…upon Sir Isaac Newton’s Principles: and made easy to those who have not studied mathematics.
This was the title of a book by the self-educated Scottish astronomer and instrument maker James Ferguson, whose work did much to popularize the understanding of science in the latter half of the 18th century.
It was first published in 1756, five years before the then much-anticipated transit of Venus. Among other things, it explained how this rare celestial event would help scientists come up with an accurate measurement of the astronomical unit – the distance of the Earth from the Sun.
Ferguson elucidated Newton’s proof of Copernicus’s heliocentric version of the solar system, gave an account of its size, and the scale of the known planets of the age, according to principles laid down by Newton and Johannes Kepler before him.
He also expounded Edmond Halley’s theory that a more precise measurement of the astronomical unit could be obtained through the observation of the Venus transit.
“Dr Halley has shown how the Sun’s distance may be known to within a 500th part of the whole, by a Transit of Venus over his Disc, which will happen on the 6th of June, in the year 1761; till which time we must content ourselves with allowing his distance to be about 81 million miles, as commonly stated by astronomers,” Ferguson wrote.
Astronomy explained… was updated in 1794 and 1799 with an extended title, reflecting revisions Ferguson’s had made before his death in 1776. The new editions featured some of the diagrams he drew up after the 1761 transit, based on data collected by the various voyages that took place round the world with the aim of viewing the event.
The new title read: Astronomy explained upon Sir Isaac Newton’s Principles: and made easy to those who have not studied mathematics: to which are added, a plain method of finding the distances of all the planets from the sun, by the transit of Venus over the sun’s disc, in the year 1761: an account of Mr. Horrox’s observation of the transit of Venus in the year 1639: and, of the distances of all the planets from the sun, as deduced from observations of the transit in the year 1761.
These editions gave credit to the English astronomer Jeremiah Horrocks, who had conducted the first known observation of the transit of Venus in 1639 and also gave the new calculation for the astronomical unit – 95,173,000 miles – noting that the figure achieved from the subsequent 1769 Venus transit observations “does not materially differ from the result of those in 1761.”
The publishers of the 1794 and 1799 editions of Astronomy explained… for some reason didn’t update the book with the major development that took place five years after Ferguson’s death – the discovery of Uranus, adding a seventh planet to the known solar system.
Observations of the 1874 Venus transit revised the astronomical unit down to around 93 million miles, pretty much the same as the figure that’s used today, achieved using modern techniques such as radio telemetry and radar.
The painting at the top of this post was by Joseph Wright and is called A Philosopher Lecturing on the Orrery, dated 1766.
An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons within the solar system. They are typically driven by a clockwork mechanism with a globe representing the Sun at the centre. The one above was designed by Ferguson to demonstrate the transit of Venus. The identity of the astronomer at the centre of Wright’s painting isn’t known but according to different commentators it’s possible it was either Newton or Ferguson.
All the editions of Astronomy explained… mentioned in this article can be found at Google Books.