Why do we get solar and lunar eclipses?

An eclipse is a shadow event, and the geometry of three bodies does the work. A solar eclipse occurs at new moon, when the Moon passes directly between the Sun and Earth and its shadow sweeps across the planet's surface. Observers inside the dark central shadow, the umbra, briefly see the Sun blotted out, while those in the lighter outer shadow, the penumbra, see only a partial bite. A lunar eclipse occurs at full moon, when Earth sits between the Sun and Moon and the planet's own shadow drapes across the lunar surface. Because Earth is far larger than the Moon, its shadow easily engulfs it, so a lunar eclipse can be watched at once by everyone on the night side of Earth.

If the Moon orbited in the same plane that Earth orbits the Sun, we would get one solar and one lunar eclipse every month. We don't, because the Moon's path is tilted about five degrees. Most months the new or full Moon rides above or below the perfect line, and no eclipse happens. Alignments occur only during two roughly month-long "eclipse seasons" each year, when the crossing points of the two orbits, called the nodes, swing toward the Sun. Total solar eclipses also rely on a striking coincidence: the Sun is about 400 times wider than the Moon but also about 400 times farther away, so the two look almost exactly the same size in our sky. That near-match lets the Moon cover the Sun's disk so precisely.

Because eclipses obey fixed geometry, they can be predicted centuries ahead. Ancient Babylonian astronomers noticed that similar eclipses recur every 18 years and 11 days, a rhythm now called the saros cycle, and modern software pins the timing to the second. Somewhere on Earth there are between two and five solar eclipses a year, but the Moon's umbra traces only a narrow path a few hundred kilometres wide, so any single location averages a total solar eclipse just once in several centuries. That rarity turns eclipses into global events: the total eclipse of April 2024 drew millions of travelers across Mexico, the United States, and Canada. Scientists still use them to study the Sun's faint outer atmosphere, the corona, normally lost in glare.

Myth: eclipses happen every month at new and full moon. Reality: the Moon's tilted orbit means alignments occur only a few times a year, during eclipse seasons. Myth: it is always safe to watch a solar eclipse with the naked eye. Reality: you need certified eclipse glasses for every partial phase; only during the brief total phase, when the Sun's disk is fully hidden, is direct viewing safe. Myth: during a lunar eclipse the Moon simply vanishes into darkness. Reality: it usually glows a coppery red, because sunlight bent through Earth's atmosphere still reaches it, the same scattering that reddens sunsets. Myth: eclipses are rare everywhere. Reality: they are common worldwide but rare at any one spot.