Wednesday's total eclipse of the Moon may be the most striking for years but observers in Britain must be content with a view of only the closing act of the drama. The Moon stands over the southern Indian Ocean as it passes through the central dark umbra of the Earth's shadow, plunging deeper into the shadow than during any eclipse since 2000. This may well result in an unusually dark eclipse, with the Moon's disc turning a deep reddish-brown as all direct sunlight is blocked.
In fact, the umbra is never black. A little light must reach the Moon from the parts of the Sun's outer atmosphere, the corona, that are not hidden by the Earth. But more sunlight is refracted and scattered around the edge of the Earth by our planet's atmosphere. Just as sunsets and sunrises appear orange or red, so this light is predominantly red.
The umbra is not illuminated evenly, though, since less of this indirect light penetrates to its core and the amount reaching different zones of the umbra is affected by varying atmospheric conditions. In particular, dust thrown up by major volcanic eruptions can render the atmosphere less transparent and the eclipsed Moon so dark that it practically disappears.
