Nature gears up for her star turn

Next month sees the first total eclipse over Britain since 19, and J. P. McEvoy's slim volume is an informative and entertaining guide for anybody who, like me, will be heading to Cornwall on August 11. My nephew Richard will be accompanying me, and already he has asked what an eclipse is, prompting me to use a globe, a ping-pong ball and a lamp to explain how the moon moves between the Sun and the Earth, thereby casting a shadow. However, as the day approaches, he may ask more awkward questions, such as how long will the eclipse last and why are they so rare over Britain. McEvoy has all the answers, and so I should be saved from embarrassment.

McEvoy begins with a fairly prosaic explanation of the mechanics of eclipses, describing the various orbits and the combination of circumstances that is required for the Moon to block the Sun. One of the most beautiful aspects of an eclipse is that the Moon and the Sun appear to be identical in size when viewed from Earth, and so the two discs fit perfectly on top of each other. This is because of a cosmic coincidence; the Sun is 400 times bigger than the Moon, but it is 400 times further away. If the Moon were smaller then it would only block the centre of the sun, and we would merely experience a partial eclipse, and if the Moon were larger then it would completely obliterate the Sun and the corona. The corona is effectively the Sun's atmosphere, which is generally impossible to see because of the brightness of the Sun, but which becomes clearly visible during an eclipse, appearing like a halo around the black disc of the Moon.

Having said that Moon and Sun are a perfect fit, both bodies along with the Earth follow elliptical paths, and so at its furthest distance from the Earth, the Moon fails completely to block the Sun, and the result is an annular eclipse. The remaining ring of sunlight is so brilliant that the phenomenon is orders of magnitude less impressive than a total eclipse. Also, the Moon is very gradually spiralling outwards, its orbit inching away from the Earth. Hence, in millions of years from now total eclipses will turn into annular eclipses. We are privileged to live in an epoch when we can witness the full wonder of a total eclipse.

McEvoy lingers on the technicalities of eclipses for longer than is probably necessary for most lay-people and becomes bogged down in details, but eventually he emerges with a series of intriguing tales that describe how eclipses have inspired scientific breakthroughs throughout history. In fact, it is likely that the science of astronomy was born out of the desire to predict eclipses. The ancients have always studied the cycles of the Sun and Moon, particularly the former in connection with the seasons, the calendar and agriculture, but these patterns were not particularly challenging. In contrast, the eclipse, being a far more complicated and less predictable phenomenon, meant that astronomers had to stretch their minds and construct models of how the Earth, Sun and Moon were interacting. For the Babylonians, the study of eclipses was driven by the belief that these celestial events signified displeasure among the gods and ensuing catastrophes. If they were aware of a forthcoming eclipse, then they could at least try to supplicate the gods.

Thanks to the meticulous astronomical records of the Babylonians, modern geophysicists have been able to learn more about the properties of the Earth. Using computers, it has been possible to go back in time, reversing the orbits of the Sun, Moon and Earth, and reversing the spin of the Earth, thereby recreating ancient eclipses. Unfortunately, initial computer estimates of the exact times of eclipses did not tally with ancient records. This was because the computer models did not take into account that the Earth's rotation is gradually slowing as a result of the gravitational interaction between the Moon and the Earth, an effect that increases the length of the day by 2.3 milliseconds per century. However, even after allowing for this effect, the results still did not tally. Scientists eventually realised that the missing ingredient was the fact that the Earth has been changing shape from a dumpy to a rounder sphere over the course of the centuries, which counteracts the effect of the Moon. During the last ice age, ice accumulated at the poles, which squeezed the rock beneath outwards and towards the equator. Since the ice melted, the rock has been oozing back towards the poles, towards the Earth's axis, which has the effect of speeding up the Earth's rotation. This is akin to a spinning ice skater who draws in her arms and spins even faster. Hence, from Babylonian astronomy, via eclipses and ice ages, emerges a significant 20th-century breakthrough.

McEvoy also tells how Edmond Halley inaccurately predicted the path of the 1724 eclipse, which meant that thousands arrived at Windsor and saw nothing but clear blue sky. He also tells of Sir Norman Lockyer's extraordinary discovery in 1870 of the element helium, achieved by studying the Sun's corona. Most fascinating of all, he recounts how Sir Arthur Eddington in 1919 raced across the world in order to observe an eclipse and prove that Einstein's theory of general relativity was correct.

Nowadays much important solar science is done by space probes, such as Soho, the Solar and Heliospheric Observatory, which is able to create artificial eclipses by moving a disc in front of its camera lens. However, the emotional and spiritual experience for the earthbound observer is as potent as ever, and the historical and scientific background provided by McEvoy can only enrich the impact of an eclipse. Get a train ticket to Cornwall, read the book on the journey, find a nice spot on a cliff overlooking the sea, and enjoy one of nature's greatest spectacles

Simon Singh is the author of Fermat's Last Theorem , published by Fourth Estate.