B00B7H7M2E EBOK (20 page)

Newton did that in an ingenious but rather convoluted way, using a technique suggested by Scottish mathematician and astronomer, James Gregory. Judging by the size of Saturn, Newton estimated that about one part in a billion of the Sun’s light hits the planet Saturn. He reasoned that Saturn doesn’t reflect all of the sunlight that hits it – so it would be incorrect to conclude that the light we see coming from Saturn represents one billionth of the Sun’s light. Instead, he thought that Saturn probably reflects only about a quarter of the Sun’s light that hits it, which would mean that the reflected sunlight coming from Saturn gives us a good indication of what one part in
four
billion of the Sun’s light looks like. Following this line of thought, if a distant star seems to have the same brightness as Saturn, it follows that the light we are receiving from that star (not reflected sunlight; the star’s own light) is also equivalent to one part in four billion of the Sun’s light. What this means –
if
all stars’ brightnesses are the same as the Sun’s – is that a star that looks (from Earth) as bright as Saturn would have to be
approximately
a hundred thousand times further away from us than the Sun is.

Newton’s measurements of the distances to some of the nearest stars were not far wide of the mark, though this method was not dependable partly because stars do differ in their absolute magnitude (their ‘close-up’ brightness), and a star’s apparent magnitude (how bright it appears from Earth) alone can’t be used as a gauge of its distance. (See box below.) Some of the stars that look brightest in the night sky are very far away, while many nearer stars are rather inconspicuous.

In 1718, Newton’s younger friend Edmund Halley discovered an important new clue to star distances. He had become fascinated with Ptolemy’s writings and the star catalogues that ancient astronomer had compiled. Halley was particularly
curious
as to whether the stars had changed positions since the time of Hipparchus and Ptolemy. He took it upon himself to compare positions recorded in Ptolemy’s
Almagest
with positions in his own lifetime in the late 17th and early 18th century.

Halley was born in 1656 and while still an undergraduate at Oxford wrote and published a book on Kepler’s laws. His book came to the attention of Flamsteed, who had a great deal of influence as the first Astronomer Royal of England (though the position wasn’t called that yet) and first head of the Royal Observatory at Greenwich. Halley left Oxford without getting his degree and, at Flamsteed’s behest, was soon on the island of St Helena in the South Atlantic, mapping the sky as seen from the southern hemisphere. When he returned to England two years later he was elected to the Royal Society. He was only 22.

Even more eclectic in his interests than Newton, Halley spent the next 30 years in an astounding variety of pursuits. He travelled extensively to meet other scientists and astronomers; he assisted Flamsteed; he got married; he commanded a warship in the Royal Navy and captained a mutinous ship across the Atlantic; he went to Vienna on two secret diplomatic missions; he served as deputy to the controller of the Mint at Chester (a position Newton helped secure for him); he studied magnetism and the winds and tides; he prevailed upon Newton to publish the
Principia
, and he financed its publication. Of course the work which brought him most fame was his study of comets. ‘Halley’s Comet’ was named after him when it reappeared in 1758, after his death, at the time he had predicted. In 1703, Halley joined the faculty of the University of Oxford, where he had failed to complete his degree, as Galileo had done at the University of Pisa.

In 1718, Halley reported that three of the stars he was studying – Sirius, Arcturus and Aldebaran – had shifted over the centuries since Ptolemy. He strongly suspected that the discrepancies between the old charts and those of his own time were too large and too isolated to be attributed to errors in ancient
measurement
. Why should early astronomers have got everything else right but this? As a follow-up, Halley proceeded to measure the shift of Sirius during the 100 years since Tycho Brahe had observed it, a measurement that confirmed his suspicions. The change had been so gradual that it could only be noticed over a span of at least several human generations.

‘Proper motion’ is the name given to this movement of stars relative to one another over the centuries – an apparent movement across the sky when viewed from the Earth. Most stars are not moving only side-to-side, of course, as though the sky were a two-dimensional surface; they are likely at the same time to be getting closer to us or further away.

In 1720, at the age of 64, Halley succeeded Flamsteed as Astronomer Royal of England, which would not have pleased Flamsteed, for Halley had acquiesced in Newton’s poor treatment of the old man. It probably would have pleased Flamsteed that his widow whisked all the instruments out of the Royal Observatory. They were legally hers because the financial arrangement at the Observatory was such that the Astronomer Royal purchased equipment out of his salary. Halley had to set to work acquiring new equipment.

Edmund Halley died in 1742 at the age of 85. One of his most significant achievements didn’t come to fruition until nearly 20 years later.

When Halley had been on St Helena in his early twenties he had seen and timed a transit of Mercury across the Sun. He was the first man ever to observe both Mercury’s first entry on to the Sun’s disc and its final exit. Halley knew of a suggestion from James Gregory that a transit would provide an opportunity to use parallax in a new way to measure distances in the solar system. The passage of a planet across the Sun shows up as a tiny black dot passing across the Sun’s face, and observers at different locations on the Earth’s surface see the planet first touch the Sun’s disc at different times.

Halley put little faith in earlier measurements of the Sun’s
and
the planets’ parallaxes and distances, including those of Cassini and Flamsteed, although his friend Newton came to agree with them. Newton also measured the orbits and the distances of the planets from the Sun, using not astronomical observations but the dynamics of the system as the basis for his calculation. He concluded that Cassini’s and Flamsteed’s results were better than his own. But as of 1700, the only real agreement among astronomers when it came to the Sun’s distance was that it was at least 55 million miles away. Halley was convinced that the transit of Venus would be a chance to make much more definitive measurements.

A transit is a relatively rare event, but Halley knew there would be a transit of Venus across the Sun in 1761. He also knew he wouldn’t be alive to witness it unless he lived to be 105. So he wrote and published detailed instructions on the best way to use observations of the transit from different parts of the world.

Sixteen years after Halley’s death, with the return of the comet he’d seen in 1682, his name became a household word. In 1761 there was indeed considerable effort, much due to his prestige, to study the transit of Venus, and similar excitement about a second transit in 1769. Both times, the globe was studded with observing parties, who knew the opportunity wouldn’t be repeated again until 1874. There are colourful stories connected with this venture which indicate that many astronomers at the time were less denizens of the ivory tower cum telescope than they were prototypes of Indiana Jones.

Frenchman Guillaume le Gentil planned to observe the 1761 transit from Pondicherry, near Madras in India. He arrived to find the town occupied by British forces. This was during the Seven Years War, England and France were enemies, and le Gentil was not welcome in Pondicherry. Rather than turn around and head for home, he settled nearby for eight years, supporting himself in part by trading while he waited for the next transit. By then the British had ceased to be an obstacle,
but
nature had no mercy on le Gentil. The Sun shone brightly before and after the transit. During the transit, alas, it was hidden by a cloud.

Jean d’Auteroche led another French observing team in Russia in 1761, and in what is now southern California in 1769. The party of four astronomers trekked overland across Mexico to reach their California observing location. D’Auteroche and two of the others died of disease shortly after their arrival. That left the fourth to undertake the treacherous return journey alone, but he brought back with him the dearly bought records of the observation.

The Reverend Nevil Maskelyne, sent by the Royal Society to St Helena to observe the 1761 transit, had a much better time of it. Maskelyne’s expenditures were in the neighbourhood of £292, out of which £141 went on his personal liquor supply.

David Rittenhouse, in America, worked for months before the 1769 transit building a temporary log observatory at Norriton, near Philadelphia. He used a collection of instruments there that included telescopes from Europe and others he had built himself, and also his own eight-day clock that ‘does not stop when wound up, beats dead seconds, and is kept in motion by a weight of five pounds’.

Charles Mason and Jeremiah Dixon, who would later establish the Mason-Dixon Line in North America, headed up another team sponsored by the Royal Society. That august body threatened them with disgrace and possible legal action if they failed to continue with their expedition to the Cape of Good Hope to observe the 1761 transit, after a French frigate attacked their ship in the English Channel and 11 crew members died. Evidently the captain of the French frigate had been unaware that in spite of the ongoing Seven Years War, Englishmen and Frenchmen were collaborating in this scientific endeavour.

Maximilian Hell, a Viennese Jesuit astronomer, observed the 1769 transit from Norway and suffered devastating damage to
his
reputation when Jerome Lalande insinuated that Hell had fiddled his observations to make them consistent with those reported by others. Karl von Littrow supported Lalande’s allegation, claiming to have found proof in the form of different ink colours in Hell’s report. Hell’s good name wasn’t restored until 1883, after there had been another transit. Among other things, it was discovered that von Littrow had been colour blind.

Sadly, the results of all this effort were less definitive than Halley and these astronomers had hoped. Precise determination of the instant the planet touched the Sun’s disc was much more difficult than anticipated. Because of the Sun’s corona and the atmosphere of Venus, Venus’s image at the beginning and end of the transit was blurry. Calculations based on the results of these observations put the distance from the Earth to the Sun at about 95 million miles or 153 million kilometres, as compared with Cassini’s measurement in 1672 of 87 million miles or 140 million kilometres, and our modern measurement of 93 million miles or 149.5 million kilometres.

Halley’s discovery of proper motion was also destined to bear fruit far beyond his lifetime. It so happens that the three stars whose proper motion he first measured – Sirius, Arcturus and Aldebaran – are some of the brightest in the sky. Was this mere coincidence? A star might look brighter than others because it really is brighter, or it might look brighter because it is closer. Halley’s discovery of proper motion gave astronomers a new clue.

Objects moving across our line of vision close to us appear to move more rapidly against the background than those further away. A child on a tricycle near us can easily outrace a car driving at a good clip off on the horizon. Logic tells us that the same will be true with stars that are moving across our line of vision. Unless stars are all the same distance, we ought to find the nearer stars appearing to move against a background of more distant stars. Most stars, in fact the vast majority of them,
show
no change of position since Ptolemy for a viewer on the Earth. Does that mean they are far more distant than those that have changed position?

Sixty-six years after Halley’s discovery, astronomer William Herschel, the discoverer of the planet Uranus, studied the proper motions of a number of stars and the way those proper motions relate to one another, and from that he was able to plot the Sun’s motion through our part of the Galaxy. Still later, the German astronomer Friedrich Wilhelm Bessel, suspecting that proper motion, rather than brightness, might be the most significant indicator of which stars are nearest us, used it as a basis for choosing which stars to try to measure with the parallax method.