What’s so special about a solar eclipse?

In the world of astronomy a total eclipse of the Sun is nature’s most spectacular show. This article will make a distinction among: • a partial eclipse of the sun, • a lunar eclipse and • the grand show that is the total eclipse of the Sun. Mainly this is about why a total solar eclipse is so special.

The concept of an eclipse is simple: in astronomy it means that the view or light of one object is blocked, or eclipsed, by another object.

In the case of a lunar eclipse, at the time of the full Moon, the Earth moves between the Moon and the Sun. So, during the eclipse the full Moon becomes dark. This is interesting, but the events proceed at a glacial pace over a period of hours. Also, everyone who is on the half of the Earth with the Moon in the sky can see it. The event is worth watching, but it is neither rare nor dramatic.

A partial eclipse of the Sun occurs when the Moon passes between the observer on the Earth and the Sun — but the full disk of the Sun is never blocked. Like the Lunar eclipse, events proceed slowly. Unlike a lunar eclipse the geographical position of the observer on the Earth will have a profound effect on the amount of the Sun that appears to be eclipsed. Although the event is only mildly interesting, it includes the possibility of real danger to the observer: during a partial eclipse people are drawn to looking at the Sun to see what is happening… and that can cause blindness.

It is extremely hazardous to ever look directly at the Sun, and at the time of solar eclipses there are always people who suffer irreparable damage to their sight.

The total eclipse of the Sun sounds similar to the partial eclipse, but it is the event called totality that makes it special. Totality occurs when the observer (perhaps you) are perfectly in line with the Moon and the Sun.

At the time of totality the entire bright disk of the Sun, called the photosphere, is completely hidden behind the Moon. Since the Moon is moving rapidly through space in its orbit around the Earth, this condition lasts for only seconds or moments.

In our Solar System the relationship between the Earth and its moon are unlike any other planetary system. The Moon is so large compared to the Earth that this is almost a double planet. The centre of gravity of the system is still below the suface of the Earth; but if the Moon were only a little larger, that point would be above the surface and this would be a double planet. What this means is that through all of human history we have been able to look up in the sky and see another world. It is so large up there that to the unaided eye it is clearly a world.

There is something else about our Moon that is remarkable. The size of the disk of the Moon in the sky is usually just slightly bigger than the disk of the Sun. The Sun is really huge, but it is 400 times farther away than the Moon. What this means is that if you can align yourself with the Moon and the Sun, the Moon will neatly and precisely cover the photosphere of the Sun. This is the total eclipse.

If you hope a total eclipse to come to you, it may be a long wait: on average eclipses happen at single locations only once in 360 years. There are total eclipses somewhere on the Earth almost every year. So, if you are willing to travel, you can probably go and see one.

A total eclipse happens only at the time of the new moon. The Moon circles the Earth about once per month. As it passes the Sun in the sky — an event called the new moon — it normally passes above or below the Sun in our sky. On the day of the new moon, you won’t see the Moon; but it is up there beside the Sun in the daytime sky. Sometimes the Moon does pass in front of the Sun causing partial or total eclipses.

On those rare days when there is a total eclipse, the Moon casts its shadow on the surface of the Earth. Totality can be observed by people within the umbra of that shadow. The umbra is an oval pool of darkness that is about 200 km in diameter and races across the surface of the Earth at speeds near 2000 km/h.

If you are to witness a total eclipse you will position yourself close to the centre of path of the umbra, called the path of totality. That is what we plan to do on July 22.

What will happen? The answer depends on many local factors including the weather. The astronomy can be calculated.

The observers must be schooled in how to safely observe a solar eclipse, and the reader is cautioned that this article provides no such advice.

As the eclipse begins, people with telescopes equipped for solar observing will notice that the silhouette of the Moon encroaches on the face of the sun. Gradually and inexorably it munches its way across the Sun. People with cameras will notice that the light is failing but it still feels like daytime because the Sun is in the sky.

Eventually all that remains of the Sun in the sky is a tiny fingernail sliver of the Sun. Now it may seem easy to stare at the sun, but the remaining bit of the photosphere is just as bright as the full sun and will quickly sear the retina of an unprotected eye. The temperature may have dropped and a wind may blow. Atmospheric effects are unpredictable but fun to observe — unless it is a dense cover of cloud.

Suddenly the ground may seem to shimmer with a low-contrast (and hard to photograph) phenomenon called shadow bands. As the Moon is about to completely hide the photosphere, the mountains on the leading edge of the Moon sever the sliver of sunlight into a series of dots called Baily’s beads.

Events are happening quickly now. If the observer is in a high location s/he may actually observe that umbra rushing across the landscape. As it arrives, daylight dwindles in seconds. For a few of those seconds the last of the beads may shine as a breathtakingly brilliant spot while at the same time the outer atmosphere of the Sun, called the corona, emerges around the black silhouette of the Moon. This momentary vision is called the diamond ring. As the Moon covers that last bead, totality has begun.

If you are a sophisticated observer, you may want to just look at the sky and the world around you and make a memorable meal of these special seconds. If you are an astronomer or a photographer you will be performing the actions you’ve rehearsed for months so you will capture the event and make no mistakes. If this is your first eclipse, maybe you are listening to your group’s leader who is quickly pointing all of the events so you don’t miss anything.

Every eclipse is different. Can you see the daylight in the sky beyond the pool of darkness? Can you see the planets Mercury and Venus, and maybe few stars? The corona is different for every eclipse. You will always recognize pictures of your eclipse. The photosphere is gone now, so it is safe to look directly at that part of the sky with your eyes or a telescope. You may see what look like red flames: these are called prominences and are active hydrogen in the atmosphere of the Sun. Likely you will see them first on the side of the Sun where the diamond ring just vanished, and then on the other side of the Moon towards the end of totality.

Suddenly it is over and the warm sunlight is flooding the landscape.

I recall that in that moment hearing someone remark, “It feels like the first day.” And it did.

© 2009 Robert J. Ballantyne

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