Europe is about to discover why a 99.5 percent eclipse is nothing like totality.
The final 0.5 percent of today’s solar eclipse will change everything.
Today, August 12, the Moon’s shadow will travel approximately 5,157 miles (8,300 kilometers) across Earth, producing a total solar eclipse along a narrow path through the Northern Hemisphere.
Totality will be visible from parts of northern Russia, Greenland, Iceland, northern Spain, and a small corner of Portugal. Depending on location, the Sun may remain completely covered for up to approximately two minutes and 18 seconds.
Much of Europe, most of Canada, northwestern Africa, and parts of the northern United States will see a partial eclipse.
But a 99.5 percent eclipse is nothing like totality.
On a typical sunny day, daylight measures around 100,000 lux. Even when the Sun is 99 percent covered, illumination can remain near 1,000 lux, comparable to an overcast day.
During totality, it can plunge to approximately 5 lux, similar to civil twilight. Most of that dramatic decline occurs during the final minute, too quickly for the eyes to adjust fully.
The sky will not necessarily become as dark as night. The exact brightness depends on the weather, the width of the Moon’s shadow, and the Sun’s height above the horizon.
Only at 100 percent coverage does the Sun’s corona become visible around the Moon. Totality can also reveal hot-pink solar prominences, bright planets, shadow bands, and sunset-like colors surrounding the horizon.
A tiny sliver of exposed Sun is still bright enough to overwhelm all of those features.
Certified eclipse glasses must remain on whenever any part of the Sun is visible. Only people inside the path of totality may remove them briefly after the Sun is completely covered.
Anyone experiencing a partial eclipse must keep their glasses on throughout the event. Regular sunglasses are not safe.