Some Background

The Greatest Spectacle in the Whole of Nature

Some Background and a Little Bit of Jargon

A total eclipse is a very spectacular event, when day is literally turned into night, and we can only speculate how the earliest inhabitants of the earth regarded the disappearance of the sun, most probably they were terrified! The oldest written record of an eclipse comes from China 4000 years ago. It concerns the fate of the two official astronomers who failed to predict an eclipse and were promptly executed – they were also drunk at the time!

Legends naturally grew up around eclipses and some of the stories are most exciting. The truth is often just as surprising. In 585 BC, a total eclipse, predicted by Thales, brought a premature end to the war between the Lydians and the Medes that had been going on for six years. Kepler, in the 17th century, seems to have been the first person to notice, or at least to record, that a pale halo of light seemed to surround the eclipsed sun. This is what we now call the corona but in those days it was thought to be something to do with the moon! It was only a hundred years ago that scientists showed the corona was really the atmosphere of the sun and it's only in the last fifty years that we have discovered the corona has a temperature of several millions of degrees - but that's another story.

It is quite remarkable that from the very earliest times, astronomers have been able to predict when eclipses were likely to occur. They were not able to make the sophisticated calculations we do today, but they did notice that eclipses recur with the same general pattern after approximately 18 years and this is called the saros cycle. We now know that the sun and moon are in almost the same positions after 6585.78 days!

Perhaps the most famous eclipse observation of all concerns tests of General Relativity carried out during the 29 May 1919 event. Einstein's General Theory postulated that rays of light would be affected by a gravitational field such that light from distant stars passing close to the sun would be bent. Two expeditions from Britain photographed the star fields around the eclipsed sun and were able to confirm that Einstein was correct.

Because the solar disk is so bright, it is very difficult to observe the atmosphere of the sun without an eclipse and, up until seventy years ago, all of our knowledge had been gained during the brief moments when the disk was eclipsed. An important breakthrough came with the invention of the coronagraph, a telescope modified to artificially produce an eclipse revealing the sun's outer atmosphere above the limb. Space platforms now allow direct observations of the disk to be made by detecting high energy radiation that does not penetrate the earth's atmosphere.

Astronomers refer to four contacts during an eclipse, when the edges of the sun and moon just touch. First contact is the moment when the outside of the moon first appears to touch the outside of the sun's disk and the partial phase begins. Over the next hour or so, as the moon gradually covers the sun, it takes a bigger and bigger bite out of it until just a thin crescent is left. A gloomy darkness descends over the land making it feel as if evening is coming. The temperature drops, the wind often blows in several directions at once and the hairs on the back of your neck stand up as a shiver goes down your spine. Then, quite suddenly, the last tiny bead of light turns reddish-pink and disappears as an eerie darkness falls from the sky. This is called second contact and is when the total phase starts. Now the sun’s outer atmosphere, the corona, is visible safely to the unaided eye, as a pearly white halo around the black disk of the moon.

The solar corona in all its glory copyright John Parkinson

As eyes adapt to the darkness, the corona appears to grow revealing streamers radiating out into space and there may be small red prominences visible low down in the solar atmosphere. If you are observing in a dark place, planets and some of the brighter stars may also be seen. The end of totality, when the first rays of sunlight return, is called third contact and appears as a blinding flash, forcing you to turn your head away. It often looks like a diamond ring, with the pale corona forming a perfect circle with a sparkling gem on one side – surely this is the rarest and most elusive diamond. Now the sun starts to be steadily uncovered, but now the crescent is the other way up. Finally, the moment when the moon leaves the solar disk is called fourth contact and we can all breathe again!

The same terminology applies to an annular eclipse except that second and third contacts refer to internal contacts at the start and end of the annular phase.

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