On April 20, 2023, the coastal town of Exmouth in Western Australia witnessed a spectacular celestial event - a total solar eclipse. For thousands of years, humans have been fascinated by eclipses, and despite the significant advancements in science and technology, predicting eclipses remains a complex task. This article aims to explore the history of eclipse predictions, the role of the Saros cycle, and modern methods of predicting eclipses.
Watching the sun and the moon
The sun and the moon are the most significant celestial objects that ancient cultures observed. Since both of them were so dominant in the sky, these ancient cultures naturally tried to anticipate and predict their movements. While the movement of the sun is relatively simple, the moon's motion is much more complex. The moon moves across the sky on an elliptical orbit around Earth and has phases. It also grows and shrinks in apparent size, making it extremely challenging to accurately describe its orbit.
Despite the challenges, many ancient cultures discovered the repeating characteristics of eclipse events and noted their occurrence in writing and art. During a lunar eclipse, where Earth blocks sunlight that would otherwise illuminate a full moon, the dimmed moon takes on a bloody hue. Many cultures attached foreboding to such events and wondered when the next such event might occur.
The not-so-mythical Saros cycle
Various cultures around the world have independently discovered that eclipses occur on an 18-year cycle. It was mentioned in written records by the Babylonians and Assyrians (of ancient Mesopotamia and modern Iraq), and oral tradition suggests the cycle was used for ceremonial purposes by Torres Strait Islanders in what is now Australia. This 18-year cycle, which can persist as a sequence for over a thousand years, is now known as a Saros cycle.
The Saros cycle represents how long it takes for the sun-Earth-moon system to return to almost exactly the same triangular configuration. So, if you see a lunar eclipse, you can expect another one 18 years later, visible from most places on Earth. However, if you observe a total solar eclipse, which occurs roughly every 375 years at a given region on Earth, you would not see a similar event 18 years later over the same part of the planet. The eclipse region should return to roughly the same position on Earth after 54 years - three Saros cycles. But only roughly, as it could be thousands of kilometers away from the previous observation spot.
From antiquity to modern-day
Despite the challenges, ancient people could predict timings for lunar eclipses and partial solar eclipses, but there is no convincing evidence of people predicting the times and locations of total solar eclipses. The famed Greek Antikythera Mechanism, an astoundingly complicated 2,000-year-old mechanical device that was used to predict the night sky, could calculate the 18-year Saros accurately. But significantly, it could not predict total solar eclipses at a precise place on Earth, just their timing.
Entering the modern era of science, the first true prediction of a total solar eclipse (both in time and location) occurred in 1715. Edmond Halley (of comet fame) correctly predicted, to within four minutes and 20 miles, a total solar eclipse that rather conveniently passed over his own house in London. He did this by making full use of Isaac Newton's new theories of gravity and orbital mechanics: the Principia.
Today, NASA uses a highly advanced form of an ancient technique - pattern recognition - to predict both solar and lunar eclipses for 1,000 years into the future. Using some 38,000 repeating mathematical terms, NASA can accurately predict eclipses, but beyond that, the moon's wobbly orbit and the influence of other celestial bodies make it difficult to make predictions beyond a certain level of accuracy.
For those who witnessed the total solar eclipse in Exmouth, it is important to remember the long and fascinating history of eclipse prediction. From the ancient Babylonians to NASA scientists, humans have been striving to understand the complex motions of the sun, moon, and Earth and to anticipate the rare and awe-inspiring moments when they align perfectly. While predicting eclipses is no longer as mysterious as it once was, it remains a testament to the human desire to explore and understand the natural world.