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.
An Observation Made Elsewhere
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.
Horrocks’ Table
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.
The Orbit of the Moon
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.
Posthumous Fame
Unfortunately, Jeremiah Horrocks died suddenly in 1641. Many of his works were lost in the chaos that ensued during the English Civil War (1642–9). Luckily his account of the transit of Venus survived and it was published in 1662. Thereafter, many of his other surviving works also found a wider audience, and his discoveries were acknowledged and acclaimed by the Royal Society as well as by some of the world’s greatest astronomers, including Isaac Newton in his third book of the Principia. John William Herschel was so impressed that he described Horrocks as “the pride and boast of British astronomy.”
Johannes Hevelius—Map Maker of the Heavens
Polish-born Johannes Hevelius (1611–87) was one of the leading observational astronomers of the 17th century. He came from a wealthy family of brewing merchants in Danzig, where he was also a town counselor and later mayor of Danzig.
However, his main interest was in astronomy. He built an observatory on the roofs of three connecting houses, the pattern of which closely followed that devised by Tycho Brahe on Hven. As well as numerous other astronomical instruments (all described in his publication of 1673 Machina coelestis) his most notable achievement was the construction of a 45-meter (150 ft) telescope of his own design.
In 1647, after ten years of observation, he produced detailed maps of the Moon, published in his work Selen
ographia, including diagrams of phases and first estimates of lunar mountain heights. He also made many observations of comets published in Prodomus Cometicus(1665) and Cometographia (1668). During his life he mapped positions for 1564 stars, and these were eventually published posthumously in 1690 by his second wife, Elizabeth Margarethe, in a catalog, Prodomus Astronomiae, and an atlas, Uranographia.
9
THE CLOCKWORK UNIVERSE
It was Christmas Eve 1642 when Hannah Newton first felt the birth pains. Her child was not expected until January or February but the contractions became regular and consistent. The child was a boy, born on Christmas Day. Hannah was a widow. She had experienced marriage, death and birth all within the space of nine months. She chose to call the child Isaac, after his deceased father.
The Newtons lived at Woolsthorpe Manor in the village of Colsterworth in Lincolnshire. It lay just off the Roman road of Ermine Street, which was at that time a part of the Great North Road from London to Scotland. The young Isaac Newton (1642–1727) was precocious; he enjoyed reading and he enjoyed constructing things. He received the rudiments of an education, but at the age of 14, when he was old enough to help with the farm, his mother took him away from school. It was a mistake. Young Isaac was discovered reading under the hedge when he should have been tending to the sheep. Sometimes he wandered around in a dream. On one occasion he walked all the way home from Grantham holding a horse’s bridle. He was so wrapped up in his own thoughts he had not noticed that the horse—which should have been attached to the bridle—had gone its own way. Isaac’s mother despaired of her son. He would never make a farmer she decided, so after discussing the matter with her relatives and with the schoolmaster at Grantham she agreed to let him try for entry to Cambridge where he could train to become a country parson.
From Farmhand to Scholar
Isaac Newton was sent back to school to prepare for a university education. He worked hard and easily qualified for entrance to Trinity College at Cambridge. In the autumn of 1661 he arrived to claim his place at the university. At first he had some problems integrating into undergraduate life, but he was fortunate to meet up with another student called John Wickens. The two decided to share a room together, and Wickens was happy to put up with the absentminded Newton and to assist with the many experiments conducted by his room-mate.
Newton obtained his Bachelor of Arts degree, but his interests ranged far outside the curriculum and he did not pass with distinction. He was able to remain at university, however, to study for a Master’s degree. In 1665 a great plague broke out in London and hundreds of people were dying by the week. Cambridge University took no chances; the authorities closed the colleges down for fear of the plague and the students were sent home to fend for themselves. Newton ended up spending most of his time at his mother’s rural home. The enforced seclusion in the countryside seemed to have a beneficial effect on him, however, and without the distractions of the university he became totally absorbed in his own ideas and able to continue with his many experiments.
Experiments with Light
In one experiment, conducted in 1666, he acquired a glass prism and passed a ray of sunlight through it. The prism split the sunlight into the colors of the spectrum. These colors had been witnessed by many before him, but Newton went on to make great discoveries about the nature of light. He described his feelings in his own words:
I procured me a Triangular glass-Prisme, to try therewith the celebrated Phenomena of Colorus. And in order thereto having darkened my chamber, and made a small hole in my window-shuts, to let in a convenient quantity of the Sun’s light, I placed my Prisme at his entrance, that it might thereby be refracted to the opposite wall. It was at first a very pleasing divertisment, to view the vivid and intense colours produced thereby; but after a while applying myself to consider them more circumspectly, I became surprised to see them in an oblong form; which according to the received laws of Refraction, I expected should be circular.
As a result of his experiments with light, Newton discovered the reason why the telescopes of his time always seemed to produce colored fringes around the image. He realized that the problem could be avoided by making a telescope that used a mirror, rather than a lens, to collect and focus the light. He actually constructed a telescope on these principles, and he was so pleased with his instrument that he sent it to the Royal Society in London for their perusal. This, and his treatise on light, both represented a step forward in the science of astronomy and the Royal Society were sufficiently impressed by these contributions to welcome Isaac Newton as one of their members.
Gravity and Mechanical Matters
But the nature of light was only one of Newton’s amazing findings during the years 1665 and 1666. The story that Newton’s theory of gravitation was inspired by the fall of an apple seems to be apocryphal, yet it must be founded in truth, for in old age he told the story to his first biographer William Stukeley (1687–1765) who related it as follows:
After dinner, the weather being warm, we went into the garden and drank tea, under the shade of some apple trees, only he and myself. Amidst other discourse, he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. It was occasion’d by the fall of an apple, as he sat in a contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself. Why should it not go sideways or upwards, but constantly to the Earth’s centre? Assuredly the reason is, that the Earth draws it. There must be a drawing power in matter: and the sum of the drawing power must be in the Earth’s centre, not in any side of the Earth. Therefore does the apple fall perpendicularly, or towards the centre. If matter thus draws matter, it must be in proportion of its quantity. Therefore the apple draws the Earth, as well as the Earth draws the apple. That there is a power, like that we here call gravity, which extends itself thro the universe.
Newton contemplated the force of gravity. He was convinced that every object in the universe had a gravitational attraction for every other object, and he felt sure that this force was governed by an inverse square law. The apple on the tree was attracted by the gravity of the Earth. Was the Moon in her passage across the sky governed by the same rule? Was the Moon drawn to the Earth by the same gravity as the apple? Newton did some calculations. He was not sure about the size of the Earth, but he found that the calculations fitted the theory “pretty nearly.” He was also unsure about which measurement he should take with regard to the distance from the apple to the Earth. Was it the few feet from the ground that he could easily measure? Was it the distance to the center of the Earth? Was it the distance to an unknown point somewhere under the Earth? He did not know the answer but he kept his calculations and his thoughts for future reference.
He also began to think about mechanics. He thought about the nature of force and acceleration. He wondered about how they were related and whether the laws of mechanics were the same on Earth as in the heavens. Why did the heavens enjoy perpetual motion when everything on Earth ran down because of friction and air resistance? The mathematics to answer his questions did not exist. The equations involved quantities in a state of flux, and he developed a method called “fluxions” to handle the problems. He had invented what we now call calculus. Newton himself described his years at Woolsthorpe:
In the beginning of the year 1665 I found the Method of approximating series and the Rule for Reducing any dignity of any Binomial into such a series. The same year in May I found the method of Tangents of Gregory and Slusius, and in November had the direct method of fluxions and the next year in January had the Theory of Colours and in May following I had entrance into ye inverse method of fluxions. And the same year I began to think of gravity extending to the orb of the Moon, and having found out how to estimate the force with which a globe revolving within a sphere presses the surface of the sphere, from Kepler’s rule of the periodical times of the planets being in a sesquialterate proportion of their distances from the centre of
their orbs I deduced that the forces which keep the planets in their orbs must [be] reciprocally as the squares of their distances from the centres about which they revolve: and thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth and found them to answer pretty nearly. All this was in the two plague years of 1665 and 1666, for in those days I was in the prime of my age for invention, and minded mathematics and philosophy more than at any time since.
His mathematical genius was soon appreciated at Cambridge, and at the recommendation of his tutor Isaac Barrow (1630–77), Newton was offered the seat of the Lucasian Professor of Mathematics. For a time his income was secure. He retired into his ivory tower and worked on his many other interests, in particular theology and alchemy. By 1666 he had solved the main mathematical problems relating to gravitation, but he never prepared his work for publication, and for nearly 20 years these momentous ideas about the universe existed only in his head.
What is Gravity?
Gravity is the force that draws everything to the ground—a fact that was well known to people long before Isaac Newton was even born. However, Newton succeeded in explaining how the force of gravity works, not just on Earth but throughout the whole universe. In the 1660s, in his garden at Woolsthorpe, Newton observed an apple falling to the ground. He began to wonder if the force on the apple was the same as the force that held the Moon in orbit around the Earth. His calculations led him to believe that it probably was, although it was nearly 20 years before he published his ideas. The force acting on the Moon and an object such as an apple are the same, and Newton was able to assert that every particle of matter in the universe exerts a gravitational attraction on every other particle of matter in the universe.
The Story of Astronomy Page 9