Stonehenge, in England. Built somewhere after 2500 BC, the Sun aligns with the stones on the summer solstice, showing an ancient knowledge of astronomy. (Public domain image from Wikimedia Commons)
The type of trigonometry that Aristarchus used to determine distances. θ is close to 0°, so this diagram is exaggerated (it's also why Aristarchus didn't measure θ perfectly - that's hard to do by eye!).
(Image by Richard Bloch)
Sunlight hits the Earth at different angles (exaggerated here). Eratosthenes looked down a well at noon, and saw no shadows. His friend in another city looked down a well at the same time, and saw a bit of shadow. By measuring the length of that shadow, and knowing the distance between their cities, Eratosthenes was able to figure out how big the Earth had to be to produce that shadow!
(Image by Richard Bloch)
Three images of the Moon taken during the total lunar eclipse of December 19, 2010. Note the Earth's shadow over the Moon is not a straight line. (Photo by Richard Bloch)
The seven days of the week and the seven naked-eye members of the Solar System. The Teutonic names were the names of the objects given long ago by Teutonic tribes living in what is now Germany.
(Table by Richard Bloch)
When you think of astronomy, it’s easy to think of a guy in a lab coat sitting at the end of a long telescope, staring at the night sky (and maybe making notes by hand). But astronomy is much, much older than telescopes and lab coats (which astronomers don’t even actually wear) – in fact, it’s one of the oldest sciences in history!
Nobody can really say where or when astronomy really began, because humans and stars have been around since before written records.
Monuments such as Stonehenge show that cultures many thousands of years ago had at least some understanding of the motions of the heavens, but the conversation about the history of astronomy really begins with the ancient Greeks.
While the Greeks certainly would have built on astronomical knowledge from before their time, they made their own major contributions to astronomy. Aristarchus (~300 BCE) was one such clever Greek. He used angles between the Earth, Sun, and Moon in the sky in order to determine their distances – not the actual numbers, but their relative distances.
For example, using his angles, he concluded that the Sun was 20 times further from the Earth than was the Moon.
Since he found the Sun and Moon appeared to be the same size in the sky, he figured that the Sun must thus be 20 times larger than the Moon. While his angles weren’t accurate, his math was – even though we now know the Sun to be 400 times the distance and size of the Moon, if we were to plug accurate angles into Aristarchus’ math, we would get the right answer!
But it didn’t matter – Aristarchus’ calculations showed that the Sun was the largest among these three celestial objects. As a result, Aristarchus was the first to propose that the Earth revolved around the Sun! Unfortunately, his Sun-centered (or ‘heliocentric’) model would not catch on for almost two thousand years.
Other Greeks made their own contributions.Eratosthenes, for example, managed to calculate the size of the Earth so accurately that our modern measurements only differ by a few hundred kilometres! His calculation assumed a spherical Earth, and indeed, back then a flat Earth was not universally accepted.
(You think Christopher Columbus really would've risked sailing his ship off the edge of the planet?) In fact, ancient astronomers noted that the Earth’s shadow on the Moon during a lunar eclipse was curved, and regarded this as evidence of the Earth’s spherical shape.
Aristotle, the great Greek philosopher, also wrote much about the natural sciences, including astronomy. He wrote that celestial objects had to move in circular orbits. Aristotle believed that circles were the most perfect shape, and so the gods, who were also perfect, would design their realm in circles. The views he put forth have come to be called Aristotelian cosmology. So revered was his stature at that time that any challenge to his views was considered to be taboo – almost on par of that of blasphemy! The idea of circular orbits and perfectly spherical planets would dominate our thinking about the universe for almost 2 000 years, with the Earth being generally accepted as the centre of the universe. This Earth-centred model is called the geocentric model of the universe.
The Greeks also mapped the night sky, and our word ‘planet’ comes from the Greek word ‘planetes,’ which means ‘wanderer.’ This is because while the stars appeared fixed relative to one another, the planets ‘wandered’ among them. The Greeks did not know that these were not stars, and so they were dubbed ‘wandering stars’.
The five visible with the naked eye were Mercury, Venus, Mars, Jupiter, and Saturn.Along with the Sun and the Moon, these seven ‘special’ objects were so notable to ancient astronomers, that the week has seven days, each named after one of the objects!
Some centuries later, an Alexandrian astronomer named Ptolemy (~100 CE) proposed a comprehensive geocentric model that explained all the motions of astronomy, which he published in his famous work, Almagest. This model was regarded as accurate for over 1 200 years.
After the decline of the Greek empire, it may seem natural to assume the Romans made some great contributions of their own. After all, our planets are all named after Roman gods (even though the Greeks and other cultures had their own names for them). But the Romans were most interested in time-keeping and calendars, although the progression of astronomical knowledge continued, most notably by Islamic astronomers in the Middle East between the 10th and 14th centuries. The contributions of medieval Islam to astronomy should not be overlooked.
Translating the works of ancient Greece, the Arabs also added their own contributions to astronomy. Arab astronomers made highly accurate observations of the night sky, and consolidated astronomical knowledge from Greece, India, and other cultures. They improved the accuracy of Greek observations of the movement of the planets, and also did extensive work on Ptolemy’s geocentric model.
An Islamic astronomer's detailing of Mercury's orbit around the Earth, in accordance with Ptolemy's geometry. All the smaller circles, called epicycles, were required to explain all of Mercury's observed motions. These motions are easily explained in a heliocentric universe. Note that this complex diagram just describes one of the planets. Imagine a single diagram for them all!
(Public domain image from Wikimedia Commons)
While some Islamic astronomers tried to improve the accuracy of the model’s geometry, in later centuries some Islamic astronomers actually began to question its validity, although most offered alterations to help keep Ptolemy’s model aligned with more accurate observations. While a heliocentric model was not yet supported, challenges to the authority of the Ptolemaic model appeared.
When the Renaissance flourished in Europe, it was the contributions of these medieval Islamic astronomers that laid the groundwork for many of the most well-known European Renaissance astronomers to make their discoveries. These astronomers included Nicolaus Copernicus (1473-1543), Galileo Galilei (1564-1642), Johannes Kepler (1571-1630), and Isaac Newton (1642-1727). Let’s see how each of these people contributed to our knowledge of astronomy, starting with Copernicus.
Nicolaus Copernicus is best known for his proposition of a heliocentric model of the Solar System. His model would at last put an end to Ptolemy’s geocentric universe. Copernicus believed that the Sun was the centre of the universe, and in his model he placed the planets in orbits around the Sun. His work, De Revolutionibus Orbium Coelestium
, which proposed this model, was completed sometime in the 1530s, but Copernicus was hesitant to publish it, out of fear that he would be ridiculed and disregarded – illustrating how strongly Ptolemy’s geocentric model had been accepted. However, Copernicus did finally decide to publish his work (through some pressure from friends), and it was printed in the year of his death – 1543.
Copernicus’ heliocentric model explained many observations of the celestial bodies and their motions which had been made by that time. Many problems with the Ptolemaic model’s ability to explain such observations did not appear in the heliocentric model. The main issue with the Ptolemaic model was that as more accurate observations of planetary motion were being made, very complicated modifications had to be introduced to Ptolemy’s model to keep the Earth at its centre. With Copernicus’ explanation that celestial objects moved with respect to the Sun, and not the Earth, in most cases no complex explanations were needed to account for the observations.
Retrograde motion of Mars. If an observer were to go outside and plot Mars' position against the stars, as Earth overtook Mars in their orbits around the Sun (seen below), the observer would see the positions change in order from 1-7 (seen above). This was a great puzzle for ancient astronomers!
(Image by Richard Bloch)
For example, when the Earth passes another planet in its orbit around the Sun, that planet appears to move backward in the sky from the path it had been taking. This is called retrograde motion. In a geocentric universe, the orbits of the planets are complex in order to explain how these planets change their apparent direction. In a heliocentric universe, however, this motion is easily explained without requiring complex orbits.
There were still flaws with Copernicus’ model, however. Copernicus assumed that all orbits were perfect circles, a belief in line with Aristotelian cosmology. Since planets actually orbit in ellipses, not perfect circles, Copernicus’ model didn’t perfectly match observations of the planets. As a result, his model still showed planets with somewhat complex orbits. However, these orbits were still much simpler than the orbits required to explain retrograde motion, and despite religious protest, the Copernican model caught on.
The second well-known astronomer was Galileo Galilei (1564-1642). Galileo also made important contributions to astronomy, and has been credited with the invention of the telescope in 1609. It is through his telescopes that Galileo made some of the most important observations in Renaissance astronomy, which challenged long-held beliefs. The heliocentric model of Copernicus was still a subject of debate during Galileo’s time. Through his telescope Galileo was able to resolve the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto (aptly named the Galilean moons for this reason). His observations and notes of the motions of these moons were evidence that they orbited Jupiter. These observations provided evidence that not everything in the heavens went around the Earth, and was a nail in the coffins of both the Ptolemaic model and Aristotelian cosmology.
The edge of the Moon. What look like slight bumps from Earth are caused by large craters and other features that make the Moon a non-perfect sphere. (Photo by Richard Bloch)
Watch Dave Scott repeat Galileo's experiment from the surface of the Moon!
(NASA Archival footage digitized by Peter Dayton)
Galileo also used his telescope to look at the edge of the Moon, where he noticed a jagged edge caused by mountains and other lunar features. This was another blow to the Aristotelian universe, and its idea of perfectly spherical bodies.
Interestingly, in 1589 Galileo would drop two balls of different masses from atop the Leaning Tower of Pisa, in an effort to show that heavier things do not fall any faster than lighter things - a direct contradiction of Aristotle’s writings. Almost 400 years later, on August 2 1971, the American astronaut David Scott repeated this experiment on the surface of the Moon with a feather and a hammer. In the absence of air resistance (which causes lighter objects to appear to fall slower), both objects hit the ground at the same time, causing Scott to remark “How about that? This proves that Mr. Galileo was correct.”
Another important astronomer was Johannes Kepler (1571-1630). Kepler made many observations of the planets using telescopes (which were becoming an essential tool for astronomers). By examining the measurements of planetary motion over time, Kepler noticed a pattern and identified three laws of planetary motion. Kepler’s work provided the conceptual framework from which the legendary Isaac Newton would derive his law of gravity.
Finally, no discussion of astronomy is complete without Isaac Newton (1642-1727). Newton, and his seminal work Philosophiae Naturalis Principia Mathematica
, would forever change the face of astronomy. In his work, Newton outlines his three laws of motion and the universal law of gravitation. His idea of gravity, that celestial bodies move by the same force that governs much motion on Earth, was incompatible with the idea that the heavens were somehow separate from us here on the ground. But Newton had done much more than simply philosophize – he provided a mathematical framework which described this force and these motions, math which to this day holds true. He furthered the works of Johannes Kepler, and showed independently of observations that Kepler’s laws were the natural consequence of gravity.
Newton’s contributions to astronomy are numerous. His work in optics and classical mechanics had wide applications in both astronomy and physics – he shares credit for the invention of calculus, and invented the reflecting telescope.
As important as Newton’s contributions are to astronomy, it’s equally important to remember that he wouldn’t have been able to contribute all these things if it wasn’t for the work that had been done before him. Like any science, astronomy builds on the discoveries and works that comes before the present – if it weren’t for the works of the Arabs and Greeks, Newton could not have formulated his law of gravity, because Kepler would not have made his laws of planetary motion, because observations wouldn’t have been recorded about objects which wouldn’t have been studied, and so on.
Even today, our knowledge of the universe is constantly changing. Astronomy remains a dynamic science. 20 years ago, children were taught that Pluto was the ninth planet from the Sun, and that the planets of our Solar System were the only known planets in the universe. Today, we count eight planets in our Solar System, and hundreds of extra-solar planets. Who knows what we’ll discover in the next 100 years, and who will make those new discoveries. What we take as fact today may one day sound like nonsense!!