21/01/2021

In Folha, Halley's Comet: a mathematical clock of space.

Registro do cometa Halley em 8 de março de 1986. Foto: Wikimedia Commons

Reproduction of Marcelo Viana's column in Folha de S.Paulo.

When I returned to Brazil to pursue my doctorate at IMPA (Institute of Pure and Applied Mathematics) in 1986, a scientific topic dominated the headlines: the return of Halley's Comet. There were enormous expectations, justifiably so, for the return of one of the most charismatic celestial bodies in the Solar System. Some even registered the trademark "Halley" to try and make money from it! In the end, the comet passed by rather far away, and its appearance was less spectacular than expected.

But, scientifically, it was a historic event. It was the first comet to be observed by space probes, such as the European Giotto, which confirmed that it is formed by a mixture of solid matter and volatile materials, which give rise to the coma (atmosphere) and the comet's tail when it approaches the Sun. And Halley's return showed, once again, the remarkable power of mathematics to describe what surrounds us.

The story is long. The first reliable record of Halley's Star was made by Chinese astronomers in 239 BC, but it is possible that it had already been noticed by the Greeks in 466 BC and even by the Babylonians in 164 BC and 87 BC. It is believed that the passage of 1301 influenced the representation of the Star of Bethlehem in the famous "Adoration of the Magi" by the Renaissance painter Giotto (hence the name of the space probe I mentioned).

But it took time for these appearances to be linked. Comets were seen as harbingers of momentous events and were even considered atmospheric phenomena (Galileo Galilei himself defended this theory). With the publication of Newton's law of gravitation in 1687, the idea that they were celestial bodies in motion gained strength.

In 1705, the English astronomer Edmond Halley (1656–1742) observed that comet appearances in 1531, 1607, and 1682 occurred at virtually identical intervals and suggested that they corresponded to the same celestial body, which was then supposed to return in 1758.

The French astronomers Clairaut, Lalande, and Lepaute refined the calculations using Newton's equations of gravitation. Their conclusions were brilliantly confirmed when the comet appeared in the sky in April 1759. Halley did not live to see it.

The fascination that comets continued to exert in the scientific age is symbolized in the phrase attributed to the American writer Mark Twain: "I came with Halley's Comet in 1835, and I hope to go with it in 1910." He died one day after the comet's perihelion, that is, its closest approach to the Sun.

To read the full text, visit the newspaper's website.