Solar eclipse: how it happens
The solar eclipse that plunged Asia into darkness for over six minutes this morning brought stargazers flocking from around the world. Here's how it happened.
By Heidi Blake
Published: 12
0PM BST 21 Jul 2009
During an eclipse, only the solar corona is visible: a much fainter white ring 600,000 miles from the Sun's surface Photo: GETTY
A total solar eclipse occurs when the Moon passes between the Earth and the Sun during New Moon, replacing the intensely bright solar disk with a lunar silhouette. Though New Moon occurs every 29 and a half days, eclipses only occur when the angle of the Moon is such that it obscures part or all of the Sun.
This morning's eclipse was part of series 136 in the Saros cycle, which governs the recurrence of eclipses over periods of around 6,585 days.
Only the solar corona was visible during the eclipse: a much fainter white ring 600,000 miles from the Sun's surface.
Between two and five solar eclipses occur each year across the world, with each existing only along a narrow corridor in the relatively small area of the Moon's shadow.
Although they occur somewhere on Earth around every 18 months, it has been estimated that total eclipses recur at any given place only once every 370 years on average.
During this morning's eclipse, the Moon's shadow moved from west to east across the Earth at over 1500mph.
Its exceptional duration of six minutes and 39 seconds at its maximum point was a result of the Moon being near perigee – its closest position to the earth.
It is impossible for an eclipse to last more than seven minutes and 40 seconds at its maximum point, and is usually much shorter: during each millennium there are typically fewer than 10 total solar eclipses exceeding seven minutes. The last was in 1973.
The longest total solar eclipse during the 8,000-year period from 3000BC to 5000AD will occur on July 16, 2186, when "totality" will last seven minutes and 29 seconds.
Due to tidal acceleration, the orbit of the Moon around the Earth becomes approximately 3.8cm more distant each year. It is estimated that in 600 million years, the distance from the Earth to the Moon will have increased by 23,500km, meaning that total eclipses will no longer be possible.