Astronomy
The Methods of Astronomy
Course Overview: Matrials
Introduction to Astronomy
Astronomy and the Universe
- Why Study Astronomy?
- Learning and Applying Formulae
- Tools of Science
- Practice with Concepts
- History of Astronomy
- Discussion Questions
- Optional Website Reading
Introduction: Why Study Astronomy?
When I consider your heavens,
the work of your fingers,
the moon and the stars
which you have set in place,
what is man that you are mindful of him,
the son of man that you care for him?
Psalm 8: 3-4
The heavens fascinate us. The moon changes shape and brightness each month; the planets move eastward most of the time, then suddenly stop and reverse their course; the sun is higher or lower in the sky at noon over the course of a year. With telescopes we can see huge clouds of gas, distant collections of billions of stars and the rare stellar supernova, an explosion so great that its light blots out the galaxy it lies in. From the motions of the heavens come our notions of time, periodicity, patterns in nature, and our conviction that the universe follows immutable physical laws. Is this conviction which makes science possible. We wonder about planets, stars, and galaxies, and about our place in this seemingly endless universe, and we are challenged to consider the cost of exploration and the ethics of colonization of these other places.
Astronomical observations are among the oldest written records we have, recorded on stone markers in Ireland and Scotland, on cuneiform tablets in Mesopotamia, on rice paper in China, on the walls of the tombs of the Pharoahs in Egypt. In the ancient world, the movements of the sun, moon, and planets were important for timekeeping, and as the source of all movement not only in the heavens but also on earth. Attempts to describe the motions and influence of these "ideal" bodies transcended study of physical reality. Astronomy and mathematics were divorced from the mechanics of motion that applied to earthly matter.
Still influenced by classical perceptions, modern astronomy takes as its subject all matter that lies beyond — or comes from — objects above the atmosphere of the earth, but recognizes that matter, and certain forces like gravity, are the same everywhere. This allows us to extrapolate from our experience on earth and predict the behavior of the planets and stars. Our goals during this year will be to
- Identify, name, and catalog the objects that belong to the study of astronomy (solar system and stellar astronomy)
- Discover the common processes and forces that govern their composition and motion (chemistry and physics)
- Discover the processes that drive their formation and eventual distruction (cosmology)
As you can see, modern astronomy rests heavily on the discoveries of other sciences, and pulls together many of the natural laws discovered in physics, chemistry, and geology.
Learning and Applying Formulae
One of the key problems many students have learning formulae is that they fail to understand what a given formula explains or tells them. In astronomy and physics, the letters or variables in the formula stand for physical concepts that are related to one another in particular ways that reflect how the universe actually works. Instead of reading
FG = GMm/r2
as a bunch of disconnected letters, try reading it this way:
The force due to gravitational attraction between two masses, given by M and m, depends directly on the product of the masses (M*m), and inversely on the square of the distance r between them (1/r2). The result must be multiplied by a constant of proportionality, G, which is universally constant in all places and at all times.
In mathematics, to say FG depends directly on M*m means that FG increases and decreases as M*m goes up or down. We say that F depends inversely on r2, which means that if r increases, F goes down.
Now think about that in terms of the physics involved. If we have a star with mass M and a planet with mass m that are a distance r apart, the gravitational force between them will be some amount FG. If we increase the mass of the planet m (or consider a planet with a larger mass) but don't change the distance, then the force of gravity must increase. In other words, if we compare the force of gravity between Sun and the Earth to the force between the Sun and Jupiter if it were at the same distance, then the force between the Sun and Jupiter would have to be greater than that between the Sun and Earth. If we moved the Earth out to the orbit of Jupiter, the force would be less than it is for Earth's actual location one Astronomical Unit from the Sun. Understanding what the formula means will help you remember its form: you know that you must divide by r2 because making increasing distance makes the force between two masses smaller.
Why bother with these calculations? Because our formula for gravitational force accounts for both mass and distance, we can compare the force between the Sun and Earth (with its current mass and distance from the Sun) to the force between the Sun and Jupiter (with its current mass and distance from the Sun). As we shall see in chapter 4, such comparisons lead to some important patterns that tell us how the force of gravity and the distance of a planet from its sun determines the rate at which the planet moves!
Practice with the Concepts
The Tools of Science: Theory and Measurement
Theory and fact
We look first at astronomy as a science, then at its methods of gathering and organizing observational data, and then at the actual observations of the phenomena appropriate to astronomy: the motions of celestial objects, the different objects themselves, and their possible origins. As you read through this introductory chapter, pay attention to the definitions of terms made by the authors, and look for any hidden assumptions about the nature of the universe that they are making. For example, they assume that an objective reality exists, and that events in that reality occur according to discoverable laws that are consistent throughout the universe. Without this premise, science as we practice it could not exist — but what evidence do we have that the universe really works this way?
Of course the assumption is useful: with it, we can assume that forces and patterns we observe in the earth's atmosphere, for example, will be similar to those we should find in Jupiter's atmosphere, or Titan's, and that studying other planets will help us understand processes in our own world. But is such an assumption valid? Are the similarities in patterns "real" or something we perceive as we try to create categories and organize data?
It is not necessary to have studied chemistry or physics to learn astronomy, but it helps to realize that most of astronomy has its basis in the the laws of motion and gravity that Kepler explored and Newton described, the chemistry rules that govern the generation of spectra from electron movement within atoms, the laws for nuclear fission and fusion that describe the formation of elements within star cores, the rules of thermodynamics and entropy that discuss the flow and transformation of energy, and the rules of relativity that describe the non-Newtonian behaviors of objects moving at speeds that are a significant fraction of the speed of light. What you study in physics and chemistry will help you learn astronomy; what you study in astronomy will help you when you come to explore the basis of the concepts in physics and chemistry.
Nothing we learn here is fixed and unquestionationable. The universe is real, but our theories or descriptions of it are human constructs, and must always be both incomplete and provisional. While we will learn the dominant theory of the origin of the Universe, the Big Bang, this theory has been under attack by Fred Hoyle and his associates, who claim that the background radiation accepted by most scientists as evidence for the original explosion is in fact the result of the constant production of hydrogen and helium by stars, in violation of the nearly universally accepted idea that matter cannot be created. Just because a theory is dominant does not mean that everyone accepts it, or that it shouldn't be seriously re-examined whenever it fails to account for new observations.
The History of Astronomy: a crash course
Astronomy is sometimes called the oldest science, and we certainly have a large number of records from Babylonian and Egyptian observations of astronomical events and predictions based on those events, dating back to before 3000 BC, to substantiate that claim.
Stele showing King Melishipak II, dated to 1186 B.C.
The star, sun, and moon are symbolic of Babylonian gods Ishtar, Shamas, and Sin.
Early astronomers were often also priests.
These observations allowed the Greek philosophers of the golden age in the 5th century BC to predict lunar and solar eclipses, based on a well-established eclipse pattern, and to predict when planets would be near one another or in conjunction.
During the fourth century BC, the Greek philosophers Plato and Aristotle wrote several scientific treatise to explain astronomical phenomena. According to Aristotle's theories, the heavy earth lay at the center of a number of concentric spheres, each sphere containing a planet on its surface. Elements below the surface of the moon were transient and moved either toward or away from the center of the earth unless compelled to move in other directions by outside forces. Objects above the sphere of the moon were made of a permanent, unchanging material that moves only in circles, and at a uniform speed.
Aristotle's theory explained the physical nature of the planets but couldn't explain their movements. During the second century AD, the Hellenistic philosopher Claudius Ptolemy used the information in the library of Alexandria, Egypt, and his own understanding of Euclidean geometry to fashion a planetary theory that accounted for the motions of the planets, but which violated the physical picture of the universe that Aristotle had created. In Ptolemy's system, planets circled imaginary points which themselves circled the earth. They sped up and slowed down. They got further away and closer to the earth. Ptolemy's planetary theory predicted the positions of the planets accurately for hundreds of years, but couldn't explain what they were made of.
The two systems existed in competition for a thousand years, until Nicolai Copernicus proposed "simplifying" the view of planetary motions by putting the earth itself into motion in his De Revolutionibus, published in 1543. His theory touched off debate and fueled religious controversies. Johannes Kepler furthered complicated the problem by throwing out the Ptolemaic circles altogether in favor of the elllipses he derived from examination of Mars' orbit. Galileo Galilei used the telescope to observe phases of Venus, craters on the moon, sunspots on the surface of the sun, and finally moons circling Jupiter. Finally, Isaac Newton tied all the pieces together with his universal theory of gravity, which accounted for Kepler's elliptical orbits with a theory based in physical forces.
During the eighteenth and nineteenth centuries, observatories catalogued different kinds of objects, but it wasn't until the end of the nineteenth century and the beginning of the twentieth century that people realized the distances involved in the observations and the wealth of different types of objects in the universe. Advances in the science of chemistry, especially in understanding the spectra of the elements, and in thermodynamics, enabled observers to classify stars by color, temperature, luminosity, and composition. Distance measurements proved the existence of whole galaxies external to our own Milky Way. The discovery of nuclear fission and fusion gave astronomers — now astrophysicists — the clue to the source of starllight: the manufacture of heavier elements from hydrogen and helium in the core of the star. Better measurements and an experiment proving the non-existence of the ether led to the development of the theory of relativity, which replaces Newtonian mechanics for objects moving at high speeds.
The latter half of the twentieth century has brought new observations from better telescopes, some land-based and some satellite-born. These in turn have fed theory and controversy over the nature of very bright objects called quasars; over the possible existence of black holes, and over the origin of the universe itself.
We will meet all these names again as we study astronomy in detail. Even the "deadends" like Ptolemy's are useful in keeping us aware of the type of reasoning we need to apply to astronomical observations.
Discussion Questions
- What are the best ways you have come up with for studying scientific materials?
- What helps you to "read" a diagram or graph so that you understand the information it contains?
- How can you best learn to understand and apply mathematical expressions of physical law?
- What is a universal physical law?
Optional Readings
Each week, I'll try to recommend some of the better web sites around for particular topics in astronomy. I will not test you on the content of these sites, so don't feel compelled to visit them! Some you may find useful in offering alternative explanations for concepts you have difficulty understanding; you'll have to pick and choose according to the amount of time you have available.
Michael Fowler has an excellent site at the University of Virginia on the history of astronomy. You can read his summary of Aristotle's contributions to astronomy or explore the rest of his site -- he has a number of lectures published on the web for his course on Einstein and Galileo.
Keep up with astronomy news by checking Science Daily or Universe Today websites for the latest developments.
One of the best online astronomy texts is Nick Strobel's introduction to astronomy, written for his Bakersfield, California, junior college course. I will be refering to this text from time to time, as it has a lot of good graphics, but be forewarned: graphics take time to load, so don't check out this site when you are in a hurry. For this week, you may want to take a look at the introductory material and scale model pictures of the solar system to get an intuitive sense of the relative sizes of the planets and their distances from the sun. These are in chapter 1, "Astronomy as a Science and a Sense of Scale"
The Astronomiae Historia site has lots of links on the history of astronomy.
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