B00B7H7M2E EBOK (21 page)

The stellar parallax shift (see box above) that ancient Greek and Hellenistic astronomers could not find does exist, just as astronomers around 1700 were sure it must. Stars do have a parallax shift produced by the Earth’s yearly journey around the Sun. But the shift is extremely tiny and difficult to detect. Certainly it isn’t possible to see it with the naked eye, so the
astronomers
in antiquity can’t be blamed for missing it. Telescopes of Galileo’s and Cassini’s time weren’t refined enough to reveal it either.

One man who attempted to detect stellar parallax – making other important discoveries in the process – was James Bradley, born in England in 1693. The star Gamma Draconis passes almost directly overhead in London, and Bradley and his friend Samuel Molyneux, a wealthy amateur astronomer, decided to try to measure its parallax motion. They attached a 24-foot-long telescope to a stack of brick chimneys on the building where Molyneux was living. By using a screw, they could adjust the telescope to keep it tilted towards the star. The result was puzzling. Instead of having to adjust the tilt most in December and June, as they had expected, the adjustment was most extreme in March and September and was so large that, even if it had occurred at the right time of year, it was highly unlikely to be caused by parallax. Bradley took advantage of an exceptionally understanding aunt, who allowed him to cut holes in her roof and floors and install a larger and more sophisticated telescope. Observations with this instrument only repeated his and Molyneux’s earlier baffling findings.

The explanation dawned on Bradley, so the story goes, while he was taking a cruise on the Thames. When the boat changed direction, a weathervane on the mast shifted. It wasn’t the wind direction that had changed, however. It was the boat’s direction in relation to the direction from which the wind was blowing. Bradley realized that the displacement of the stars he was studying was similarly caused by the changing motion of the Earth. Just as the wind direction seemed to shift according to the direction the boat was moving, so starlight seemed to shift according to the direction of the Earth’s motion.

Bradley knew he had not found stellar parallax. What his observations demonstrated was that the Earth orbits the Sun and the speed of light is not infinite, both of which were already well accepted. Bradley named the effect he had found ‘aberration’
and
announced the discovery to the Royal Society in 1729. Aberration produces a 20½ arcsecond shift (see
Figure 4.4
) in the apparent positions of stars over a year. Bradley also found that the Earth wobbles due to the fact that its shape is not perfectly spherical, and he gave the wobble the name ‘nutation’. Aberration and nutation were not what Bradley had set out to find; but these discoveries were actually helpful steps on the road to discovering the tiny displacement of true annual stellar
parallax.
In any search for annual stellar parallax, one needed to take into account these other reasons why the positions of stars change with the seasons. The negative side of Bradley’s discoveries was even more significant. He had shown that the parallaxes of stars could not be more than one second of arc. Had they been as large as one arcsecond, he knew he would have been able to detect them. This meant stars were much further away than was generally supposed.

In 1742, Bradley succeeded Halley as Astronomer Royal.

Figure 4.4

The arcsecond originated as an ancient Mesopotamian measurement. A circle has 360 degrees. Each degree can be divided into 60 minutes of arc or ‘arcminutes’. Each minute of arc can be divided into 60 seconds of arc or ‘arcseconds’.

The measurement of an arcsecond isn’t a measurement of true size. If you hold your finger up at arm’s length against the sky, its width covers about two degrees of arc. But this finger held at arm’s length would cover a branch on a nearby tree, the Concorde flying overhead, the entire Moon, or (out of sight with the naked eye) an enormous number of galaxies. Clearly not all objects in the sky that have the ‘angular size’ of two degrees of arc are the same true size. How ‘large’ two degrees of arc are depends upon how deep into space you’re looking. The Moon’s angular size is about half a degree of arc. The Sun’s angular size is approximately the same as the Moon. Yet these two bodies are definitely not the same true size.

In the picture below, each of the circles has the same angular size viewed from Earth, covering the same number of degrees of arc, yet they are not the same true size. (Recall Aristarchus’s study of the Sun and Moon, shown in
Figure 1.4.
)

One arcsecond is the angular size the width of your finger would have if you were able to hold your finger up about 5,000 feet (1,500 metres) above you.

The 18th century saw a rapid increase in the number of observatories in Europe. Among the sciences, only in medicine were there more people professionally involved in research. Not all of them were peering through telescopes, for the meticulous cataloguing of what others found required many clerical workers. Funding came from governments, universities, scientific bodies, even religious societies – the expense often justified in terms of the practical spin-off for navigation, mapping and surveying, and the prestige such institutions gave to their sponsors.

The educated public loved astronomy and followed it avidly. Travelling lecturers drew large audiences, less technical books were best-sellers, telescopes for amateur use sold well, as did globes. Natural theology – the argument that nature and the harmony of the universe are eloquent proof of the existence of God and tell us what God is like – was preached from many pulpits, given voice in hymns and secular poetry, and favourably discussed in academic circles. Though astronomy had ceased to be a required part of the curriculum in most universities, it was widely considered essential to a proper gentlemanly education.

In England, the Royal Observatory continued to be financed from government coffers, a considerable investment. The Observatory’s demand for equipment as well as the Royal Society’s great interest in the improvement of all scientific instruments, including telescopes, helped support and encourage
a
healthy local optical industry. The Industrial Revolution, which started in England in the 18th century, brought advances in the design and construction of machines and in precision engineering in general. Astronomy reaped enormous benefits from this progress and also, with the increasing expertise of its instrument-makers and demand for their products, contributed substantially to it. The finest telescopic instruments came from England, and English manufacturers were suppliers to all Europe, setting the standard and style of the profession. Earlier astronomers had often designed and built their own instruments, and some of the more notable among them would continue to do so, but increasingly there began to be a division between those who manufactured telescopes and those who used them. Telescope builders were not necessarily considered a lower breed than practising astronomers. Some were elected to the Royal Society.

In the early 19th century, the Industrial Revolution spread to Europe and the United States. London opticians were still producing reflecting telescopes, the type pioneered by Newton and others in the late 17th century (see
Figure 4.5
), at affordable prices. English amateurs were making some excellent instruments themselves. However, Britain’s Prime Minister William Pitt and his government dealt London’s optical industry an almost mortal blow by imposing a punitive tax first on windows and a little later on all glass. After Swiss glass-maker Pierre Guinand moved from Switzerland to Munich in 1804, bringing with him a new method of mixing molten glass, Germany took the lead in refining the art of telescope design and manufacture. Bavarian-born Josef von Fraunhofer, who had worked for a time as Guinand’s assistant, greatly improved the refracting telescope (again, see
Figure 4.5
). Friedrich Wilhelm Bessel pioneered the use of the meridian circle, which made it possible to measure two coordinates of a star at the same time, improving greatly the accuracy of observations, as did advances in the design of astronomical clocks. Mathematician Karl
Friedrich
Gauss, then only 18 years old, at Göttingen in 1804, invented the method of ‘least squares’ which enabled an astronomer to choose the best observations in a less arbitrary manner.

Figure 4.5

This sketch shows the basic difference between a refracting and a reflecting telescope. There are many varieties of each.

A refracting telescope.

The large lens (object glass) in one end gathers light from the stars and bends or ‘refracts’ it down the length of the tube to focus on a smaller magnifying lens (eyepiece).

A reflecting telescope.

A curved mirror at the bottom end of an empty tube gathers light from the stars and reflects it to focus on a second mirror suspended at the first mirror’s focal point. The second mirror reflects the image to the eyepiece.

It’s possible to make a reflecting telescope much larger than a refracting telescope, because the large mirror can be supported from the back as a lens cannot.

Finally, by the late 1830s, improved technology and theoretical understanding had converged to the extent that it was possible for three astronomers to detect annual stellar parallax. Clearly it was a discovery whose time had come. The three measurements occurred independently but almost simultaneously.

Friedrich Wilhelm Bessel, in Königsburg, Germany, was the first to announce his findings, in 1838. Reasoning that proper motion, rather than brightness, might be the most significant indicator of which stars are nearest, he chose 61 Cygni, a dim star with a large proper motion (5.2 arcseconds a year). (See
Figure 4.4.
) The figure at which he arrived for its annual parallax was 0.3136 arcseconds. Knowing the distance the Earth had travelled in its orbit in order to produce this displacement allowed him to calculate the distance from the Earth to the star – 3.4 parsecs (11.2 light years) – which is, for comparison, 600,000 times greater than the distance from the Earth to the Sun. Bessel’s measurement was a considerable step up on the cosmic distance ladder.

Meanwhile Thomas Henderson from Scotland, observing from South Africa, chose to study Alpha Centauri. He made his choice on the basis of brightness rather than proper motion. Alpha Centauri is the third brightest star in the night sky. Though Henderson actually measured Alpha Centauri’s parallax before Bessel measured 61 Cygni’s, Henderson didn’t announce his results until he got back to Britain early in 1839 – thus losing out to Bessel in the pages of history. A year later, Friedrich von Struve, born in Germany but working in Tallin (now in Estonia), announced that he had measured the parallax for Alpha Lyrae (aka Vega), the fifth brightest star in the night sky.

The parallaxes measured for all three of these stars were small:

For 61 Cygni, a parallax of 0.3136 arcseconds (see
Figure 4.4
), a distance from us of 3.4 parsecs (11.2 light years). We now know that 61 Cygni is a double star.