An Annular Solar Eclipse: the Basics
Simply put, annular eclipses occur when the new Moon passes across the face of the Sun, but never completely hides the solar disk. At mid-eclipse (annularity), there is always a ring of brilliant sunlight surrounding the Moon. Indeed, the word ‘annular’ derives from medieval Latin ‘annularis,’ which means ‘pertaining to a ring.’
But why is it that when the Moon crosses the middle of the solar disk, it sometimes completely covers the Sun, while at other times it doesn’t? It’s all about changing distances between the Sun, the Moon, and Earth.
The Sun is roughly 400 times farther from Earth than the Moon, and it’s also about 400 times larger than Luna. This means that, on average, the Moon and Sun appear to be almost the same size (apparent diameter) in the sky. But the word ‘average’ is the key. The Earth’s orbit around the Sun is elliptical, as is the Moon’s orbit around Earth. So the distance between these three bodies is constantly changing, which means their apparent diameter is also constantly changing.
Consider the consequences of two orbital extremes. When Earth’s orbit takes it farthest from the Sun, and the Moon’s orbit draws it closest to Earth, the apparent diameter of the Moon is larger than the Sun’s. This means the Moon can completely cover the solar disk and, at mid-eclipse, the result is totality.
On the other hand, when Earth is closest to the Sun and the Moon’s orbit takes it far from Earth, the Moon’s apparent diameter is smaller than the Sun’s. Consequently, the Moon appears too small to completely cover the solar disk and, at mid-eclipse, a brilliant ring (annulus) of sunlight surrounds the Moon. This leads to the nickname “Ring of Fire” for annular eclipses.
The duration of annularity varies tremendously, and it all depends on the locations of the Moon and Earth in their respective orbits. At one extreme, when the apparent diameters of the Moon and Sun are almost equal, annularity can last for a second or less (as it did on May 9, 1948). At the other extreme, with the Moon farthest from Earth and the Sun at its closest, annularity of more than 12 minutes is possible, though this won’t happen again until January 3080.