The Story of Astronomy (11 page)

In 1635, at the age of 17, Jeremiah Horrocks returned to his native Lancashire. He had decided that what he wanted from his life was to be able to study the stars and the planets. He needed books on astronomy to achieve his aims and he also needed instruments. The most important astronomical instrument of the day was the telescope; it had been in use for about 20 years and was beginning to become far more readily available. Horrocks tells us that he purchased a “half crown” telescope, probably at a local fair. He knew that better instruments were available, however, and in May 1638 he wrote that
“I have at last obtained a more accurate telescope.”

We now discover that Jeremiah Horrocks was more than simply an astronomer. He was also a poet. The most prominent astronomer before this time who could also claim to be a poet was the philosopher Omar Khayyam (1044–1122). But when it came to putting his thoughts into verse Jeremiah Horrocks was the equal of his Persian predecessor. He was very thrilled with his new telescope and wrote about it thus:

Divine the hand which to Urania's power
Triumphant raised the trophy, which on man
Hath first bestowed the wondrous tube by art
Invented, and in noble daring taught
His mortal eyes to scan the furthest heavens.
Whether he seek the solar path to trace,
Or watch the nightly wanderings of the Moon
Whilst at her fullest splendour, no such guide
From Jove was ever sent, no aid like this
In brightest light such mysteries to display;
Nor longer now shall man with straining eye
In vain attempt to seize the stars. Blest with this
Thou shalt draw down the Moon from heaven, and give
Our Earth to the celestial spheres, and fix
Each orb in its own ordered place to run
Its course sublime in strict analogy.

During his time at Cambridge, Horrocks corresponded with Herbert Gellibrand (1597–1637), the professor of astronomy at Gresham College in London. Gellibrand, acting in good faith, suggested to Horrocks that he purchase a copy of a book by a Belgian astronomer called Philip Lansberg (1561–1632). Horrocks followed this advice, and he spent the next year trying to fit his observed motions of the planets to Lansberg's tables. Horrocks met with no success in this endeavor, but in 1636 he befriended a fellow amateur astronomer called William Crabtree (1610–44), who was working at Broughton near Manchester. It was Crabtree who suggested to him that he use the
Rudolphine Tables
, the work of Johannes Kepler and Tycho Brahe.

The incident illustrates how long it took for scientific works to circulate in the early 17th century. Gellibrand apparently knew nothing of the works of Kepler and Brahe, or he would certainly not have recommended Lansberg's tables to Jeremiah Horrocks. We now know that the
Rudolphine Tables
were far in advance of anything else available at that time, but the first people in England to use them were the amateur astronomers Jeremiah Horrocks and William Crabtree. The Copernican system also took a long time to circulate. The Ptolemaic system was still in use by the astrologers at this time, but Horrocks had little regard for it. He soon discovered that the
Copernican system was far superior and he put his feelings into verse:

Why should'st thou try, O Ptolemy, to pass
Thy narrow-bounded world for aught divine?
Why should thy poor machine presume to claim
A noble maker? Can a narrow space
Call for eternal hands? Will thy mansion
Suit great Jove? or can he from such a seat
prepare his lightnings for the trembling Earth?
Fair are the gods you frame forsooth! nor vain
Would be their fears if giant hands assailed them.
Such little world were well the infant sport
Of Jove in darker times; such toys in truth
His cradle might befit, nor would the work
In after years have e're been perfected,
When harlot smiles restrained his riper powers.

In 1639, at the age of 21, Jeremiah Horrocks left his home in Toxteth. He moved to the small village of Much Hoole about 18 miles (29 km) north of Liverpool. Nobody has been able to find the reason for Horrocks' move. Some have suggested that he was offered the post of curate at the chapel of Much Hoole, while others have suggested that he was employed as a tutor at Carr House in
Bretherton. It would be fascinating to discover that the reason was a romantic attachment, but there is no solid evidence to support this theory, either. All we know is that from the summer of 1639 his correspondence to his friend William Crabtree was addressed from Much Hoole and not from Toxteth.

In a survey taken in 1641 the population of Much Hoole was recorded as 235 adults. The choice of residences was limited, with the vernacular-style farmhouses being low and thatched, with smoking peat fires. The dwellings usually had pigs and chickens running in and out of the doorway. They were a far cry from the ideal residence for an educated astronomer. Apart from the church itself the only building in the village suitable for an observatory was Carr House at Bretherton, the home of the Stones family. This was situated about half a mile (about 0.8 km) away from the church. Much Hoole is the last place where we might expect to meet merchants from great trading centers like London and Amsterdam, but there is sometimes a tendency to overplay the isolation of the English village. Above the doorway at Carr House is an inscription in stone that proves the trading connections to be a fact:

Thomas Stones of London haberdasher and Andrewe Stones of Amsterdam marchant hath builded this howse
of their own charges and giveth the same unto their brother John Stones: Ano Domini 1613 Lavs.

The Stones family were evidently haberdashers. There is a strong connection between haberdashery, drapery and cloth dealing. Horrocks' contacts John Worthington and William Crabtree were both involved in the clothing industry.

As Horrocks continued his observations of the planets he discovered to his great delight that Venus seemed to be on course for a conjunction with the Sun—in other words, the planet seemed to be following a path that would take it across the face of the Sun. This was an event so rare that it had never been recorded before. Horrocks knew that his observations might be marred by cloudy skies, so he asked both William Crabtree and his brother at Toxteth to try to make the observation as well. He asked William Crabtree to inform Henry Gellibrand in London about the event, but it appears that the request did not arrive in time.

The year was 1639, and from his calculations Horrocks did not expect the transit of Venus to take place before 3 o'clock on the afternoon of November 24. It appeared from the tables of other astronomers, however, that it
might occur somewhat sooner, and in order to avoid the possibility of disappointment, he began to observe the Sun from about midday on November 23. As expected, he saw no sign of the image of Venus. The next day he continued his vigil until he tells us he was
“called away by business of the highest importance, which could not with propriety be neglected.”
This phrase has given rise to the possibility that he had to give a sermon just as the transit was about to begin. He tells us nothing else about this important business, but since it was Sunday it is reasonable to deduce that he needed to perform a Sabbath duty of some kind. Nevertheless, the task cannot have been too time-consuming for he was back at his telescope again in just over an hour. This would have been just enough time for Horrocks to get to the church, perform his duties and then return to his observations.

When Horrocks returned to his observations he was overjoyed to see that a dark, round spot was already fully entered upon the image of the Sun. It was without doubt the silhouette of Venus that he had been anticipating. He did not want to be accused of seeing nothing more than a sunspot, however—even though that observation alone would have put him in the company of his mentor Johannes Kepler. Thus later, Horrocks went to great lengths in his treatise to explain that Venus appeared on the Sun's disc as a perfectly circular dark spot. As well as being a
perfect circle, the spot moved across the Sun much faster than a sunspot; there was no doubt that he was observing the planet Venus. He measured the size of the dark spot as accurately as he could and he drew it in the exact position it appeared on his image of the Sun. He drew two more images and recorded the times as 3.15 pm, 3:35 pm and 3:45 pm. The image moved by one diameter in the first 20-minute interval, but slightly less in the second interval. Then the Sun set over the Ribble marshes. He knew the value of accurate measurements and he wanted his observation to be as precise as it could possibly be. He was working to angles within seconds of arc. He estimated that the diameter of Venus was 1′ 12″ and he estimated his error was 4 or 5 seconds of arc.

At Broughton, William Crabtree was also trying to observe the event. He had been very unfortunate with the weather, however. The skies were overcast for the greater part of the day and the Sun was not visible. Crabtree had almost given up on the task when, a little before sunset, at about 35 minutes past 3 o'clock, at the same time as Horrocks was making his observation, the Sun suddenly burst out from behind the clouds. Crabtree rushed into his house and he began to observe at once. To his great joy he saw the rare spectacle of Venus passing across the
Sun's disc. In a passage that does much to illuminate the personalities of both men Horrocks recorded the feelings of his friend:

Rapt in contemplation, he stood for some time motionless, scarcely trusting his own senses, through excess of joy; for we astronomers have as it were a womanish disposition, and are overjoyed with trifles and such small matters as scarcely make an impression upon others; a susceptibility which those who will may deride with impunity, even in my own presence, and, if it gratify them, I too will join in the merriment. One thing I request: let no severe Cato be seriously offended with our follies; for, to speak poetically, what young man on Earth would not, like ourselves, fondly admire Venus in conjunction with the Sun
. Pulchritudinem divitiis conjunctam?
[beauty conjoined with wealth] What youth would not dwell with rapture upon the fair and beautiful face of a lady whose charms derive an additional grace from her fortune?

Jeremiah Horrocks wrote up his account of the transit of Venus, making full use of his careful observations. Firstly he was able to calculate more accurate values for the orbit of Venus and secondly he had a very good estimate of the angular diameter of Venus at the planet's closest
approach to the Earth. But Horrocks went much further than this. Some may say in fact that he went too far, for he used his result to try to estimate the distance from Earth to Venus, and hence calculate the scale of the solar system. To assist him in his endeavors, Horrocks had at his disposal an account of the transit of Mercury, observed a few years before him by the French astronomer and mathematician Pierre Gassendi (1592–1655).

He then formulated what we might call Horrocks' hypothesis—that every planet has the same angular diameter when it is seen from the Sun. Horrocks knew that some of the planets did not seem to fit his hypothesis, the most obvious being Mars, which was much too small. The hypothesis never reached the status of a law but it was not an unreasonable postulate. It gave a value for what Horrocks called the parallax of the Sun, a measure closely related to what we would call the astronomical unit—the mean distance from the Earth to the Sun.

The table shown overleaf was compiled by Horrocks to enable him to compare his estimate of the solar parallax with those made by other astronomers. In this table Horrocks shows the solar parallax in terms of radius and not diameter, and they are therefore only half of the values quoted elsewhere. An extra column is
included to show the distance in millions of kilometers, and the bottom line has been added to show the modern accepted values.

Thus, with the transit of Venus and his estimate of the solar parallax, Jeremiah Horrocks was the first Englishman to contribute significantly to the history of astronomy. But his main contribution was to come. He wanted to find a better method of calculating the orbit of the Moon. Apart from briefly visible bodies such as comets and meteors, the Moon was the fastest-moving object in the
sky seen by early astronomers. Many had tried to explain its motion but without success. It was well known that if an accurate theory of the Moon's motion could be found then it would be an invaluable aid to navigation. Ptolemy had attempted to tackle the task, and his system at least enabled astronomers to forecast an eclipse, but if his theory had been correct then the Moon would have appeared about four times larger than it actually was at some points of its orbit. Horrocks knew that as a first approximation the Earth–Moon system was similar to a sun–planet system. The Earth was at one focus of an ellipse. The ellipse was perturbed, however, by the gravity of the Sun. Horrocks proposed a system whereby the orbit of the Moon oscillated throughout the year, and he set about trying to find the constants of this oscillation. His theory of the motion of the Moon was the most advanced of his time. It was used by the first Astronomer Royal, John Flamsteed (1646–1719), and his successors at the Greenwich Observatory, and it survived for almost a century before a better theory was found.