Solar eclipses, 28 October 2013!The reason the Moon can exactly cover up the sun during an eclipse is a lucky 1.accident of geometry: !Though the Sun in 400 times bigger than the Moon, it is also 400 times A.farther away. !They look the same size as each other in the sky. !a.On all the other objects with moons in the solar system, the moon is either B.too small to cover up the sun or so big that it conceals the sun's chromosphere and corona. !During an eclipse, the sun's rays are blocked by the moon, which casts an 2.outer shadow and an inner shadow:!The inner shadow is a cone whose point just barley touches earth. !A.If the point of the inner shadow falls on earth and you are in it, you see a a.total eclipse.!The sky must also be clear. !1.If the tip of the shadow does not quite reach earth, you see a ring of b.sunlight around the moon– an annular eclipse. !A pinpoint annular eclipse is an annular eclipse where you never see a 1.complete ring of sunlight around the moon. This is because the Moon's mountains selectively block the light. !Which mountains block the light constantly changes because the A.Moon and Sun are moving throughout the eclipse. !If the tip of the cone doesn't touch earth, all you see if part of the moon c.covering part of the sun.!This is a curiosity, but not as impressive as a total eclipse. !1.Total eclipses of the sun occur in a range of places, and each follows an arc 3.across the Earth. !There are several eclipses each decade which are worth going to. !A.The others are in places which are hard to get to or have a low chance a.of clear skies on the day of the eclipse.!For example, Antarctica, the Arctic, or the middle of the Pacific ocean. !1.Clouds are a big problem even for more accessible areas: !2.Sometimes the sky is just cloudy. !A.Other times, the amount of light energy the eclipse blocks is B.enough to cause vapor clouds to form about 10 minutes before totality and disappear 10 minutes after totality. !This effect can be minimized by viewing the eclipse over a body a.of water, which stabilizes temperatures. !This heat sink effect is the reason why places near the coast 1.like California have such temperate weather. !To determine the likelihood of clear skies on the day of an eclipse, C.you can consult a climatologist.!Climatologists specialize in predicting the likelihood of weather a.decades in the future.!The next solar eclipse worth going to is in 2017. !b.It enters the United States at Oregon, goes over the great plains, and 1.exits at Georgia. !The best place to see this eclipse will be in the deserts of eastern A.Oregon. !The stages of an Eclipse: !4.At the very beginning of the eclipse, the moon slowly passes in front of the A.edge of the sun. This can only be seen by looking at the sun with special filters. The world is not yet noticeably darker. !An hour into the partial part of the eclipse, the moon covers a substantial B.part of the sun. The shape of the sun is different. !Three minutes before totality, there is only a small sliver of sun light. The C.light gets much dimmer, and becomes orange in color. !The sky becomes orange because the only light reaching Earth comes a.from the edge of the sun. This light must go through a lot of solar atmosphere which scatters out the blue light. !A similar effect happens at sunset. !1. If you look down at the ground, you can see that some shadows are b.fuzzy and others are sharp: !Shadows are normally fuzzy because light from a large disk (the sun) 1.can come from different angles. Objects cannot block out light coming from all of these angles. !The sharp shadows are due to part of the remaining sliver of the sun 2.acting like a point source of light. !It is standard procedure to take pictures of these different shadows 3.during an eclipse. !In the last two or three seconds before totality, you can see the corona of D.the Sun plus a small bit of the Sun's photosphere showing at the edge of the moon.!This is called the diamond ring effect (because it resembles a diamond a.ring). !Normally, when you look at the sky at this point you see nothing. !b.In the 1979 eclipse, there was a thin cirrus cloud covering the sky. This 1.scattered the light of the sun's photosphere into a halo, allowingastronomers to see the moon's shadow (which was the areas without the halo). !It was also possible to see how orange the light of the sun looked A.in the distance. !When the sun is completely covered by the moon you can see the corona, E.the chromosphere, and sometimes prominences. This totality lasts for at least two minutes and as long as seven minutes. !The corona has a different shape at every eclipse. !a.Coronal mass ejections (bursts of solar wind and magnetic fields in 1.the corona) are sometimes seen. !The chromosphere is a shocking pink from hydrogen under the extreme b.conditions near the sun. !Prominences vary: they are sometimes stationary, sometimes moving, c.and sometimes they follow magnetic lines.!Prominences are bits of the photosphere in the Sun's corona. !1.Based on how large they are relative to the sun's disk, they are about 2.108 times the size of the Earth. !Prominences can also be seen with special devices like a 3.spectrohelioscope on a spacecraft. !With our current technology, photographs of the corona and the d.chromosphere cannot replicate how they appear during an eclipse. !The corona has a pearly color that is impossible to capture on film or 1.ccds:!The most specialized cameras can capture intensities of about A.5000 to one, but the range of illumination the the corona is about a million to one.!The film over exposes the inner parts, and under exposes the B.outer parts. !Your eyes are better at this. !C.The chromosphere has an emission line spectrum that cameras 2.translate as pink. !Eclipse viewings:!5.Eclipse viewings are reunions for astronomy enthusiasts from a wide range A.of areas. !They tend to call local astronomy clubs to learn the best place for a.viewing, and then they all go to that place. !People go to multiple eclipse viewings because they are addictive. !1.Sometimes, eclipse viewings bring people who it would be impossible to b.meet otherwise: !When Professor Sperling went with a group of astronomers to see an 1.eclipse in Tanzania, they were joined by the Tanzanian government, including the president. !Eclipses also capture a great deal of local interest, a lot of it negative: !B.Most local governments spread alarmist misinformation about eclipses a.to their own
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