The Tilt That Hides Eclipses

There is a new moon every month, yet a solar eclipse happens only a few times a year. The reason is a tilt. The Moon's orbit sits about 5 degrees off the plane Earth travels on, so most months its shadow passes above or below us and nothing happens.

The two orbital planes meet along a line, and the Moon's path crosses the Earth's at the two points on that line — the nodes. An eclipse can only happen when a new or full moon lands near one of them, because only there do Sun, Earth and Moon line up closely enough for a shadow to reach. That is why eclipses arrive in seasons rather than monthly.

Solar eclipses come in four kinds: total, annular, partial and hybrid. An annular one happens when the Moon is too far from us to cover the Sun's disc, so a ring stays lit. Lunar eclipses come in three: total, partial and penumbral, and the penumbral kind passes only through Earth's outer shadow.

That outer shadow dims the Moon so slightly that most people looking up cannot tell anything is happening. This is why we do not announce penumbral eclipses in your feed. If the night sky looks ordinary to you, trust your eyes. The event was real, but it was never going to be visible.

An eclipse is not a rare kind of shadow. It is an ordinary shadow that happens to be aimed well. The tilt decides how often one arrives, the Moon's distance decides whether it can cover the Sun completely, and where you happen to be standing decides how much of it you see at all.