Understanding who, where, when we are ...
Tracing our Understanding of the Cosmos
Astronomy, one of the oldest sciences, explores the universe – the cosmos as a whole comprising space, time and matter. People were already observing the starry sky in ancient civilisations. Outstanding scholars such as Aristarchus, Aristotle and Ptolemy, and, over 1,000 years later, Copernicus in particular, laid the foundations for our understanding of the universe. In the 17th century, Galileo, Kepler and Newton revolutionised astronomy by recognising fundamental relationships. Today, the theory of cosmic evolution explains the formation of the Earth around 4.6 billion years ago from a cloud of dust and gas. Astronomy remains a fascinating window into the mysteries of the universe and our own origins.
It is easier (for me, at least) to grasp this understanding if one considers the beginnings of astronomy within their historical context. In the following, these are first outlined briefly, drawing on the leading authorities (and their major works).
Historia est testis temporum, lux veritatis, vita memoriae, magistra vitae
(Cicero)
or
Videmus nunc per speculum in aenigmate tunc autem facie ad faciem.
(1. Korinther, chapter 13, Vers 12)
Aristoteles (384 - 322 BC)
„On the Heavens“ (Latin: „De Caelo“) is the major work of the Greek polymath, in which he sets out his understanding of the cosmos. In this book, Aristotle examines the nature and constitution of the heavens and the celestial bodies. He distinguishes between the unchanging heavens, through which the stars and planets move in their orbits, and the sublunar region, which encompasses the world of change and becoming. In this work, Aristotle develops his theory of natural motion and explains the various movements of the celestial bodies.
The concept of the spheres plays a central role here. „On the Heavens“ is a seminal work of ancient natural philosophy and astronomy that had a decisive influence on ancient conceptions of the universe.
Claudius Ptolemeus (ca. 100 bis 160 AD)
Ptolemy compiled the first comprehensive summary of the astronomical knowledge of his time in his work ‘Mathematicae Syntaxis Libri XIII’, which was discovered by the Arabs in the ninth century and reached Europe under the title ‘Almagest’. It was to become the standard work on mathematical astronomy from the second to the 17th century.
Ptolemy was regarded as the founder of the geocentric model of the universe, in which the Earth was at the centre of the universe. Some 400 years earlier, Aristarchus had already proposed a heliocentric model of the universe; however, due to a lack of empirical evidence, prevailing scientific preferences and political and religious influences in the ancient world, this model was not pursued further and had fallen into oblivion. It was not until Nicolaus Copernicus that it was rediscovered and further developed.
From the mid-twelfth century onwards, an Arabic version of the *Almagest* was translated into Latin by Gerhard of Cremona. The best-known translation was produced by Regiomontanus, the German astronomer and mathematician (whose real name was Johannes Müller).
A summary of Regiomontanus’s translation, known as the Epitome, was printed in 1496 and remained one of the most important foundations of astronomy for decades. Nicolaus Copernicus also owned a copy.
In the Middle Ages, Ptolemy’s ideas continued to be accepted in Europe, and reference was made to the ancient philosophers, particularly Aristotle. The role of the Church in relation to advances in astronomy from Ptolemy to Copernicus was complex and multifaceted.
The Ptolemaic world view was supported and promoted by the Roman Catholic Church; after all, it fitted well with the religious notion that the Earth was the centre of the universe. This was only to change with the advent of the modern era through Copernicus.
Nikolaus Kopernikus (1473 – 1543)
In his magnum opus “De revolutionibus orbium coelestium” (On the Revolutions of the Celestial Spheres, 1543), he describes how the Earth and the other five planets visible to the naked eye (Mercury, Venus, Mars, Jupiter and Saturn) revolve around the Sun, thereby establishing (once again, see Aristarchus of Samos) the heliocentric model of the universe.
Copernicus had already published his initial theses on the heliocentric model of the universe earlier (probably around 1509) in his ‘Commentariolus’ (De hypothesibus motuum coelestium a se constitutis commentariolus), though he had not yet provided a mathematical foundation for them. One of three copies of the manuscript – which only resurfaced in 1877 – is held at the Austrian National Library in Vienna. Initially, the only evidence of the book’s existence was a note by Tycho Brahe in one of his works (left: the title page of the Vienna manuscript; right: a page from “De revolutionibus …”).
The Copernican revolution, triggered by ‘De Revolutionibus’, ultimately marks the transition from the Middle Ages to the modern era.
Tycho Brahe (1546 – 1601)
Tycho Brahe was a prominent 16th-century Danish astronomer who made a name for himself in the astronomical community of his time through his precise and extensive observations of the heavens. His accurate records of planetary and stellar movements were groundbreaking and laid the foundations for later astronomical research. Whilst Brahe himself believed in a geocentric model of the universe, his observations helped to refine and improve the models of the time.
Tycho Brahe observed (on 11 November 1572) and documented (in 1573) a supernova (‘Nova Stella’) in the constellation Cassiopeia (now known as ‘Brahe’s Star’ or SN1572).
Tycho Brahe’s observation of SN 1572 called the prevailing world view into question. In his work ‘On the Heavens’, Aristotle had put forward the hypothesis that changes occurred only on Earth, whereas events in the heavens remained eternally unchanged. This hypothesis had become established by the time of Tycho Brahe and had become part of generally accepted knowledge. Through the observation of the supernova, Aristotle’s hypothesis was empirically disproved, thereby making the need for a new world view evident.
In 1577, Tycho Brahe also observed a comet (Tycho’s Comet, 12 November 1577, C/1577 V1). It had a tail 50° long and was “as bright as the Moon” (by way of comparison: the full Moon (and also the Sun) has a (mean apparent) diameter of approximately 31 arcminutes in degrees. That corresponds to only about half a degree!). The comet plays a significant role in the history of comet research, as it was the first to provide conclusive proof that it was located outside the Earth’s atmosphere. Until then, it had been believed that comets were a ‘phenomenon of the atmosphere’. These observations contributed to a shift in the conception of an unchanging sky.
Brahe also developed new astronomical instruments, including the quadrant, to enable precise measurements of celestial bodies. After Brahe’s death, Johannes Kepler took over his observational data and used it to formulate his famous Keplerian laws of planetary motion.
Tycho Brahe’s work laid the foundation for the scientific revolution in astronomy and helped to challenge the geocentric world view. His meticulous observations and precise data records served as the basis for the work of his contemporaries and subsequent astronomers, who advanced the heliocentric model and modern astronomy.
Galileo Galilei (1564 – 1642) and Johannes Kepler (1573 - 1643)
Galileo Galilei and Johannes Kepler were two of the most significant astronomers and scientists of the early 17th century, who made a decisive contribution to the development of astronomy. Their work helped lay the foundations of modern astronomy.
Galileo was the first to use the telescope for observing the heavens. In 1609, he built a telescope and began to explore the sky. He discovered craters and mountains on the Moon, the phases of Venus and the moons of Jupiter. These observations disproved some geocentric ideas, as they showed that celestial bodies do not all revolve around the Earth.
Galileo also observed Saturn and its rings, although he was unable to identify them precisely. He criticised the Aristotelian world view and Ptolemaic astronomy, which were still dominant in his time. His observations supported the Copernican theory of a heliocentric world system in which the Earth and the planets revolve around the Sun.
Sidereus Nuncius is the title of a treatise that Galileo published in March 1610, establishing his reputation as an astronomer. The Sidereus Nuncius was printed in Latin in a run of 550 copies. In it, Galileo published his first astronomical observations made with a telescope.
Galileo emphasised the importance of empirical research and the experimental method. He argued that theories should be supported by observations and experiments. Eventually, Galileo came into conflict with the Catholic Church, as his ideas were considered heretical. He was condemned by the Inquisition in 1633 and forced to publicly recant his theories.
Almost at the same time, Johannes Kepler developed his famous laws of planetary motion, according to which the planets orbit the Sun in elliptical paths (Kepler’s First Law), the line connecting a planet to the Sun sweeps out equal areas in equal intervals of time (Kepler’s Second Law), and the orbital periods of the planets are related to their distances from the Sun (Kepler’s Third Law).
Kepler’s discovery of elliptical orbits led him to carry out precise calculations of Mars’s motion. His models helped to explain and refine the observational data collected by Tycho Brahe.
In his magnum opus “Astronomia nova aitiologetos seu physica coelestis”, Johannes Kepler primarily sets out the new method by which he arrived at his laws of planetary motion. He bases his findings on observations and mathematical interpretations. Whilst Copernicus and other scholars before him had based their work on purely geometric representations of planetary motion, “New Astronomy Causally Explained, or the Physics of the Heavens” contains the first approaches to celestial mechanics, as the full title already suggests. However, with his attempt to derive a universal validity of physical laws on Earth as well as in space, Kepler was far ahead of the thinking of his contemporaries.
Ultimately, Kepler’s laws led to a paradigm shift in astronomy. They enabled more accurate calculations and predictions of planetary motions, laying the foundation for later developments in celestial mechanics and theories of gravitation.
The work of Galileo Galilei and Johannes Kepler was crucial to the development of astronomy. Galileo’s telescope and observations helped to support Nicolaus Copernicus’s heliocentric theory and challenge the old geocentric ideas. Kepler’s laws and his methodology provided precise mathematical models for the motion of the planets, which revolutionised our understanding of the solar system and contributed to the later development of Isaac Newton’s theories of gravity. Their work laid the foundations for modern astronomy.
Isaak Newton (1642 - 1727)
The gravitational force, which is directly proportional to mass and inversely proportional to the square of the distance, was finally to confirm Kepler’s laws.
The „Principia“ (Philosophiae Naturalis Principia Mathematica, often referred to as *Principia Mathematica* or simply *Principia*), published in 1687, is regarded as one of the most important scientific works in history and laid the foundations for modern physics. The image shows Newton’s personal copy with his handwritten notes. Newton’s achievements in the field of optics were also outstanding – the varying refraction of light in refracting telescopes led him to develop the first reflecting telescope as early as 1668 (still known today as the Newtonian telescope).
William Herschel (1738 - 1822)
William Herschel was a prominent 18th-century astronomer who made numerous outstanding contributions. In 1781, Herschel discovered the planet Uranus, which led to an expansion of the known solar system and greatly enhanced his reputation within the scientific community. Herschel carried out extensive surveys of the sky and compiled the first systematic catalogue of deep-sky objects, including galaxies, nebulae and star clusters. This contributed significantly to our understanding of the structure of the universe. He also ultimately discovered the existence of infrared radiation by determining the temperatures of the colour spectra of sunlight. This was an important step towards realising that the light spectrum encompasses more than just the light visible to the human eye.
Herschel contributed to our understanding of the nature of stars by systematically studying their colours, brightnesses and distances. He also developed theories about the structure of the Milky Way.
John Herschel (1792 - 1871)
William Herschel’s son continued his father’s work and contributed to research into star clusters, nebulae and the southern hemisphere. He was also a pioneer in the photography of celestial objects.
In 1820, together with his father and others, he founded the Astronomical Society, which became the Royal Astronomical Society in 1831. Among other discoveries, John Herschel found that the Magellanic Clouds are not nebulae, but consist of myriads of stars. In addition to other publications, he produced eleven catalogues of double stars, and a catalogue of 5,079 nebulae and star clusters was published in 1864. A catalogue of 10,300 double and multiple stars appeared posthumously in 1874. John Herschel also introduced the Julian date into astronomy.
Henrietta Swan Leavitt (1868 - 1921)
Henrietta Swan Leavitt worked at the Harvard Observatory analysing astronomical data and, in 1912, discovered the period-luminosity relationship of the so-called Cepheid variables. This fundamental relationship, known as Leavitt’s Law, enables astronomers to determine the distances to Cepheid stars by relating their observed brightness to their absolute brightness. As Cepheids can be observed in galaxies far more distant than our own Milky Way, they play a crucial role in determining distances to other galaxies and have contributed significantly to the development of the cosmic distance ladder.
Henrietta Swan Leavitt at the Harvard College Observatory
Edwin Hubble (1889 - 1953)
Edwin Powell Hubble was born in Missouri in 1889. He showed an early interest in science and astronomy. After studying at the University of Chicago and obtaining his doctorate at Yale University in 1917, Hubble served in the army during the First World War.
After the war, he returned to astronomy and began his career at the Mount Wilson Observatory in California, which was one of the world’s leading observatories at the time. His work and research laid the foundations for his later groundbreaking discoveries.
In the 1920s, Hubble began conducting systematic investigations of apparent nebulae. In doing so, he realised that these nebulae were in fact independent galaxies outside our Milky Way. This discovery revolutionised the understanding of the universe at the time and opened up new horizons for astronomy.
Hubble’s most groundbreaking discovery was the Hubble’s Law, which he presented in 1929. By observing galaxies and their redshifts, he found that there is a clear correlation between a galaxy’s distance and its redshift. This suggested that the universe is expanding. The realisation that galaxies are moving away from one another laid the foundation for the Big Bang theory.
The Hubble constant (Ho=70 km/s/Mpc), named after him, describes the expansion of the universe.
Mathematically, the Hubble constant is denoted as Ho and has units of kilometres per second per megaparsec (km/s/Mpc). A megaparsec is an astronomical unit for distances in the universe and corresponds to approximately 3.26 million light-years.
The Hubble constant is a key parameter in cosmology, particularly within the context of Hubble’s law, which describes a linear relationship between a galaxy’s distance and its redshift. This relationship was empirically established by Edwin Hubble in the 1920s.
Hubble`s Law mathematically is:
where:
- v is the speed at which a galaxy is moving away from us (measured by the redshift),
- H0 is the Hubble constant,
- and d is the distance to the galaxy.
The precise determination of the Hubble constant has been and remains the subject of intensive scientific research, as its accurate measurement is crucial for determining the age of the universe and other cosmological parameters
The cosmological redshift describes the observation that the spectrum of light from distant galaxies is shifted towards longer wavelengths, which is attributable to the expansion of the universe (similar to the Doppler effect, whereby receding sources exhibit longer-wavelength radiation – towards the lower end of the acoustic or optical red spectrum). This insight helped to overcome the notion of a static, unchanging universe and led to a more dynamic picture consistent with the Big Bang theory.
Overall, Edwin Hubble’s achievements have fundamentally changed the way we understand the universe. His discoveries have not only influenced astronomy and cosmology, but have also broadened and deepened humanity’s fundamental view of the world. Edwin Hubble is therefore rightly regarded as one of the most important astronomers of the 20th century.