Tracing the path towards totality
Dec 14, 2016 3 comments
Taken from the December 2016 issue of Physics World
Sun Moon Earth: the History of Solar Eclipses from Omens of Doom to Einstein and Exoplanets
2016 Basic Books $26.99hb 256pp
On 21 August 2017 a total solar eclipse will cast its sweeping shadow across the US. Starting around 10 a.m. in Oregon on the west coast, it will end all too soon, a mere 90 minutes later in South Carolina in the east. An awe-inspiring cosmic display awaits those who are either lucky enough to live along that swath of land, or who make the effort to get there. Depending on where it’s observed from along its path in the US, totality will last from anywhere between 1 and 2.5 minutes, but for the rest of Northern and Central America, the eclipse will be partial. Nonetheless, the sight of the new Moon forming a dark crescent shadow on the solar surface will be a celestial spectacle for any viewer. In his book Sun Moon Earth: the History of Solar Eclipses from Omens of Doom to Einstein and Exoplanets, author and astronomer Tyler Nordgren charts the evolving history and science of the natural phenomenon that is a solar eclipse.
Through a narrative that flows effortlessly between personal experiences and scientific facts, Nordgren – a professor of physics at the University of Redlands in California, US – engages the reader with fascinating scientific discoveries that this cosmic coincidence makes possible. Eclipses are a good reminder that planetary motions follow well-defined laws of physics, for the most part. It is these latter exceptions, when the laws are broken, that have driven some of the most significant discoveries of our time, including the modifications to Newton’s laws of gravitational motion in the form of Albert Einstein’s theory of relativity, which was substantiated thanks to Arthur Eddington’s observations of starlight during the eclipse of 1919.
Nordgren also describes just how addictive it can become once you witness a total solar eclipse – see one and you are already planning for the next. Eclipse chasing can become a costly affair as you often travel to remote locations. As a scientist who has been leading a team to observe total solar eclipses since 1995, I understand all too well the eclipse-addiction syndrome. For my group – the Solar Wind Sherpas – an eclipse offers us the unique opportunity to probe a small section of the sun’s corona (of just a few solar radii) that is closest to its surface, which currently cannot be observed by any other instrument. With each eclipse, opportunities arise for testing new ideas and new instrumentation.
For the most part, it is the fleeting beauty of this event that makes the experience so compelling. Lasting a maximum possible seven minutes, the magic is always over too soon. The prospect of bad weather – that sometimes looms as a literal dark cloud – is omnipresent and makes observing solar eclipses even more of a challenge. The Solar Wind Sherpas have experienced the entire spectrum of emotion, from utter disappointment when the view was clouded out, to extreme joy when clear skies prevailed. But the addiction persists as we continue to strive to uncover some of the secrets of the Sun and its corona.
Astronomers and non-scientists are often equally obsessed with eclipses. But for the former, it is the unique opportunities that a solar eclipse offers – to test certain theories or trial new technologies – that is tempting. Beyond the sheer visual awe of an eclipse, the celestial setting that comprises the Sun, Moon and Earth serves as an excellent laboratory tool. Light is a ubiquitous astronomical signal that can be detected using everything from a telescope to a spectrograph to the naked human eye and has been studied through the centuries.
According to Nordgren, the world’s first eclipse-chaser happened to be a scientist – Jacques d’Allonville, or Chevalier de Louville, a member of the Royal Academy of Sciences in Paris – who travelled to London to see the total solar eclipse of 22 April 1715. This eclipse had been predicted by astronomer and mathematician Edmund Halley, using his friend Isaac Newton’s laws of motion. Incidentally, this event was also one of the first times that the public had been asked to engage in the science being done. Halley distributed posters across the country asking people to record the time and duration of the event using their pendulum clocks and to mail him their results. Halley’s aim was to make better measurements of the Moon’s orbit, thereby improving his ability to predict future eclipses.
A strikingly similar example of citizen science followed some 200 years later, when a total solar eclipse was predicted to pass over New York City on 24 January 1925. A number of scientists urged the public to witness totality and record the eclipse’s time and duration, in what the New York Times deemed “cosmic detective work.”
The book ends by offering the author’s insight into the evolution and the ultimate fate of our Sun and solar system. Nordgren teases us with the basic question, and worry, of whether eclipses will “forever” be present for us to marvel at. Currently, no other planet in our system has the privilege of experiencing a total solar eclipse. Our Sun, Moon and Earth can continue to boast of their unique alignment, but for how long? We can breathe a sigh of relief for now, as this fortuitous combination of sizes, distances and orbits that allows for a total solar eclipse to occur will last for at least the next few hundred million years.
Nordgren captures the scientific significance of total solar eclipses in a manner that is readily accessible to most readers. Certain concepts mentioned in the book – including the rather complicated story behind the modification to Newtonian gravity, which could not account for discrepancies in the orbit of Mercury, and eventually led to Einstein’s theory of relativity – might be difficult for some to follow. However, this does not deter the reader from carrying on. Nordgren’s book is extremely timely, and hopefully many of its readers will be compelled to witness the beauty of the corona next year.
About the author
Shadia Habbal is a professor of astronomy at the University of Hawaii’s Institute for Astronomy, in Manoa, US. So far she has led 14 scientific eclipse expeditions