What Is the Penumbra

If you are standing in the Moon’s penumbra and look at the Sun, you will witness a partial solar eclipse.

During some eclipses, the Sun, the Moon, and the Earth do not form a perfectly straight line, so only the penumbra falls on the Earth’s surface while the umbra, the shadow’s dark center, is cast into space. In that case, the eclipse is classified as a partial solar eclipse, and there are no places on Earth where the Moon obscures the Sun completely.

However, even a total solar eclipse can look like a partial solar eclipse in some locations. The total phase of the eclipse can only be seen from within the Moon’s umbra, which only covers a small area on the Earth’s surface. Since the Moon’s umbra is always surrounded by a penumbra, the eclipse appears partial if you are just outside the spot where the umbra hits. For example, the total solar eclipse in August 2017, also known as the Great American Eclipse, appeared as a partial eclipse in most areas in the US and Canada.

Within the Moon’s penumbra, only part of the Sun’s disk is obscured. This means that the penumbra has a considerably lighter shade than the umbra, where the Sun is completely covered. However, the penumbral shadow’s light level is not uniform. It is much darker in the areas adjacent to the umbra than at the edges.

This variation is caused by the changing magnitude of the eclipse in different areas within the penumbra. In the penumbra’s most central regions, just outside the umbra, the Sun appears nearly completely obscured. This means that only a small portion of the Sun’s rays reach us, causing a dark penumbral shadow. On the other hand, in the outskirts of the areas covered by the penumbra, the Moon barely touches the Sun’s disk. This means that much more sunlight passes through, making for a very light, and sometimes unnoticeable, shadow.

The reason why the penumbra is so much larger has to do with the distance of the Moon from Earth and the fact that the Moon is smaller than the Sun. With growing distance from the Moon, the diameter of its umbra decreases as the Moon appears to diminish in size. This results in a roughly V-shaped umbra (see illustration).

Imagine the distance you would have to travel in a spaceship in lunar orbit to reach the umbra’s edge from its center – it would be a journey spanning thousands of miles. In contrast, you typically only have to move about 100 km (60 mi) across the Earth’s surface to reach penumbral territory from the umbra’s center.

In contrast, the area where the Sun appears partly eclipsed increases in size as you move away from the Moon. From the spaceship hovering just a few miles above the lunar surface, the Moon appears much larger than the Sun, so you would have to travel a relatively short distance to cross the penumbral area, where the Sun appears partly eclipsed. On Earth, however, where the Sun and the Moon look as if they were roughly the same size, that journey would be much longer.

During most eclipses, the region covered by the penumbra is even further enlarged by the fact that in most locations the Sun does not appear in the zenith position (straight above), so its rays hit the Earth’s surface at a shallower angle.

In contrast, the main phase of the transit, which takes us through the planet’s antumbra as it passes in front of the Sun, takes several hours in most cases. The antumbra’s diameter increases with growing distance from the object casting the shadow, so it is very wide when it reaches Earth.

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