What Is a Lunar Eclipse? The Moon Lit by Every Sunrise at Once

The Earth's shadow has two parts, a central umbra where the sun is completely blocked and a surrounding penumbra where it is partly blocked. A total eclipse occurs when the moon passes entirely into the umbra, and geometrically it should then receive no direct sunlight at all. It remains visible because the Earth has an atmosphere, which refracts sunlight around the edge of the planet and into the shadow. That light has travelled a long path through the atmosphere, which scatters short wavelengths away strongly and transmits long ones, which is the same process that makes a sunset red, so what reaches the moon is overwhelmingly red light. Standing on the moon during the event, an observer would see the Earth as a black disc surrounded by a thin ring of red, and that ring is the sum of every sunrise and sunset occurring around the planet simultaneously. The depth of the colour varies with the state of the atmosphere, and eclipses following large volcanic eruptions are noticeably darker because the extra dust and aerosol block more light.

Several distinct events are grouped under the name and they look quite different:

• •Total, where the whole moon enters the umbra and turns red, which is the spectacular case
• •Partial, where only part of the moon enters the umbra, producing a visible dark bite whose curved edge is direct evidence that the Earth is round
• •Penumbral, where the moon passes only through the outer shadow, producing a subtle dimming that many observers cannot detect at all
• •Variation in colour and brightness between total eclipses, graded on a five-point scale reflecting atmospheric conditions
• •Duration varying with how centrally the moon crosses the shadow, with the longest totalities approaching an hour and three quarters
• •Visibility from an entire hemisphere at once, which is why lunar eclipses are far more commonly seen than solar ones despite occurring less frequently

The moon passes between the Earth and sun, and the Earth passes between sun and moon, once each per orbit, so eclipses ought to occur every month. They do not because the moon's orbit is tilted by about five degrees relative to the Earth's orbit around the sun, so at most full moons the moon passes above or below the shadow. Eclipses occur only when a full moon coincides with the moon crossing the plane of the Earth's orbit, at points called nodes, and those alignments come around roughly twice a year in periods called eclipse seasons. The pattern of eclipses repeats on a cycle of about eighteen years and eleven days, known since antiquity as the saros, because after that interval the geometry very nearly returns to the same configuration. That cycle allowed Babylonian astronomers to predict eclipses long before anyone understood the mechanism, which is a striking example of prediction without explanation.

Before the mechanism was understood, eclipses were interpreted across many cultures as attacks on the moon, as omens concerning rulers, or as events requiring ritual response, and several traditions record noise-making intended to drive off whatever was consuming it. Once understood, they became useful. The curved edge of the Earth's shadow on the moon was one of the arguments Aristotle gave for a spherical Earth, and it remains among the most direct pieces of everyday evidence for it. The fact that the shadow is always larger than the moon gave ancient astronomers a way to estimate relative sizes and distances, which Aristarchus attempted in the third century before the common era. Because an eclipse is seen simultaneously across a hemisphere, it provided a shared time signal that allowed longitude differences between distant places to be estimated by comparing local times of the event, which was one of the few methods available before accurate clocks. Historical records of eclipses are still used to measure the long-term slowing of the Earth's rotation.

Sunlight refracted through the Earth's atmosphere reaches the moon inside the shadow, and the long atmospheric path strips out short wavelengths, so what arrives is red, and an observer on the moon would see a black Earth ringed by every sunrise at once. A five degree orbital tilt is why this is not monthly. The curved shadow edge is direct evidence the Earth is round, and eclipses supplied a shared time signal for longitude.