A Point of View: Crowd-sourcing comets

Astronomers in the 17th Century understood the value of sharing information in order to plot the path of comets. Now modern science is using the internet to follow their example, says historian Lisa Jardine.

On three consecutive nights this week I was one of the many amateur stargazers across the northern hemisphere scanning the skies for a brand new comet, unromantically named C/2011 L4 Pan-Starrs, which has just become visible over Britain.

Judging from posts on comet Pan-Starrs' Facebook page (in 2013 a comet has its own social network complete with "likes", comments and shares), I am not alone in having been thwarted by the weather.

Happily, a second comet - comet ISON - is set to pass between the Earth and Sun in the autumn, and astronomers expect that it will shine brightly enough to be visible even during the day. Perhaps then I will get my view of a fiercely blazing celestial body with a glowing tail.

In earlier times, a succession of comets was greeted with less equanimity than today. When two comets passed over London in quick succession in 1680-81 there were many who were more superstitiously fearful that they were harbingers of doom. Spectacular comets had appeared in the night sky in 1664 and 1665. Had not these presaged the visitation of the plague on London and the Great Fire the following year?

In early November 1680, a comet appeared, so bright that it was visible by daylight, and was tracked heading steadily in the direction of the Sun until the end of the month. In mid-December, another comet appeared in the early morning sky, this time heading away from the sun. Its tail was particularly long and spectacular.

The diarist John Evelyn held the properly scientific view that comets "appear from natural causes". "Yet", he added in his diary entry for 12 December 1680, "they may be warnings from God, as they commonly are forerunners of his animadversions."

The scientific members of the Royal Society - of whom Evelyn was one - put aside superstition and busied themselves with due professional diligence observing and plotting the trajectory of the first, and then the second comet.

Christopher Wren, Robert Hooke and Isaac Newton charted and recorded its nightly positions. Edmund Halley, on his way to Rome on Royal Society business, observed its progress across the night sky with Cassini at the Paris Observatory.

Meanwhile, astronomers across Europe also watched and plotted, collaborating with their British counterparts by sending in their calculations to the Royal Society for collation with their own results in an early example of something like crowd-sourced data collection.

At the Royal Observatory in Greenwich - established by Charles II in 1675 - the first Astronomer Royal, John Flamsteed was appointed by the king to produce better star charts to increase the accuracy of navigation at sea. He assiduously followed the progress of the comets night after night. Flamsteed would spend more than 40 years assembling meticulous records for his star catalogue, which would eventually triple the number of entries in the previously used sky atlas.

In spring 1681, after close study of the data he had compiled, Flamsteed proposed that the two comets observed in November and December of 1680 were not two comets at all, but rather one comet travelling first towards the Sun and then sharply away from it. Newton disagreed. "To make ye comets of November and December but one is to make that one paradoxical," he told Flamsteed.

Sometime between the spring of 1681 and the autumn of 1684, however, Newton changed his mind. As obstinately as he had opposed Flamsteed's suggestion, he was now convinced that it had indeed been a single comet that had rounded the sun in a tight, hairpin turn in November and December 1680. And he proposed that comets, like planets, moved around the sun in large, closed elliptical orbits under the influence of the new force he was in the process of formulating mathematically - what we now call gravity.

As the revolutionary theory of gravitational attraction took shape, the comet of 1680 became an important test case in Newton's argument. Characteristically, however, Newton refused to give Flamsteed any credit for having been the first to propose to him that comets moved under the influence of a central, attracting force.

So Flamsteed was furious when he learned in 1685 that Newton had got hold of all his data, in order "to determine ye lines described by ye comets of 1664 and 1680 according to ye principles of motion observed by ye planets".

Flamsteed's observations had been obtained by dubious means by Edmund Halley, who was once Flamsteed's assistant at the Greenwich Observatory. Halley was now a pivotal figure of the Royal Society and the fact-checker and financial backer for the preparation of Newton's ground-breaking book, Principia Mathematica.

The scientific community-wide collaborative observations of the 1680 comet, including Flamsteed's purloined data, were subsequently printed as important evidence in Newton's Principia in 1687. There Newton established (among other things) the inverse-square law of gravitational attraction.

Twenty years later, Flamsteed would again obstruct scientific progress by refusing to publish his by now huge amount of accumulated data, in the form of star charts for the use of sea captains. Declaring that he was unwilling to risk his reputation by releasing unverified data, he kept the incomplete records locked up securely at Greenwich.

Newton maintained that Flamsteed was a public servant, and therefore his work was public property, but to no avail.

So in 1712, Newton, by now president of the Royal Society, together with the indefatigable Halley, again obtained the data by subterfuge, and published a pirated edition of a new star catalogue. Undeterred, Flamsteed managed to retrieve 300 of the 400 copies printed and destroy them. His own star catalogue, the Historia Coelestis Britannica, was eventually published posthumously by his wife and co-observer at the Observatory, Margaret Flamsteed, in 1725.

The enthusiastic collaboration among the wider community of 17th Century astronomers, across nations and continents, continuing to exchange astronomical observations even when their countries were at war with one another, is in stark contrast to Flamsteed's relentless withholding. His refusal to release his valuable data, and his insistence that his work was his personal intellectual property, slowed progress on an important scientific project.

By contrast, the sharing of data among European astronomers who took part in the tracking of the 1680 comet looks surprisingly modern. It is directly comparable with the current drive, particularly within the scientific community, towards open data and data-sharing.

Elegantly echoing the activities of these early, ground-breaking astronomers, what we now refer to as "crowd-sourcing" has recently been shown to be able to determine the trajectory of a comet as spectacular as the one observed in 1680.

In October 2007, Comet 17P/Holmes briefly became the brightest object in the solar system, arousing the interest of amateur astronomers worldwide. Using search engines, Dustin Lang from Princeton University and David Hogg at the Max-Planck-Institute in Germany gathered more than 2,000 images of the comet from all kinds of online sources.

They ran the pictures through Astrometry.net, external, which can recognise images of the sky and measure star patterns, and identified more than 1,000 that had captured the progress of Comet Holmes. They were then able to superimpose a large number of the comet images, and to arrange them as a sequence by carefully aligning the stars.

Many of the images were time-stamped, so that when they were superimposed the comet's precise path across the sky was clearly visible. Finally, Lang and Hogg compared their orbital data with observed information from Nasa's Space Laboratory in California, and found a close match.

"You can do high-quality quantitative astrophysics with images of unknown provenance on the web," Lang and Hogg conclude. "Is it possible to build from these images a true sky survey? We expect the answer is 'yes'."

It is to be hoped that long-running, ill-tempered quarrels over data, like that between Newton and Flamsteed, are a thing of the past. And that it is the collegial and good-natured collaborations among the astronomers of Europe in the final decades of the 17th Century that will in future serve as a model for the global teamwork that underpins so much of today's scientific activity.

There is some anxiety currently in the academic community, especially in the humanities, over government insistence that publicly funded research must in future be open access. I declare myself to be a strong advocate for collaboration and sharing of data in all fields of intellectual endeavour. There may be transitional difficulties. But we are, in the end, all part of a common quest for greater knowledge and understanding.

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