Every New Moon puts the Moon roughly between Earth and the Sun. Every Full Moon puts Earth roughly between the Sun and the Moon. If the Moon's orbit sat exactly in Earth's orbital plane (the ecliptic), that "roughly" would be "exactly," and we'd get a solar and a lunar eclipse every single month. We don't — because the Moon's orbit is tilted about 5° relative to the ecliptic. Picture two hula hoops sharing a center but tilted a little relative to each other: most of the time, one hoop is above or below the other, and they only cross at two points.
Those two crossing points are called nodes. An eclipse can only happen when a New or Full Moon lines up with being near one of those nodes — a window called eclipse season, which rolls around roughly twice a year. Most New and Full Moons miss the alignment entirely, with the Moon passing a little too high or too low relative to the Sun-Earth line to cast or catch a shadow.
The two eclipse types are mirror images of each other. A solar eclipse happens at New Moon: the Moon passes directly between the Sun and Earth, and the Moon's shadow falls on Earth's surface. A lunar eclipse happens at Full Moon: Earth passes directly between the Sun and the Moon, and Earth's shadow falls on the Moon.
Any object blocking sunlight casts a shadow with two parts. The umbra is the region of complete shadow — from inside it, the Sun is fully blocked. The penumbra is the surrounding region of partial shadow — from inside it, part of the Sun still peeks around the edge of whatever's blocking it.
| You're in the... | Solar eclipse | Lunar eclipse |
|---|---|---|
| Umbra | Total solar eclipse | Total lunar eclipse |
| Penumbra | Partial solar eclipse | Partial lunar eclipse |
You're standing in the Moon's umbra during a New Moon. What are you witnessing?
Earth is much bigger than the Moon, so Earth's umbra is much bigger than the Moon's. During a lunar eclipse, the Moon can sit fully inside that large umbra for over an hour, and anyone on Earth's night side can watch it happen — a lunar eclipse is a whole-hemisphere event.
A solar eclipse works the opposite way. The Moon's umbra is small by the time it reaches Earth's surface — often just a strip a hundred miles or so wide — and it sweeps across the globe at roughly 1,500 km/hour as Earth rotates and the Moon moves. Only people standing in that narrow, fast-moving strip ever see a total solar eclipse; everyone else sees a partial eclipse or nothing at all.
Both conditions have to line up: the Moon has to be New or Full, and it has to be eclipse season (the nodes pointed toward the Sun). Try every combination.