Lunar eclipses

At the Moon's distance Earth's dark shadow is about 9,200 km wide, more than two and a half Moons.

Earth's shadow on the Moon

A lunar eclipse happens when Earth sits exactly between the Sun and the Moon and Earth's shadow falls on the Moon. That arrangement is only possible at full Moon, since that is the only time the Moon is on the far side of Earth from the Sun. Because the Moon is lit by sunlight and we only need to see the Moon, a lunar eclipse can be watched from the whole night side of Earth at once. NASA puts it simply: each lunar eclipse is visible from half of Earth.

Earth's shadow has two parts. The umbra is the dark inner cone where the whole Sun is hidden. Around it lies the penumbra, where Earth hides only part of the Sun. Their sizes follow from geometry. Earth is 12,742 km across, but the Sun is much bigger than Earth, so the umbra narrows as it stretches away. At the Moon's average distance the narrowing is about 3,545 km, leaving an umbra roughly 9,200 km wide. The Moon is 3,475 km across, so the umbra is about 2.6 Moons wide. That is why the total part of an eclipse can last more than an hour and a half.

NASA lists three kinds of lunar eclipse. In a total eclipse the whole Moon passes into the umbra. In a partial eclipse only part of it does, and a curved dark bite grows and shrinks on the face without covering it all. In a penumbral eclipse the Moon passes only through the penumbra; it dims so slightly that, in NASA's words, you might miss it if you did not know it was happening. The model sorts eclipses the same way using the true angular sizes: the Moon is about 0.52° wide, the umbra about 1.4° and the penumbra about 2.5°.

Why a totally eclipsed Moon turns red

Even in the umbra the Moon rarely goes black. Some sunlight passes through the thin ring of Earth's atmosphere at the edge of the planet and bends into the shadow. On the way, the air scatters blue light much more than red, the same effect that makes our sky blue and our sunsets red. What reaches the Moon is the leftover orange and red. NASA describes it as all the world's sunrises and sunsets projected onto the Moon. The more dust or cloud there is in Earth's atmosphere, the darker and redder the Moon looks.

Why not every month

If the Moon's orbit lay flat in the same plane as Earth's orbit, every full Moon would be eclipsed. It is tilted by 5.145°, so most full Moons pass above or below Earth's shadow. An eclipse needs a full Moon close to one of the two nodes, the points where the Moon's orbit crosses the plane of Earth's orbit. Twice a year the line through the nodes points roughly at the Sun; these stretches are called eclipse seasons. Using the true sizes, an umbral eclipse needs the full Moon within about 11° of a node. In the model, keep the day at full Moon and move the Node line slider to see where eclipses stop.