Ken Tapping, June 5, 2013
On 10th May those lucky enough to be in Northern Australia or in the Southern Pacific Ocean got a treat. They saw the Moon pass in front of the Sun, covering most of the solar disc, except for the outer edge, forming for a minute or so a fiery ring in the sky. These events are known as “annular eclipses”. Annulus is Latin for “ring”.
We on Earth enjoy something unique in the Solar System and maybe even in the Universe. In our skies the Moon looks the same size as the Sun. Of course the Sun is enormously larger than the Moon, but it is by some strange cosmic coincidence proportionally further away. Also, because the orbits of the Earth around the Sun and the Moon around the Earth lie almost in the same plane, the Moon sometimes moves between us and the Sun, blocking out it out, giving us an “eclipse of the Sun”. When the bright, solar disc is covered, we get to see parts of the Sun usually lost in the glare. We see prominences – great arches of glowing gas supported in magnetic fields, and the pearly streamers of the solar corona, gas so hot, millions of degrees, that it emits little light. Astronomers flock to eclipses to make observations that are impossible at any other time.
There are two issues that make these events rarer. Firstly, the Moon’s path around the Earth is elliptical, not circular, so its distance from us varies. When the Moon is at its closest it blocks out the solar disc and some of the surrounding sky, so we get longer eclipses but might not see the solar structures at the edge of the bright disc quite as well. However, when the Moon is at its farthest point from us it is too small in our sky to completely cover the solar disc, so we don’t get to see those interesting structures in the solar atmosphere. What we see then is fascinating but not as scientifically useful. The middle part of the solar disc is covered, leaving the edge, as that glowing ring those lucky enough to be in the right place saw on 10th May.
This brings us to the other downside of our solar eclipses. Because the Moon looks almost exactly the same size as the Sun in our sky, there is only a tiny bit of the Earth’s surface where we see the Sun completely covered. Because the Moon is moving in its orbit, this tiny spot of shadow moves across the Earth’s surface forming the eclipse track. Observers go to points on the track, set up their instruments, and wait for the Moon’s shadow to pass over them. This will take around two minutes.
Solar eclipses happen when the Moon passes between the Sun and Earth. Another possibility is for the Earth to pass between the Moon and Sun. When this happens, the Earth’s shadow falls on the Moon. However, the Earth is nearly four times the diameter of the Moon, so we look much larger in the Moon’s sky than it looks in ours. Our shadow is much larger, so the Moon takes much longer to pass through it: hours rather than minutes. We would expect this to render the Moon completely dark, but our atmosphere bends sunlight around the curve of the Earth and onto the Moon. If our atmosphere is clean, the Moon looks a lovely copper colour; on the other hand, if it is particularly polluted, the Moon will look a dull, ashy, grey.
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