The long tradition of astronomy

03 January 2020 | Research - Insights

In the past, astronomers were perplexed by the occurance of eclipses, but after careful monitoring of the moon and sun, they learned how to predict their cycles

| By Dr Abel Yang |

For a few minutes in the afternoon of 26 December, the moon passed in front of the sun, leaving a ring of fire around it. This rare annular solar eclipse will only next be visible in Singapore in the year 2063.

In modern times, we know what solar eclipses are, and astronomers in Singapore eagerly anticipated the eclipse as a once-in-a-lifetime special event. This was not the case in the past, especially in agrarian societies. 

For people whose lives were closely intertwined with the cycles of the sun and moon, an eclipse could be a terrifying experience. Imagine the sun, whose light you rely on for your warmth and your fields, disappearing for a few minutes in the day when you least expect it.

Naturally, one of the first things that people in civilised society did was to try to predict eclipses, which turned out to be far simpler than it seems. All you have to do is to pay close attention to the motion of the moon and the sun, which anyone could easily do. 

Ancient Greek and Babylonian astronomers, using only their eyes, worked out that the Earth, the Moon and the Sun line up in a straight line every 18 years (known in modern times as the Saros cycle), when a new eclipse might occur.

Ancient astronomers have long observed cycles in the sky for timekeeping. The ancient Babylonians and Chinese, among others, figured out that 235 lunations is almost exactly 19 solar years. This is known as the Metonic cycle — named after Meton of Athens from 5th century BC. It tells us that in order to keep a lunar calendar in step with a solar calendar, an extra month will need to be added every two to three years. This explains why the Chinese lunar calendar sometimes has 13 months in a year.

To figure out such cycles require many years of observation, possibly even longer than a human lifetime. The ancient Babylonians thus made systematic records of astronomical observations, with known records that span from 652 BC to 61 BC. The Babylonians were not the only civilisation to make records, the Chinese did so too, recording 1600 eclipses from as early as 720 BC.

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The next annular solar eclipse visible in Singapore will not occur for more than 40 years

These systematic records enabled ancient astronomers to determine and verify the Saros cycle and Metonic cycle, as well as develop more accurate cycles for timekeeping. In an agrarian society, a reasonably accurate calendar is a necessity to know when to plant and to harvest, and it comes as no surprise that astronomy played a special role in these ancient civilisations. As the oldest of the natural sciences, with origins in prehistory, astronomy is also a pioneer in data science.

Astronomical observations and data have been critical in many scientific discoveries closer to modern times. Between 1580 and 1597, the Danish astronomer Tycho Brahe made extremely detailed observations of the night sky, cataloguing the positions of planets and other astronomical objects. Over the next decade, Tycho’s assistant and successor Johannes Kepler used the data to calculate and compare various models of the orbit of Mars before finally publishing two laws of planetary motion in 1609.

These were empirical laws, derived from observations but with little physical basis. However, they were good enough to correctly predict the transit of Mercury in 1631, an eclipse-like event where Mercury passes in front of the Sun. 

A consistent physical theory of the solar system would come in 1687 when Sir Isaac Newton described the three laws of motion and the law of gravity from which he mathematically derived Kepler’s three laws of planetary motion; Kepler’s third law was published in 1621. Today, we know of Newton’s laws of motion and gravity as one of the most foundational topics in physics, but their origin and confirmation was from the heavens.

In the 21st century, astronomy and astrophysics keep the tradition of detailed systematic observations, now with modern technology. Large telescopes scan the sky every night, generating huge amounts of data on stars and galaxies, as well as potentially hazardous asteroids that could hit the earth and destroy a city. Astronomical archives contain data on hundreds of millions of stars and galaxies.

Like the astronomers of old, modern astronomers do not simply look at the sky. In fact, most of the work of modern astrophysics involves calculating theories and analysing data, just like Kepler and many others did before. What has changed is how data is collected, stored and distributed. Large astronomical archives are now publicly available online with far greater detail than before, enabling countries and institutions without their own research observatories to do cutting edge astrophysics. Unlike Kepler who spent years trying to get access to Brahe’s data, we can now just download our data off the internet. In fact, citizen science projects now engage and enable the public to participate in cutting-edge research.

NUS Physics offers an astrophysics specialisation for the students taking an undergraduate degree in Physics. Students not only get to learn about astrophysics, cosmology and particle physics among other topics, they also get hands-on experience analysing real scientific data, either from large astronomical surveys or the Large Hadron Collider in CERN, Geneva. Although my colleagues and I work to throw light into the beginning of the universe and matter, our students end up highly employable in many other sectors because of their skills in scientific computing, statistics, mathematical modelling and data analysis.

Part of the allure of astronomy comes from the innate human sense of curiosity and wonder. Since the dawn of civilisation, people have looked up at the stars, and if you, too, have sometimes looked up at the stars in wonder, you are not alone. That astronomy could also be used to better our lives made it all the more a worthwhile pursuit then and now. Astronomy is not one of those rich country pursuits that we cannot afford. Rather, it is a highly practical tradition that, with modern technology, has never been more accessible.

About the author

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Dr Abel Yang is a lecturer at the Department of Physics in the National University of Singapore where he teaches astrophysics and practical astronomy.