Lunar Eclipse
A lunar eclipse happens when the full Moon passes through Earth's shadow, dimming it and often turning it a coppery red for up to a few hours.
Definition
A lunar eclipse happens when Earth moves between the Sun and a full moon so Earth's own shadow falls across the Moon, often turning it a deep reddish color for up to a few hours; it can only happen at full moon. In a total eclipse the whole Moon enters Earth's dark inner shadow, the umbra, and glows red because sunlight bent through Earth's atmosphere still reaches it; in a partial eclipse only part of the Moon enters the umbra, and in a penumbral eclipse the Moon only crosses the faint outer shadow and dims slightly, an event visible from Earth's entire night side at once. This requires the full moon to fall near a node of the Moon's orbit, which tilts about $5.1^\circ$ from the ecliptic, and because Earth's umbral cone runs about $1.4$ million km long and spans roughly $2.6$ lunar diameters at the Moon's distance, total lunar eclipses are common and last far longer than solar totality. The eclipsed Moon's brightness, rated on the Danjon scale, depends on how much dust and volcanic aerosol sit in Earth's stratosphere, and lunar and solar eclipses alternate within the same eclipse season, both repeating on the $18$-year, $11$-day saros cycle.
Example
During a total lunar eclipse the Moon can glow deep orange-red, sometimes called a blood moon, and it is perfectly safe to watch with just your eyes for as long as an hour. Earth's umbra at the Moon's distance is about $9{,}000\ \text{km}$ wide, more than twice the Moon's own diameter, so totality can last over $1$ hour $40$ minutes and the whole event from first contact to last can stretch past $5$ hours. Seen from the Moon itself, a total lunar eclipse would look like a total solar eclipse of Earth, with Earth's black disk ringed by a thin red circle of refracted sunlight, and lunar surface temperatures drop by well over $100^\circ\text{C}$ within the hour or so of totality, a change spacecraft use to study the Moon's surface thermal properties.
Key Insight
A lunar eclipse is the one time Earth's shadow genuinely falls on the Moon; the Moon's regular monthly phases are not caused by Earth's shadow at all, only by how much of its sunlit half faces us. Aristotle used lunar eclipses as evidence that Earth is a sphere, since the edge of Earth's shadow on the Moon is always a circular arc no matter the eclipse's angle, and only a sphere casts a circular shadow from every direction. The argument still works today with nothing more than your own eyes and a bit of patience.