A few weeks before the total solar eclipse of 3 May 1715, Londoners could buy a printed map of southern England with a broad diagonal band drawn across it. The band showed where the Moon’s shadow would fall. Alongside it were the predicted times of the eclipse and an explanation of what was going to happen and why.
The map was made by Edmond Halley, and it is one of the first examples of a scientific prediction issued to the general public in advance, in a form anyone could check. It also came with a request: readers who saw the eclipse were asked to record the timing and duration and send the results back.
Why It Was Possible
Eclipses had been predicted for a very long time. Babylonian astronomers had identified the saros cycle, the eighteen year period after which similar eclipses recur, and could say when an eclipse was likely. Ptolemy’s Almagest devotes a whole book to eclipse calculation, and medieval Islamic and European astronomers refined the tables considerably.
Predicting that an eclipse will occur is one problem. Predicting exactly where on the surface of the Earth the shadow of totality will pass, and at what time, is a much harder one. It requires knowing the position of the Moon in its orbit to high precision, the size of the shadow it casts, and the geometry of that shadow’s intersection with a rotating spherical Earth.
The lunar theory in Newton’s Principia is what made the calculation tractable. The Moon’s motion is the classic difficult case in gravitation, since it is pulled by both the Earth and the Sun, and its orbit is consequently full of irregularities. Newton worked on it for years and reportedly complained that it was the only problem that ever made his head ache. The section of Book III dealing with the Moon is the most laboured part of the entire work.
Halley had a particular claim on the result. It was he who had persuaded Newton to write the Principia in the first place, and who paid for its publication out of his own pocket when the Royal Society ran out of money. The eclipse map is, in a direct sense, that investment paying a dividend.
What the Map Said
Halley computed the path and had it engraved across a map of England, with the shadow band running from the Cornish coast up through the Midlands and out over the North Sea near Norfolk. London sat inside the band, which had not happened since 1140 and will not happen again until 2151.
The accompanying text is explicit about the purpose. Halley wrote that he published it so that the sudden darkness would not frighten the population, who might otherwise take the event for an omen, and so that observers might be prepared to make useful measurements.
That first motive was not condescension. Unexplained darkness in the middle of the day had historically produced panic, and a printed map showing that learned men knew in advance exactly when and where it would occur was a demonstration of what natural philosophy could now do.
The Result
The eclipse arrived. Halley observed it from the rooms of the Royal Society in London, timing the duration of totality at three minutes and thirty three seconds. Reports came in from across the country, many from clergymen and country gentlemen who had bought the map.
Comparing the predicted path with the observed one, Halley’s northern and southern limits were off by around twenty miles, and his timings were about four minutes early. For a calculation of that difficulty, made in advance, it was an impressive performance and a public one.
He then did something equally important. He printed a second map after the event, showing the true path in comparison with his prediction, along with the corrected path for the next eclipse, due in 1724. Publishing the size of his own error, and using it to improve the next forecast, is the practice that makes prediction into a self correcting method rather than a display of authority.
The Man Who Made It
Halley is remembered for the comet, and the comet is a fair summary of his working style. In 1705 he examined the records of comets seen in 1531, 1607 and 1682, noticed that the orbital elements were nearly identical, and proposed that they were three appearances of a single object on a closed orbit of about seventy six years. He predicted its return for 1758.
He died in 1742. The comet arrived on Christmas Day 1758, and has carried his name ever since. It was the first demonstration that Newtonian gravitation governs objects beyond the planets and the first successful prediction of a specific astronomical event decades in advance.
His other work covers an unusual range. At twenty he sailed to St Helena and catalogued 341 southern stars, producing the first accurate star chart for the southern hemisphere. He mapped the trade winds and the magnetic variation of the compass across the Atlantic, publishing the results as charts with lines connecting points of equal value, an innovation in data presentation that we now take for granted. He published a mortality table for the city of Breslau that became the basis of life insurance mathematics. In 1718 he compared contemporary star positions with those in Ptolemy’s catalogue and discovered that some stars had moved, establishing that the fixed stars are not fixed.
He also proposed using transits of Venus across the face of the Sun, observed from widely separated points on Earth, to determine the distance to the Sun. He knew he would not live to see the transits of 1761 and 1769, and wrote out the method for the astronomers who would.
The Map as an Object
What makes the 1715 broadsheet interesting beyond the astronomy is its form. It is a piece of printed science made for sale to the public, combining a technical result with a map, a timetable, and an explanation in plain language.
The eighteenth century produced a great deal of this material, and it marks the point at which science stopped being a correspondence between specialists and started being something a literate person could buy at a shop and follow. The engraved scientific image, whether a diagram, a chart, or a portrait, became the ordinary means of transmitting results, and the techniques behind it are the subject of the printmaking methods developed in the previous century.
The Point of the Exercise
An eclipse prediction is an unusually strict test of a theory. There is no room for interpretation and no way to adjust the result afterwards. The shadow either crosses Cornwall at the stated hour or it does not, and thousands of people are watching. Two centuries later an eclipse would be used the same way, to test a prediction about the bending of starlight.
Ptolemy’s system could predict that eclipses would happen. Newton’s gravitation, applied by Halley, could predict where the shadow would fall on a specific field in a specific county. The difference between those two capabilities is the difference between a model that reproduces the sky and a model that explains it.
Halley published his prediction, published his error, and used the error to sharpen the next attempt. That sequence, more than any individual number on the map, is what the 1715 eclipse demonstrated to a public that had never seen science operate in the open before.
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