that's where the word 'month' comes from; 'month' and 'Moon' are cognates, words that have similar etymological histories.

New Moon happens when the Sun, Moon, and Earth are all more or less in a line. The Moon's orbit is actually tipped a bit to the Earth's, so sometimes new Moon happens when the Moon is 'below' the Sun, or 'above' it.

At New Moon, the Moon stays near the Sun, so it rises at sunrise, and sets at sunset. This makes it extremely difficult to see; it is, after all, sitting next to the brightest object in the sky.

Seven or so days after new Moon we get to our first milestone: the Moon is now one-quarter of the way around its orbit. It's 90° away from the Sun in the sky, which means we're looking straight down on the terminator, the Moon's day/night line. It cuts right down the middle of the visible face of the Moon, so it's half lit, with the sunward side of the Moon visible and the other side dark.

Our next big step comes two weeks after new Moon, when it's moved halfway through its orbit. It is now opposite the Sun in the sky, 180° around. The Earth is between the Moon and Sun, so we're looking at the fully-illuminated half of the Moon. This is the full Moon.

Three weeks or so after new Moon, and a week after full, the Moon is once again half lit, the terminator splitting the Moon's face in two even halves. This is the 'third quarter' Moon, because the Moon is ¾ of the way through its cycle.

Then, finally, we're back where we started. One month after new Moon, the Moon has traveled 360° around the sky, and is once again as close to the Sun as it can get. It's new Moon, and the cycle starts up again as it has for time immemorial.

An interesting thing happens if you move your perspective from the Earth to the Moon. The phases of the Moon we see from Earth depend on the angle of the Moon and Sun in the sky. But on the Moon, the angles are exactly 180° reversed; at new Moon, when the Moon is between the Earth and Sun, the Earth is opposite the Sun as seen from the Moon. It's full Earth!

Have you ever looked at the thin crescent Moon and seen the ghostly face of the rest of the unlit side? That's because it's not really unlit: the nearly full Earth is reflecting sunlight on the Moon, lighting up the otherwise dark part. The Earth is bigger and more reflective than the Moon, so it's actually 50 times brighter than a full Moon! This glow is called Earthshine.

The Moon is one of the most beautiful and most gratifying objects in the sky to observe. It's different every day! Yet it's also the same, because we see, more or less, the same half of it, the same face all the time.