Chapter 1 A History of Astronomy and Cosmological Ideas
Periods of Western Astronomy Western astronomy divides into 4 periods Prehistoric (before 500 B.C.) Cyclical motions of Sun, Moon and stars observed Keeping time and determining directions develops Classical (500 B.C. to A.D. 1400) Measurements of the heavens Geometry and models to explain motions Renaissance (1400 to 1650) Accumulation of data led to better models Technology (the telescope) enters picture Modern (1650 to present) Physical laws and mathematical techniques Technological advances accelerate
Cosmology is the study of the Universe and its components, how it formed, how its has evolved and what is its future. Modern cosmology grew from ideas before recorded history. Ancient man asked questions such as "What's going on around me?" which then developed into "How does the Universe work?", the key question that cosmology asks. Cosmology is as old as humankind. Once primitive social groups developed language, it was a short step to making their first attempts to understand the world around them. Very early cosmology, from Neolithic times of 20,000 to 100,000 years ago, was extremely local. The Universe was what you immediately interacted with. Cosmological things were weather, earthquakes, sharp changes in your environment, etc. Things outside your daily experience appeared supernatural, and so we call this the time of Magic Cosmology.
Later in history, 5,000 to 20,000 years ago, humankind begins to organize themselves and develop what we now call culture. A greater sense of permanence in your daily existences leads to the development of myths, particularly creation myths to explain the origin of the Universe.
Historians tend to exaggerate the capabilities of ancient Egyptians, when, in fact, they were a practical culture. The development of cosmology in ancient Egypt followed practical lines. Early man's impressions of the night sky formulated into various myths which then later became the core of Egyptian religion. A great deal of effort was made by the priesthood to calculate and predict the time and place of their god's appearances. These skills led to the division of the day and night into twelve sections each, the development of a lunar calendar and the development of a solar calendar of 12 30-day months with a special 5-day unit to bring the total to 365 days. Egyptian legend declares that the sky goddess Nut gives birth to Ra once a year, catalysing calendar development The Sun (Ra) is shown entering her mouth, passing through her star speckled body and emerging from her birth canal nine months later (from the spring equinox to the winter solstice). Thus, Ra becomes a self-creating god, i.e. the Universe is self-creating and eternal.
Cosmology in Mesopotamia was much more sophisticated. Babylonians believed in a six-level universe with two heavens above the sky, the heaven of the stars, the earth, the underground of the Apsu, and the underworld of the dead. The Earth was created by the god Marduk as a raft floating on the Apsu. The gods were divided into two pantheons, one occupying the heavens and the other in the underworld. Babylonian astronomy is noted for their detailed, and continuous, records of astronomical phenomenon such as eclipses, positions of the planets and rise and setting of the Moon. These records date back to 800 B.C. and are the oldest scientific documents in existence. Babylonian astronomers also developed several arithmetic tools to aid in the prediction of eclipses and planetary motion. Their system of stellar names and measurement system was passed onto later civilizations, however the Babylonians never developed a cosmological model in which to interpret their observations. Greek astronomers will achieve this goal using the Babylonian data.
Ancient Greek Astronomers The underlying theme in Greek science is the use of observation and experimentation to search for simple, universal laws. We call this the time of Geometric Cosmology. Through the use of models and observations, they were the first to use a careful and systematic manner to explain the workings of the heavens Limited to naked-eye observations, their idea of using logic and mathematics as tools for investigating nature is still with us today Their investigative methodology is in many ways as important as the discoveries themselves.
Amoung the many philosopher-scientists, there were two main schools of thought. One school is attributed to Plato, and finds that Nature is a structure that is precisely governed by timeless mathematical laws. According to Platonists we do not invent mathematical truths, we discover them. The Platonic world exists and physical world is a shadow of the truths in the Platonic world. The other school. Pythagorians, held that mathematical concepts are mere idealizations of our physical world. The world of absolutes, what is called the Platonic world, has existence only through the physical world. In this case, the mathematical world is the same as the Platonic world and would be thought of as emerging from the world of physical objects. Both Plato and Pythagoras influenced the first logically consistent cosmological worldview, developed by the Greeks in the 4th century B.C. This early cosmology was an extrapolation of the Greek theory of matter proposed by Empedocles. This theory states that all matter in the Universe is composed of some combination of four elements: Earth, Water, Fire, Air.
Early Ideas: Pythagoras Pythagoras taught as early as 500 B.C. that the Earth was round, based on the belief that the sphere is the perfect shape used by the gods Euclid, a Greek mathematician, proved that there are only five solid shapes that can be made from simple polygons (the triangle, square and hexagon). Plato, strongly influenced by this pure mathematical discovery, revised the four element theory with the proposition that there were five elements to the Universe (earth, water, air, fire and quintessence) in correspondence with the five regular solids.
Elements had a natural tendency to separate in space; fire moved outwards, away from the earth, and earth moved inwards, with air and water being intermediate. Thus, each of these five elements occupied a unique place in the heavens (earth elements were heavy and, therefore, low; fire elements were light and located up high). Thus, Plato's system also became one of the first cosmological models. For example, hot air rises to reach the sphere of Fire, so heated balloons go up. Note that this model also predicts some incorrect things, such as all the planets revolve around the Earth, called the geocentric theory. The geocentric cosmology becomes the first of many anthropocentric universes.
Early Ideas: Aristotle (384-322 BC) By 300 B.C., Aristotle presented naked-eye observations for the Earth s spherical shape: Shape of Earth s shadow on the Moon during an eclipse
Early Ideas: Aristotle He also noted that a traveler moving south will see stars previously hidden by the southern horizon Aristotle held that the universe was divided into two parts, the terrestrial region and the celestial region. On Earth, all bodies were made out of combinations of four substances, earth, fire, air, and water, whereas in the region of the universe beyond the Moon the heavenly bodies were made of a fifth substance, called aether. He noted that crystalline spheres made of aether held the celestial bodies. :
Early Ideas: The Size of the Earth Eratosthenes (276195 B.C.) made the first measurement of the Earth s size He obtained a value of 25,000 miles for the circumference, a value very close to today s value
Early Ideas: Distance and Size of the Sun and Moon The sizes and distances of the Sun and Moon relative to Earth were determined by Aristarchus about 75 years before Eratosthenes measured the Earth s size Once the actual size of the Earth was determined, the absolute sizes and distances of the Sun and Moon could be determined
Early Ideas: Distance and Size of the Sun and Moon These relative sizes were based on the angular size of objects and a simple geometry formula relating the object s diameter, its angular size, and its distance
Early Ideas: Distance and Size of the Sun and Moon Aristarchus, realizing the Sun was very large, proposed the Sun as center of the Solar System, but the lack of parallax argued against such a model
Measuring the Diameter of Astronomical Objects α l = 2πd 360 l linear size of object d distance to object α angular size of object
Planets and the Zodiac The planets (Greek for wanderers ) do not follow the same cyclic behavior of the stars The planets move relative to the stars in a very narrow band centered about the ecliptic and called the zodiac Motion and location of the planets in the sky is a combination of all the planets orbits being nearly in the same plane and their relative speeds about the Sun
Planets and the Zodiac Apparent motion of planets is usually from west to east relative to the stars, although on a daily basis, the planets always rise in the east
Retrograde Motion Occasionally, a planet will move from east to west relative to the stars; this is called retrograde motion Explaining retrograde motion was one of the main reasons astronomers ultimately rejected the idea of the Earth being located at the center of the solar system
Early Ideas: The Geocentric Model Because of the general east to west motion of objects in the sky, geocentric theories were developed to explain the motions Eudoxus (400-347 B.C.) proposed a geocentric model in which each celestial object was mounted on its own revolving transparent sphere with its own separate tilt The faster an object moved in the sky, the smaller was its corresponding sphere This simple geocentric model could not explain retrograde motion without appealing to clumsy and unappealing contrivances
Early Ideas: The Geocentric Model
Ptolemy of Alexandria (2 A.D.) Ptolemy of Alexandria improved the geocentric model by assuming each planet moved on a small circle, which in turn had its center move on a much larger circle centered on the Earth The small circles were called epicycles and were incorporated so as to explain retrograde motion
Ptolemy of Alexandria Ptolemy s model was able to predict planetary motion with fair precision Discrepancies remained and this led to the development of very complex Ptolemaic models up until about the 1500s Ultimately, all the geocentric models collapsed under the weight of Occam s razor and the heliocentric models prevailed
Non-Western Contributions Islamic Contributions Relied on celestial phenomena to set its religious calendar Created a large vocabulary still evident today (e.g., zenith, Betelgeuse) Developed algebra and Arabic numerals Asian Contributions Devised constellations based on Asian mythologies Kept detailed records of unusual celestial events (e.g., eclipses, comets, supernova, and sunspots) Eclipse predictions
Astronomy and Cosmology in the Renaissance Nicolaus Copernicus (1473-1543) Could not reconcile centuries of data with Ptolemy s geocentric model Consequently, Copernicus reconsidered Aristarchus s heliocentric model with the Sun at the center of the solar system
Heliocentric models explain retrograde motion as a natural consequence of two planets (one being the Earth) passing each other Copernicus could also derive the relative distances of the planets from the Sun
However, problems remained: Could not predict planet positions any more accurately than the model of Ptolemy Could not explain lack of parallax motion of stars Conflicted with Aristotelian common sense
Astronomy in the Renaissance Tycho Brahe (15461601) Designed and built instruments of far greater accuracy than any yet devised Made meticulous measurements of the planets
Tycho Brahe (1546-1601) Made observations (supernova and comet) that suggested that the heavens were both changeable and more complex than previously believed Proposed compromise geocentric model, as he observed no parallax motion!
Tychonian Universe
Astronomy in the Renaissance Johannes Kepler (15711630) Upon Tycho s death, his data passed to Kepler, his young assistant Using the very precise Mars data, Kepler showed the orbit to be an ellipse
Kepler s 1 Law st Planets move in elliptical orbits with the Sun at one focus of the ellipse
Kepler s 2nd Law The orbital speed of a planet varies so that a line joining the Sun and the planet will sweep out equal areas in equal time intervals The closer a planet is to the Sun, the faster it moves
Kepler s 3rd Law The amount of time a planet takes to orbit the Sun is related to its orbit s size The square of the period, P, is proportional to the cube of the semimajor axis, a
Kepler s 3rd Law This law implies that a planet with a larger average distance from the Sun, which is the semimajor axis distance, will take longer to circle the Sun Third law hints at the nature of the force holding the planets in orbit
Astronomy and Cosmology in the Renaissance Galileo (1564-1642) Contemporary of Kepler First person to use the telescope to study the heavens and offer interpretations The Moon s surface has features similar to that of the Earth The Moon is a ball of rock
The Sun has spots The Sun is not perfect, changes its appearance, and rotates Jupiter has four objects orbiting it The objects are moons and they are not circling Earth Milky Way is populated by uncountable number of stars Earth-centered universe is too simple
Evidence for the Heliocentric Model Venus undergoes full phase cycle Venus must circle Sun
Galileo is credited with originating the experimental method for studying scientific problems Deduced the first correct laws of motion Was brought before the Inquisition and put under house arrest for the remainder of his life
Isaac Newton Isaac Newton (16421727) was born the year Galileo died He made major advances in mathematics, physics, and astronomy
Isaac Newton He pioneered the modern studies of motion, optics, and gravity and discovered the mathematical methods of calculus It was not until the 20th century that Newton s laws of motion and gravity were modified by the theories of relativity
The omnipresence of God pervaded the Newtonian cosmos. The divine presence operated as an immaterial "aether" that offered no resistance to bodies, but could move them through the force of gravitation. Newtonian gravitational theory practically demanded a continual miracle to prevent the Sun and the fixed stars from being pulled together. Newton envisioned an infinitely large universe, in which God had placed the stars at just the right distances so their attractions cancelled, as precisely as balancing needles on their points. The French philosopher René Descartes, on the other hand, had proposed a nonmathematical model. He suggested that the universe consists of huge whirlpools ("vortices") of cosmic matter. Our solar system would be only one of many such whirlpools. Descartes' mechanical, mechanistic cosmology was highly acceptable within the general seventeenth-century conception of the world as a machine. His explanations, though, were but qualitative re-descriptions of phenomena in mechanistic terms. During the course of the eighteenth century, vortex theory proved unable to calculate the observed planetary motions. Meanwhile, the rival Newtonian theory advanced from one precise quantitative success to another.
René Descartes --Cartesian Vortex Universe -- 17th Century Static (evolving), steady state, infinite No empty space can exist and that space must consequently be filled with matter. The parts of this matter tend to move in straight paths, but because they lie close together, they can't move freely, which according to Descartes implies that every motion is circular, so the aether is filled with vortices. Due to centrifugal force, matter tends towards the outer edges of the vortex, which causes a condensation of this matter there. (1644, principia philosophiae)
The solar system contains many bodies, and the calculation of the orbit of any planet or satellite is not simply a matter of its gravitational attraction to the body around which it orbits. In addition, other bodies have smaller, but not negligible, effects (called "perturbations"). Still unexplained were large anomalies in the motions of Jupiter and Saturn, and an acceleration of the Moon's orbital speed around the Earth. The French mathematical astronomer Pierre-Simon Laplace resolved these in 1785 and 1787 Mécanique Céleste, Here he proposed that all physical phenomena in the universe could be reduced to a system of particles, exerting attractive and repulsive forces on one another. In the book, Laplace advanced an idea that became known as the "nebular hypothesis." He suggested that our solar system, and indeed all stars, were created from the cooling and condensation of a massive hot rotating "nebula" (a gassy cloud of particles). The nebular hypothesis strongly influenced scientists in the 19th century. The German philosopher Immanuel Kant argued against the Romantics. For him, the Newtonian solar system provided a model for the larger stellar system. He reasoned that the same cause which gave the planets their centrifugal force, keeping them in orbits around the Sun, could also have given the stars the power of revolving. And whatever made all the planets orbit in roughly the same plane could have done the same to the stars. Nebulousappearing objects in the heavens became, in Kant's mind, island universes, like colossal solar systems.
In his 1785 paper "On the Construction of the Heavens," William Herschel wrote that our Milky Way is a very extensive, branching, compound Congeries of many millions of stars. Herschel's drawing shows a cross section through the Milky Way, our galaxy, as determined from his observations. Herschel proposed to determine the position of the solar system in the stratum of stars by " 'counting the number of stars in different directions. This number, Herschel argued, assuming the stars to be equal in brightness and equally scattered, would be proportional to the distance to the edge of our galaxy in each particular direction. Approaching the beginning of the 20th century, the worldview pioneered by Herschel was vastly different from that of Aristotle or even Copernicus. No longer were human beings necessarily at or very near the center of the universe. The Milky Way was now understood to be an optical effect, with our solar system immersed in a much larger stratum of stars, a roughly disk-shaped stellar system. Possibly other island universes were scattered throughout a possibly infinite space.
Harlow Shapley's galaxy measured the size of the Galaxy with the aid of Globular clusters around 100 kpc which was far larger than any previous estimate. It might indeed be the entire universe. Shapley had aslo showed that globular clusters were not independent island universes. Other nebulae (concentrations of stars and dust), especially spiral-shaped ones, might still lie outside our galaxy. But if they were similar in size to our now enormous galaxy, they seemed implausibly large. Separate island universes were not impossible, but they seemed less likely since Shapley had multiplied the size of our galaxy to many fold. Edwin Hubble (1924) found that the distance [to M31] one of the spiral-shaped to be something over 300,000 parsecs. This was roughly a million light years, and several times more distant than Shapley's estimate of the outer limits of our own galaxy. Before the 1920s ended, astronomers understood that the spiral nebulae lie outside our own galaxy. In the previous decade Shapley had multiplied the size of the universe by about ten times. Hubble multiplied it by another ten - if not more. Hubble's universe was no longer the one all-comprehending galaxy envisioned by Shapley. The universe was understood to be composed of innumerable galaxies spread out in space, farther than the largest telescope could see. Hubble next would show that the universe is not static, as nearly everyone then believed, but is expanding.
In the theories of relativity and the quantum, physicists starting with Albert Einstein had given entirely new and astounding explanations of energy, matter, gravity, even space and time. As astronomers tried to apply these new tools to cosmology, they were struck with their own revelations. In his gravitational field equations, Einstein was just then providing a compact mathematical tool that could describe the general configuration of matter and space taking the universe as a whole. The peculiar curvature of space predicted in the equations was quickly endorsed in famous experiments, and by the early 1920s most leading scietists agreed that Einstein's field equations could make a foundation for cosmology. The only problem was that finding a solution to these simple equations that is, producing a model of the universe. In fact a few astronomers had been looking for other solutions to Einstein's equations. Back in 1922, the Russian meteorologist and mathematician Alexander Friedmann had published a set of possible mathematical solutions that gave a non-static universe. Einstein noted that this model was indeed a mathematically possible solution to the field equations. The Belgian astrophysicist Georges Lemaître had also published a model of an expanding universe, in 1927.