Part 3: Understanding the heavenly bodies – The rise of modern Astronomy

The study of how the stars, the Sun, Earth, Moon and the planets, i.e. the heavenly bodies, move across the sky is an important part of human history. One can even argue that civilization of any kind would never have developed without people’s understanding of how the stars move across the sky, how the Sun’s position during the year is related to the seasons, how the Moon changes its appearance during each month and when a Sun or Moon eclipse occurs. With this knowledge, people could move from nomadic lives to building more permanent settlements using agriculture to grow fruits and farming to rear cattle in large quantities. It also allowed them to navigate vast distances on land and sea and to begin trading with other settlements, thus creating the first known civilizations.

In the previous article, I continued the story of how many curious and clever individuals decoded step by step the heavenly bodies over the last centuries and millennia. We stopped our journey through time at the Renaissance period, or more precisely, at the 15^th century A.D. To this point, the so-called Ptolemy model of the universe, that is the solar system as we know it today and the stars you can see with the naked eye during a clear night sky, was the best model known to the western world. I am cautious to say the “western world” as there is evidence that the Mayan culture was an extremely sophisticated one that had developed a complicated solar model of their own. However, there are very few written records available to give a detailed picture and research is still ongoing. Since there was a great deal of communication between the West and the East and Far East during the ancient period and throughout the Middle Ages, some of the knowledge compiled in Asia and China finds its way back to the then known West, i.e. Europe.

The corrected picture - The Copernican revolution

The fantastic long term success of the Ptolemy model of the universe, as it lasted 1400 years, was due to its accurate prediction of the movements of the Sun, Moon, stars and the planets as well as that it fitted in the religious belief that the Earth was the center of the universe. To really understand its endurance for so long, another fact must be mentioned: there was no better model available which had the same accuracy. During the last centuries of the Middle Ages, Arabian astronomers, due to new techniques, became better in observing and recording the movement of the planets.

The steadily improved records started to show some irregularities of the planets’ movements which the existing model could not account for. This prompted an adjustment of the Ptolemy model and lead to adding more epicycles (see part 2 for more information) to the model. Over the centuries, the model ended up with 240 epicycles! This complicated the Ptolemy model so much so that according to tradition, the Spanish monarch Alphonso X (1221-1284), who initiated a massive revision of the tables recoding the planetary movements, is said to have complained: “If I had been present at the creation, I would have recommended a simpler design for the universe”.

Well, let’s see if we can find a less complicated model of the universe. To do so, we will have to go back in time to antiquity and ancient Greek.

Aristarchus of Samos – The Sun-centered universe

Around the year 260 B.C. the Greek astronomer Aristarchus from the island of Samos, proposed a radically different model of the universe known to the ancient Greeks and up to the time of the Renaissance. In his model, he puts the Sun in the center of the universe, not Earth. This kind of model is called heliocentric (from Greek helios meaning “Sun”). This may seem ‘trivial’ by modern standards but looking back, it was a gigantic and totally counter-intuitive move.

Figure 1: Aristarchus's 3rd century BC calculations on the relative sizes of the Earth, Sun and Moon, from a 10th century CE Greek copy.

Aristarchus was all alone with his idea. His contemporaries rejected the idea and so it was lost in time. The reason for adapting such a different view may have been on pure geometrical reasoning. Aristarchus discovered that by measuring the angles between the Moon, Earth and the Sun, the Sun was much bigger and further away from the Earth than the moon (see Figure 1). The Greeks were masters of geometry, the science of measuring distances and angles between objects. Geometry was a pure mathematical method; it was almost a religion. Since the Sun was much bigger than Earth, Aristarchus may have concluded that the Sun must be in the center and all the other spheres, that is, Earth, Moon, planets and stars, must revolve around it.

Nicolaus Copernicus – The challenger

It was not until the Renaissance when German-born Polish astronomer Nicolaus Copernicus (1473-1543) suggested an alternative to the Ptolemy geocentric model of the universe (see Figure 2). At the time of the Renaissance the Ptolemy model was becoming increasingly inaccurate in explaining the differences in the improved measuring of the planet’s orbits (see Part 2 for explanation of orbit). Many astronomers became tired of revising the Ptolemy model to fit the new data which involved difficult calculations. In this climate of desperately desiring a change, Copernicus adapted the heliocentric model of the universe conceived by Aristarchus more than 1200 years earlier (see above).

Figure 2: Nicolaus Copernicus, Born February 19, 1473, Toruń (Thorn), Royal Prussia, Poland; Died May 24, 1543 (aged 70), Frombork (Frauenburg), Warmia, Poland.

The heliocentric model (see Figure 3) places the Sun in the center of the universe and all other objects follow circular orbits around the Sun. According to the adapted model by Copernicus:

1. All planets, Moon and stars orbit the Sun in perfect circles.
2. The Earth is many times farther away from the Sun than the length of its own diameter.
3. The motion of the Sun is caused by the motion of the Earth in relation to the Sun.
4. The rotation of the firmament is caused by the Earth’s rotation around is axes once every day.  

In Ancient Greece, the circle is the most perfect form in Geometry and the ancient Greeks marveled at the beauty of circles, as did other ancient civilizations. Believing that motion follows perfect forms like those made by circles, the heliocentric model did not, however, more accurately plot the orbit of the heavenly objects than the outdated geocentric model of Ptolemy. Why? Well, the motion of the planets is not in perfect circles (I will talk about this in more detail in the paragraph about Johannes Kepler below).

Figure 3: Model of the universe (solar system) according to Copernicus.

This is the reason why the Copernican model was not accepted by his contemporaries. In fact Copernicus waited until his deathbed to publish his infamous “ De Revolutionibus Orbium Caelestium” (Latin meaning: “Concerning the Revolution of the Heavenly Spheres”).

It is ironic that all along, high Catholic Church officials urged Copernicus to publish his work about the heliocentric model of the universe. Later however, the Church categorically opposed his model as sacrilegious and forced supporters long after Copernicus into isolation or death.

What is the benefit of such a model you may rightfully ask? Copernicus was motivated by finding simple answers to complicated problems. This is, in fact, the aim of all science. By putting the Sun in the center he could describe the retrograde-motion of the planets (see part 2 of the series for more detail) more elegantly and simply.  Copernicus concludes:” The apparent retrograde and direct motion of the planets arises not from their motion but from the earth's. The motion of the earth alone, therefore, suffices to explain so many apparent inequalities in the heavens.”

Figure 4: Retrograde motion of the planets according to Copernicus.

Let’s have a look at Figure 4 undefined undefined undefined undefined undefined undefined. The retrograde motion of the planet Mars is shown in a sequence of 7 observation events from Earth. Since Mars is orbiting the Sun on a longer orbit, Earth is moving faster around the Sun. Earth is catching up with Mars and finally overpowers it. During this flyby, an observer on Earth would see Mars following a loop in the sky which is purely an optical illusion produced by the different speeds of Mars and Earth.

Besides the elegance of the heliocentric model, Copernicus had to adjust it in order to improve his accuracy. He introduced epicycles (see part 2 of the series for more detail) in his model, the very thing which made the Ptolemy model so cumbersome and unaesthetic. The world has to wait another hundred years to fix the heliocentric model to come to its real glory.

Tycho Brahe - The master of observation

Figure 5: Danish astronomer Tyco Brahe (1546-1601).

As a young adult, so the story goes, Tycho Brahe, Tycho for short, engaged himself in countless duels provoked mainly to prove his noble birth as well as his superior intellect. At one of these fights, he lost part of his nose. Wasting no time, he simply designed an artificial replacement out of silver and gold.

Figure 6:Tycho Brahe’s famous observatory Uraniborg was located on the island Hven.

Besides his unorthodox character, Tycho revolutionized astronomy by introducing extremely accurate and comprehensive astronomical observations. In his Uraniborg observatory, Tycho Brahe was granted an estate on the island of Hven (a small Swedish island in the Baltic Sea) and the funding to build the Uraniborg (see   Figure 6 ), an early form of a research institute, where he conducted all his precise measurements of stars and planets. Tycho conducted his observations without the aid of a telescope (see Figure 7), which was invented shortly before his death and first used by Galileo Galilei around 1610.

Figure 7: Tyco Brahe takes measurement of the planet Mars with his Mural quadrant at his famous Uraniborg observatory.

A mural instrument measures an angle of an astronomical object from the horizon. The higher the object, i.e. star or planet, is in the sky the bigger the angle measured by the device (see Figure 8 undefined undefined undefined undefined undefined undefinedfor a visualization of the meridian angle). Usually the device was mounted on or built into a wall. For astronomical purposes, these walls were oriented so they lie precisely on a meridian. The meridian (from Latin meridianus meaning “belonging to mid-day”, from Latin meridies meaning “mid-day”) is also called latitude and is an imaginary line connecting the North Pole with the South Pole.

Figure 8: Visualizing the meridian angle, i.e. altitude, of a celestial object.

A mural instrument that measures angles from 0 to 90 degrees is called a mural quadrant. Each object in the sky has different altitudes that are the apparent distance from the horizon. An observer can for example see a star with an angle of 60^o degrees from the horizon (see Figure 8 as a visualization of the situation). A mural quadrant would measure the angles as shown in Figure 9 .

Figure 9: Measuring the altitude with a mural quadrant.

Tycho’s extremely accurate measurements led, for example, to the measurement of an alignment of Jupiter and Saturn in 1563. However, it came two days later than Copernicus’s model had predicted. During his career Tycho accumulated a massive archive of data, particularly relating to the position of the planets. It is precisely this new material of observational data which would change our picture of the solar system forever.

Johannes Kepler - Not quite perfect circles

Figure 10: Astronomer Johannes Kepler (1573-1630).

In the year 1600, one year before his death, Tycho took the young German astronomer, Johannes Kepler (pronounced kepla) as his apprentice (see Figure 10 ) . This would turn out to be a most significant event for modern astronomy, which we will talk about in this section. Born in 1571 in south-west Germany, he died after an eventful life in 1630.

Kepler was a gifted mathematician and astronomer who used his position as Tycho’s apprentice to gain access to the vast amount of accumulated data concerning the position of the planets.  Tycho’s data of the planets positions was the most accurate available in the world at this time. Equipped with this, Kepler pored himself over the data and tried to find patterns which would lay out the path a planet takes through the sky.

To his surprise and after many unsuccessful attempts Kepler realized that the orbit of a planet is not a perfect circle but an ellipse. An ellipse (from the Greek elleipis, meaning “missing” or “not complete”) is an elongate circle. Figure 11 shows an ellipse compared to that of a circle. One side of an ellipse is longer than the other. This deviation from the “perfect” object, which was the ideal of ancient Greek and the guide throughout the Middle Ages, allowed Kepler to answer all the questions about how the planets really move.

Figure 11: An ellipse in comparison to a circle.

Breaking with millennia-old tradition was not an easy task for Kepler. However, Kepler was a man of deep faith in God and His creation and equally in the power of data. This combination allowed him to see the truth. Kepler adopted and refined the heliocentric model of Copernicus. This time the heliocentric model won indisputably against the geocentric model (see part 2 of the series), that is, only in the view of science.

In two of his major works “ Astronomia Nova” (1609) and “ Harmonices Mundi” (1619) Kepler described three fundamental laws of planetary motions, which are still valid today. They laid the groundwork for modern celestial mechanics, the science describing all motions in the universe. Kepler discovered relationships between the geometrical structure of an ellipse and the speed of the planets during their orbits.

Figure 12: The “real” motion of a planet around the Sun.

Now, how does the new model look? Let’s see: All the planets move around the Sun in elliptical orbits in the following order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus and Pluto (ok. Since August 24^th 2006 Pluto was downgraded to a dwarf planet). The Sun is not quite in the center of all the planets. In reality, the Sun is a little off-centered. That means sometimes a planet is closer to the Sun during one orbit, called the perihelion (from Greek peri meaning “beside, close, near” and Greek helios meaning “sun”), and sometimes it is further away from the Sun, called aphelion (from Greek apo helios meaning “sun”). Please see Figure 12 to visualize the just described situatio n. meaning “opposite, away from” and Greek

And back to Galileo Galilei – New Frontiers

In this part of the article series we continued our journey through the history of decoding the motions of the heavenly bodies with the Copernican revolution. The astronomer Copernicus boldly suggested that the Sun is the center of the universe and not Earth, as believed for more than a thousand years since the Greek astronomer and philosopher Ptolemy. Placing the Sun in the center made the heliocentric model much easier than the geocentric model of Ptolemy. It was also able to describe more compellingly and elegantly, some key features of planetary motion. Besides this, however, it was not really more accurate than its rival. This would change with the work of Danish astronomer Tycho Brahe and his astonishingly accurate observational data of the motion of the planets. This wealth of data allowed the German astronomer Johannes Kepler, a contemporary of Galileo Galilei, to finally complete the Copernican revolution with an ultimate victory for the heliocentric model of the universe. And, the victory remained undisputed by the Church.

In the year 1610, Galileo Galilei pointed a telescope the first time to the sky (see the first part of the series). With this pivotal moment in human history, Galileo was able to confirm the heliocentric model with “visual” data from his telescope. He confirmed this by observing the phases of Venus (Yes, the planet Venus has phases just like our Moon) and confirming that Venus is orbiting the Sun, not Earth.

Galileo succeeded in showing that the Sun is much larger than all planets put together. He showed that the planets orbit the Sun in vast distances measuring in millions and billions of miles. He showed that the stars we see in the night sky are really suns on their own, just like our Sun. He showed that the stars are even farther away than the planets from the Sun, many times farther away.

Looking back, Galileo finally completed the Copernican revolution and the quest to decode the motion of the heavenly bodies.

In the next part of the series, I invite you to join me through a tour of our solar system as we know it today. Believe me, it will be a fantastic journey!

Read Part 2 HERE