The umbra is the dark inner core of a shadow, where the light source is blocked completely.
Not attributable — a direct Latin borrowing, in English from the 1590s for a phantom or shade, and in the eclipse sense by the 1670s.
// Where it sits
Narrows with distance, which is why totality is a track rather than a region.
What it really means
The umbra is the part of a shadow where the light source is gone completely — not dimmed, blocked. Stand inside the Moon's umbra and the Sun is entirely covered: that's a total eclipse, and there is nowhere else you can experience one.
The geometry explains why it's such a rare thing to be under. The umbra is a cone, and it narrows as it extends away from the Moon. By the time it reaches Earth — a quarter of a million miles later — it has tapered to a patch covering a relatively small area, which is why totality arrives as a track a few dozen miles wide rather than a region. Everyone else, across most of a continent, is standing in the penumbra watching a partial eclipse and wondering what the fuss is about.
And the cone doesn't always arrive at all. The Moon's orbit is tilted about five degrees to Earth's orbital plane, so at most new moons the umbra passes above or below us and nothing happens. That tilt is the main reason we aren't having a total eclipse somewhere every month — and even when the alignment holds, the Moon is sometimes far enough away that the cone falls short and leaves a ring instead.
One pleasure in the etymology. Latin umbra is simply shadow — and it has been unusually productive. It gives penumbra (almost-shadow), antumbra, and two words nobody connects to eclipses: umbrella, literally little shade, originally carried against the sun rather than rain; and umbrage, which travelled from shadow to suspicion to taking offence.
Where it comes from
One root generates a whole technical family — penumbra ('almost-shadow'), antumbra — plus the everyday descendants umbrella (literally 'little shade', originally for sun) and umbrage (shadow, then suspicion, then offence).
Myths & misconceptions
The Moon's shadow is a big dark cone that easily covers Earth.
It barely reaches. The umbra narrows with distance, and by the time it arrives it covers a relatively small area — which is why totality is confined to a narrow track while most of a continent sees only a partial eclipse from the penumbra.
A total solar eclipse should happen every new moon.
The Moon's orbit is tilted about five degrees to Earth's orbital plane, so the umbra usually misses Earth entirely. The alignment has to be near-exact for the umbral cone to make contact at all.
Compare & contrast
Umbra vs antumbra
The antumbra begins where the umbra's cone tapers to a point and ends. Inside the umbra the Moon covers the Sun entirely, giving a total eclipse; inside the antumbra the Moon is too small to cover it, so a ring remains. Same geometry, one distance apart.
| // | Umbra | Antumbra |
|---|---|---|
| Sun is | Fully covered | Ringed |
| Eclipse type | Total | Annular |
| Position | Before the cone's tip | Beyond it |
| Moon appears | Larger than Sun | Smaller |
How it connects
Tell it apart
Questions people ask
- What is the umbra?
- The darkest inner part of a shadow, where the light source is completely blocked. Observers inside the Moon's umbra see a total solar eclipse — the Sun entirely covered.
- Why is the path of totality so narrow?
- Because the umbra narrows as it extends away from the Moon. By the time it reaches Earth it covers only a relatively small area, so totality is a track a few dozen miles wide while the surrounding penumbra covers most of a continent.
- Why aren't there eclipses every month?
- The Moon's orbit is tilted by about five degrees relative to Earth's orbital plane, so on most new moons the shadow passes above or below us entirely. The alignment has to be nearly exact for the umbral cone to touch Earth at all.