An annular eclipse. Image credit: SmrgeogOn 1st September 2016, a spectacular "ring of fire" appeared in the sky over parts of Africa, as the Moon almost—but not completely—blocked out the Sun. This was what is known as an annular eclipse of the Sun. It happens when the Moon lies farther away from Earth than during a total eclipse. In an annular eclipse, we see a bright circle of sunshine surrounding the black disc of the Moon. The best views of the eclipse were seen in Tanzania, where the event lasted about three minutes. It could also be seen in Gabon, Congo-Brazzaville, Democratic Republic of Congo, Madagascar and the French Indian Ocean island of Réunion.
A "ring of fire". Image credit: Kevin Baird
The Moon's shadow passing across AfricaThe Moon does not orbit Earth in a perfect circle—its orbit is slightly elliptical (oval-shaped). This means that its distance from Earth varies from between around 360,000 and 400,000 kilometres (225,000–250,000 miles). When the Moon is farther away, the Moon's diameter looks to viewers on Earth a little smaller than the Sun's, causing the Sun behind it to look like an annulus (the word used in maths for a ring).
September's annular eclipse appeared to viewers along a narrow strip of Africa (traced by a red spot in this animation) as it travelled across the sky. At the same time, it appeared as a partial eclipse over a much larger region (the grey area).
Diagram of partial and total eclipses. image credit: NASAThis diagram shows the Moon passing in front of the Sun to bring about a total eclipse. In a partial eclipse, part of the Sun still remains visible, and the light dims only a little for the duration of the eclipse. If the Moon were on exactly the same orbital plane as Earth, there would be a total eclipse every month. But because the Moon's orbit is slightly tilted relative to Earth's, its shadow at New Moon usually misses our planet. One or two total eclipses viewable from somewhere on Earth occur each year.
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