Galileo Galilei

Galileo was born on February 15, 1564 in Pisa. By the time he died on January 8, 1642 (but see problems with the date, Machamer 1998, pp. 24–5) he was as famous as any person in Europe. Moreover, when he was born there was no such thing as ‘science’, yet by the time he died science was well on its way to becoming a discipline and its concepts and method a whole philosophical system.

Galileo and his family moved to Florence in 1572. He started to study for the priesthood, but left and enrolled for a medical degree at the University of Pisa. He never completed this degree, but instead studied mathematics notably with Ostilio Ricci, the mathematician of the Tuscan court. Later he visited the mathematician Christopher Clavius in Rome and started a correspondence with Guildobaldo del Monte. He applied and was turned down for a position in Bologna, but a few years later in 1589, with the help of Clavius and del Monte, he was appointed to the chair of mathematics in Pisa.

In 1592 he was appointed, at a much higher salary, to the position of mathematician at the University of Padua. While in Padua he met Marina Gamba, and in 1600 their daughter Virginia was born. In 1601 they had another daughter Livia, and in 1606 a son Vincenzo.

It was during his Paduan period that Galileo worked out much of his mechanics and began his work with the telescope. In 1610 he published The Starry Messenger, and soon after accepted a position as Mathematician,a non-teaching post at University of Pisa and Philosopher to the Grand Duke of Tuscany. A facsimile copy of The Library of Congress’ manuscript of The Starry Messenger and a symposium discussing details about the manuscript, may be found in Hessler and DeSimone 2013. Galileo had lobbied hard for this position at the Medici court and even named the moons of Jupiter, which he discovered, after the Medici. There were many reasons hewanted move, but he says he did not like the wine in the Venice area and he had to teach too many students. Late in 1610, the Collegio Romano in Rome, where Clavius taught, certified the results of Galileo’s telescopic observations. In 1611 he became a member of what is perhaps the first scientific society, the Academia dei Lincei.

In 1612 Galileo published a Discourse on Floating Bodies, and in 1613, Letters on the Sunspots. In this latter work he first expressed his position in favor of Copernicus. In 1614 both his daughters entered the Franciscan convent of Saint Mathew, near Florence. Virginia became Sister Maria Celeste and Livia, Sister Arcangela. Marina Gamba, their mother, had been left behind in Padua when Galileo moved to Florence.

In 1613–4 Galileo entered into discussions of Copernicanism through his student Benedetto Castelli, and wrote a Letter to Castelli. In 1616 he transformed this into the Letter to the Grand Duchess Christina. In February 1616, the Sacred Congregation of the Index condemned Copernicus’ book On the Revolution of the Heavenly Orbs, pending correction. Galileo then was called to an audience with Cardinal Robert Bellarmine and advised not to teach or defend Copernican theory.

In 1623 Galileo published The Assayer dealing with the comets and arguing they were sublunary phenomena. In this book, he made some of his most famous methodological pronouncements including the claim the book of nature is written in the language of mathematics.

The same year Maffeo Barberini, Galileo’s supporter and friend, was elected Pope Urban VIII. Galileo felt empowered to begin work on his Dialogues concerning the Two Great World Systems. It was published with an imprimatur from Florence (and not Rome) in 1632. Shortly afterwards the Inquisition banned its sale, and Galileo was ordered to Rome for trial. In 1633 he was condemned. There is more about these events and their implications in the final section of this article, Galileo and the Church.

In 1634, while Galileo was under house arrest, his daughter, Maria Celeste died (cf. Sobel 1999). At this time he began work on his final book, Discourses and Mathematical Demonstrations concerning Two New Sciences. This book was smuggled out of Italy and published in Holland. Galileo died early in 1642. Due to his conviction, he was buried obscurely until 1737.

 

Edmund Halley

Born November 8th, 1656 in Haggerston, England to a prosperous family, young Edmund Halley received a private education before enrolling in St. Paul's school. While in school, Edmund excelled in mathematics and astronomy. At the young age of 17, he enrolled at The Queen's College, Oxford to study under the Astronomer Royal of the time John Flamsteed

By 1676, Halley had dropped out of university to begin making his own contributions to the field of astronomy. After publishing his star chart of the southern oceans in 1678, he was granted a Master of Arts from Oxford by decree of King Charles II. Anchoring his position as a great astronomer, Halley was also elected fellow of the Royal Society as one of the youngest members.

At the young age of 22, he sailed to St. Helena where he discovered a star cluster in Centaurus and mapped the stars of the southern hemisphere, allowing sailors to navigate the world's oceans. Halley devised a clever way to calculate the distance from the Earth to the Sun. By carefully timing how long it took Venus to cross the Sun's disk, he was able to give a real distance to the astronomical unit; about 93 million miles. He used this newfound information to accurately calculate the size of the solar system for the first time.

Pouring over ancient Greek recordings of stars, Halley compared their observations to what he saw in the night sky some 1800 years later. He discovered that the stars the Greeks had cataloged were not in the same position as the stars he observed, but had moved. Halley concluded that the stars are not fixed in one position as once thought, their motion only apparent to the observer after many centuries.

Continuing his pioneering work in observational astronomy, Halley published in 1705 A Synopsis of the Astronomy of Comets, in which he described the parabolic orbits of 24 comets that had been observed from 1337 to 1698. He showed that the three historic comets of 1531, 1607, and 1682 were so similar in characteristics that they must have been successive returns of the same visitant—now known as Halley’s Comet—and accurately predicted its return in 1758.

His Early Career and Travels

In 1675, Halley became the assistant for John Flamsteed who was first Astronomer Royal at Greenwich Observatory. One of Halley’s many tasks was to assign numbers to stars using Flamsteed’s number system for identification and cataloguing purposes. A year later, he travelled to the volcanic tropical island Saint Helena, in the South Atlantic Ocean. He brought with him a large sextant and some telescopic sights so he could set up an observatory and study and catalogue the stars in the Southern Hemisphere. It was during his stay in St. Helena that he carefully observed the transit of Mercury across the sky. He realized that Venus, moving in the same way, could then be used to calculate the size of the Solar System.

Halley returned to England in 1678 and a year later he went to Danzig at the request of the Royal Society to help resolve a dispute between Robert Hooke and Johannes Hevelius. As Hevelius did not utilize a telescope in his observations, Robert Hooke questioned his findings. Halley stayed with Hevelius to observe his findings and verify his conclusions

That same year in 1679, Halley published “Catalogue Stellarum Australium”, a catalogue of the Southern Hemisphere stars which he observed while in St. Helena. His publication was so extensive that it included 341 stars that could be viewed only in the Southern Hemisphere. Flamsteed gave him the title “The Southern Tycho” in reference to the well-known sixteenth century astronomer, Tycho Brahe. Halley also attained his Master’s Degree from Oxford and was elected as a Royal Society Fellow aged 22.

Halley conducted many lunar observations which took up most of his time. Aside from his lunar studies, he also became interested with problems relating to gravity. One issue in particular that concerned him was finding a proof for the laws of planetary motion. In 1684, he travelled to Cambridge to talk the issue over with Sir Isaac Newton, only to find out that Newton had already managed to solve the problem, stating that the planets’ orbits would be elliptical. Halley naturally wanted to see the calculations Newton used but Newton wasn’t able to locate them. Newton then wrote a short treatise entitled “On the Motion of Bodies in an Orbit” in 1684 which explained his calculations. This work was later expanded on by Newton, becoming the famous work “Philosophiæ Naturalis Principia Mathematica” in 1687 which Halley helped to publish.

Later Life and Comet Predictions

In 1698, he was given permission to take command of the sailing ship Paramour and travel to the South Atlantic Ocean to find out more about the laws that govern the variation of the compass. The first expedition was cut short due to unrest among the crew and a second expedition began in 1699. His magnetic charts of Atlantic Ocean and parts of the Pacific Ocean were published in “General Chart of the Variation of the Compass” in 1701.

In 1704, Halley finally became Savilian Professor of Geometry at the University of Oxford, after missing out on this prestigious appointment earlier in his career. He published “Synopsis Astronomia Cometicae” (A Synopsis of the Astronomy of Comets) in 1705. This work detailed the parabolic orbits of 24 comets that had been observed from 1337 to 1698. It also stated that the comet sightings of 1531, 1607, and 1682 were of the same comet, which orbits the sun every 75-76 years and he correctly predicted it would return in 1758. When the comet did return it became known as Halley’s Comet.

He devised a method of observing the transit of Venus across the sun which would next be observed in 1761 and 1769. These observations would then be used to make an accurate calculation of the distance from the Earth from the sun.

Halley became Astronomer Royal at Greenwich Observatory, succeeding John Flamsteed in 1720.

His death

He died on January 14, 1742 aged 86 and sadly, did not live to see the return of the comet that was named in his honor. Halley’s Comet will next appear in the night sky in the year 2062.

 

Tycho Brahe

Tycho Ottesen Brahe was born into a highly aristocratic, very wealthy family on December 14, 1546. Tycho was the second of the couple’s 12 children. Something rather remarkable happened to Tycho in his second year of life – he was kidnapped by his uncle and aunt, Jørgen Brahe and Inger Oxe, when his parents were away from home. Tycho’s uncle and aunt were childless, and they believed that Jørgen was entitled to a lawful son and heir to his estates. Tycho’s natural parents eventually agreed to this, so Tycho was raised by his uncle and aunt as if he were their own son.

  Tycho’s foster mother, Inger, had come from an academic family and she persuaded her husband that Tycho should receive an academic education. Tycho began school aged six or seven, a grammar school where he probably learned the classical languages, mathematics, and the Lutheran religion.

In April 1559, aged 12, Tycho matriculated at the University of Copenhagen. He studied a general classical curriculum for three years, during which time he became increasingly absorbed in astronomy. He bought a number of important books in the field, including Johannes de Sacrobosco’s On the Spheres, Peter Abian’s Cosmography, and Regiomantus’s Trigonometry.

Tycho’s interest in astronomy began with the solar eclipse of August 21, 1560. In Copenhagen this eclipse was barely noticeable – less than half of the sun was covered. The eclipse inspired Tycho not because it was spectacular, but because astronomers had predicted exactly when it would happen. Tycho was fascinated, and wanted to learn how he too could make predictions like this.

Using just a basic, fist-sized celestial sphere and string, Tycho discovered that tables of predictions of planet positions sourced from the works of both Ptolemy and Nicolaus Copernicus were rather unsatisfactory.

In August 1563, aged 16, Tycho began his first logbook of astronomical observations. He observed a one-in-twenty-year conjunction of Jupiter and Saturn, and again noted errors in both Copernicus’s and Ptolemy’s predictions. Using Ptolemy’s data tables, the conjunction timing was wrong by a month!

It became Tycho’s goal to produce truly accurate predictions of planetary positions based on accurate observations. Although he did not quite succeed in his ambition to make all his measurements accurate to within one arc minute, many of them did meet this standard, and his observations were a phenomenal five times more accurate than his peers made.

Tycho made his first significant discovery on November 11, 1572. Observing the night sky from an uncle’s home, Tycho was amazed to see a new light brighter than Venus in the sky.

He studied the new heavenly body for a year. He deduced that it was a star because, unlike closer bodies such as the planets, its position relative to the other stars did not change.

In 1573, Tycho’s name became well-known in astronomical circles when he published De nova stella – The New Star. Although other people had also observed the new star, Tycho published the most comprehensive study of it. Tycho’s new star gradually faded until, after a year, it was no longer visible to the naked eye.

The Great Comet of 1577 made people fearful, because comets were seen as bad omens.

Tycho recorded the comet’s positions between November 13, 1577 and January 26, 1578, after which he could no longer see it.

Tycho used Hipparchus’s parallax method to measure the comet’s distance from the earth.

Unfortunately, there was insufficient parallax for him to pin down the distance, but he was able to say that:-

The comet was much farther away from our planet than the moon is – at least six times as far. This refuted the popular idea that comets traveled within the earth’s atmosphere.

The comet’s tail always pointed away from the sun.

The comet’s path was associated with the sun, not the earth.

The comet had another far-reaching consequence for Tycho and science. It prompted him to begin making observations with a view to producing his own star catalogue to replace Ptolemy’s ancient work.

Tycho accurately recorded the positions of 777 stars by 1592, and he eventually amassed data for 1,006 stars. Tycho’s catalog was later worked on and published by Johannes Kepler. 

In Prague, Tycho gave Johannes Kepler a job as his assistant. Together, they began working on a new star catalog, but it was slow work. Tycho Brahe died aged 54 on October 24, 1601 in Prague. The catalog was eventually published by Kepler in 1627 as the Rudolphine Tables. These were by far the most accurate astronomical data tables ever published, with planetary data and 1,006 star positions. The majority of stars were cataloged to within one arc minute accuracy, which had been Tycho’s ambition.

Kepler's Laws of Planetary Motion

Kepler was assigned the task by Tycho Brahe to analyze the observations that Tycho had made of Mars. Of all the planets, the predicted position of Mars had the largest errors and therefore posed the greatest problem. Tycho's data were the best available before the invention of the telescope and the accuracy was good enough for Kepler to show that Mars' orbit would precisely fit an ellipse. In 1605 he announced The First Law:

Planets move in ellipses with the Sun at one focus.

The radius vector describes equal areas in equal times. (The Second Law)

Kepler published these two laws in 1609 in his book Astronomia Nova.

For a circle the motion is uniform, but in order for an object along an elliptical orbit to sweep out the area at a uniform rate, the object moves quickly when the radius vector is short and the object moves slowly when the radius vector is long.

On May 15, 1618 he discovered The Third Law:

The squares of the periodic times are to each other as the cubes of the mean distances.

This law he published in 1619 in his Harmonices Mundi . It was this law, not an apple, that led Newton to his law of gravitation. Kepler can truly be called the founder of celestial mechanics.

 

Nicolaus Copernicus

Nicolaus Copernicus was a Polish astronomer known as the father of modern astronomy. He was the first modern European scientist to propose that Earth and other planets revolve around the sun, or the Heliocentric Theory of the universe. 

Prior to the publication of his major astronomical work, “Six Books Concerning the Revolutions of the Heavenly Orbs,” in 1543, European astronomers argued that Earth lay at the center of the universe, the view also held by most ancient philosophers and biblical writers. 

In addition to correctly postulating the order of the known planets, including Earth, from the sun, and estimating their orbital periods relatively accurately, Copernicus argued that Earth turned daily on its axis and that gradual shifts of this axis accounted for the changing seasons. 

Nicolaus Copernicus was born on February 19, 1473 in Torun, a city in north-central Poland on the Vistula River. Copernicus was born into a family of well-to-do merchants, and after his father’s death, his uncle–soon to be a bishop–took the boy under his wing. He was given the best education of the day and bred for a career in canon (church) law. At the University of Krakow, he studied liberal arts, including astronomy and astrology, and then, like many Poles of his social class, was sent to Italy to study medicine and law. 

The cosmology of early 16th-century Europe held that Earth sat stationary and motionless at the center of several rotating, concentric spheres that bore the celestial bodies: the sun, the moon, the known planets, and the stars. From ancient times, philosophers adhered to the belief that the heavens were arranged in circles (which by definition are perfectly round), causing confusion among astronomers who recorded the often eccentric motion of the planets, which sometimes appeared to halt in their orbit of Earth and move retrograde across the sky. 

In the second century A.D., the Alexandrian geographer and astronomer Ptolemy sought to resolve this problem by arguing that the sun, planets, and moon move in small circles around much larger circles that revolve around Earth. These small circles he called epicycles, and by incorporating numerous epicycles rotating at varying speeds he made his celestial system correspond with most astronomical observations on record. 

The Ptolemaic system remained Europe’s accepted cosmology for more than 1,000 years, but by Copernicus’ day accumulated astronomical evidence had thrown some of his theories into confusion. Astronomers disagreed on the order of the planets from Earth, and it was this problem that Copernicus addressed at the beginning of the 16th century. 

Sometime between 1508 and 1514, Nicolaus Copernicus wrote a short astronomical treatise commonly called the Commentariolus, or “Little Commentary,” which laid the basis for his heliocentric (sun-centered) system. The work was not published in his lifetime. In the treatise, he correctly postulated the order of the known planets, including Earth, from the sun, and estimated their orbital periods relatively accurately. 

In “Six Books Concerning the Revolutions of the Heavenly Orbs,” Copernicus’ groundbreaking argument that Earth and the planets revolve around the sun led him to make a number of other major astronomical discoveries. While revolving around the sun, Earth, he argued, spins on its axis daily. Earth takes one year to orbit the sun and during this time wobbles gradually on its axis, which accounts for the precession of the equinoxes. 

Nicolaus Copernicus died on May 24, 1543 in what is now Frombork, Poland. He died the year his major work was published, saving him from the outrage of some religious leaders who later condemned his heliocentric view of the universe as heresy. 

It was not until the early 17th century that Galileo and Johannes Kepler developed and popularized the Copernican theory, which for Galileo resulted in a trial and conviction for heresy. Following Isaac Newton’s work in celestial mechanics in the late 17th century, acceptance of the Copernican theory spread rapidly in non-Catholic countries, and by the late 18th century the Copernican view of the solar system, it was almost universally accepted.