ASTR 1210 (O'Connell) Study Guide
1. INTRODUCTION: SCIENTIFIC DISCOVERY
AND THE SCALE OF THE UNIVERSE
The European Southern Observatory Very
Large Telescope, Chile.
This introductory lecture places astronomy in the broader context of
science. It discusses the nature of science, how science is
distinguished from other modes of thought, the difference between
science and technology, and some of the main results of science.
Astronomy has had a strong influence on other sciences and defines the
limits of the scientific universe. We will illustrate some of the
mind-boggling cosmic spatial and temporal scales revealed by
A. What is Science?
My definition: science is the systematic understanding of empirical
"Empirical" = experienced or observed
Science is both a body of knowledge and a way of thinking.
Scientific values & methods are relatively new in human
history, starting ca. 1500.
- Logical; rational
- Clear, orderly; choose the simplest alternative ("Occam's razor")
- Cumulative development
New interpretations are required to be consistent with all established
Given all that we now know about nature, this is a tremendously challenging requirement!
The scientific method in practice:
- Science is based on an empirical criterion
of truth: ideas are tested by objective experiment/observation in
the real world.
- Evaluation of evidence must be rigorous, objective, and public.
- Any valid scientific proposition is falsifiable by
empirical tests. I.e. it is well enough defined that you can specify
in advance those outcomes that would show the idea to be wrong.
- The results of tests must be repeatable.
- Confidence in ideas grows with the number of independent
tests or validations. "Independence" implies verification not only
by different people but also by different methods of
inquiry or lines of evidence.
Scientific methods were not invented or prescribed wholesale by
some individual or group. Rather, they emerged over a period of several
centuries as the set of analytical techniques that proved most
successful in the struggle to understand nature.
The usual formalized statement of "The Scientific Method"
(e.g. Hypothesis ===> Prediction ===> Experiment ===> Interpretation
===> Repeat) is misleading:
Scientists do not necessarily work from "hypotheses." They frequently
simply explore the characteristics of new phenomena, without
any particular intent to distinguish between interpretations but with
the hope that they might prove interesting or important.
A number of sciences, including astronomy, deal with phenomena that
cannot be manipulated by humans. Experiments are impossible in
these cases. These sciences must rely instead on passive
For astronomy, the only exception is that we can,
with difficulty, sample and experiment with the surfaces and
atmospheres of some of the other bodies in the solar system.
Science and "Truth"
Scientific ideas are not claimed to be absolute
truths. They are always provisional, to a greater or lesser
degree. They are regarded as approximations to the truth,
subject to revision as new evidence emerges. They almost
always systematically improve with time, by discarding older,
less successful interpretations.
Science and "Common Sense"
The characteristics of scientific thinking should sound familiar to
you: this is really nothing more than slightly refined, standardized,
and tough-minded "common sense." Scientific thinking deviates
from "common sense" mainly in that it insists on validating ideas in
the context of a vast store of existing knowledge that goes
far beyond everyday experience.
The subjects and conclusions of science, however, often
differ drastically from the "common-sense perspective" that we
develop in our everyday lives. Humans directly experience only
a very small range of the distances, timescales, masses,
densities, velocities, and energy releases that prevail in the
universe around us, and those are often completely
non-intuitive for us.
In fact, most of science deals with phenomena that
are "invisible" in everyday life even if they, like electrons
and bacteria, are actually ubiquitous in the world around us. They
have been recognized by scientists only gradually through systematic,
painstaking observations and measurements.
We will lay the groundwork for going beyond the "common-sense
perspective" to a cosmic perspective in
Section G below.
B. Alternative Modes of Thought
It is worthwhile distinguishing science from some other modes of
E.g. Greek philosophy, Plato
Deductions about the physical world are made from abstract, a
priori postulates. It is assumed that empirical evidence
Fine for mathematics, badly flawed for real world.
Explanations are sought by appeal to a spiritual realm beyond the
natural world ("super"-natural).
Prior to the development of science, important physical phenomena were
usually interpreted as the product of deliberate and purposeful
manipulation by supernatural agencies, with motives largely beyond
human understanding. Knowledge of these agencies was possible only
through intentional revelation by them.
By contrast, science deals only with the natural world.
It seeks explanations of natural phenomena in terms of other natural
Supernatural explanations are not acceptable to scientists because
there is no way to test them.
Objective empirical tests are impossible for the supernatural world.
Science cannot predict supernatural behavior or intervention in
the natural world since, by definition, the laws of nature don't
However, science could in principle detect supernatural
intervention in the natural world a posteriori (after the
fact) because it would appear as a breakdown in the laws of nature.
The fact that discontinuities in natural law have never been verified
shows that direct intervention is rare at current levels of
detection. Frequent intervention would produce an arbitrary and
largely unintelligible world.
Revelation produces a hierarchy of privilege. Access to the
truth, and the corresponding power, is normally accorded only to a few
(priests, prophets, gurus). Non-democratic.
Ideas are tested by appeal to the pronouncements of an individual or
small group. These cannot be questioned or superseded. Again,
a non-democratic system.
The most familiar, and dangerous, variety of authoritative
"truth" is found in dictatorships. But authority is also the basis of
the case law system ("precedent") in American jurisprudence.
Despite the public impression, science is not based on
authority. Instead, it operates by consensus. Any scientist
is entitled to question and retest any idea at any time and attempt to
convince others to change their minds. Science is democratic.
Scientific "authorities" are respected not because of
who they are but instead because of what they say.
Their ideas are continuously subjected to new tests. The best idea
Young scientists often aspire to overthrow "authorities" like
Einstein; they almost always fail because their ideas are
not as good when tested against the evidence.
Examples: astrology, extra-sensory perception (ESP), psychic
forecasts, telekinesis, UFO abductions, and ghosts. Also, much
Pseudo-scientific investigations apply some, but not all, of
the established methods of science. They fall short of the basic
criteria listed above in some important way.
Hallmarks: uncritical acceptance of poorly documented phenomena; lack
of experiments or inadequate experimental controls; inexperienced
observers; logical fallicies; non-repeatability; often self-deception;
often for-profit publicity/entertainment; sometimes deliberate fraud.
Almost always: lack of consistency with established principles
or facts (e.g. foreseeing the future contradicts the verified
principles of special relativity).
The evidence is always inadequate
The issue is not that there are no good scientific explanations
for claimed pseudo-scientific phenomena. At any time, there
are thousands of well-documented phenomena that science does
not properly understand. These are what active scientists devote
their careers to studying. By contrast, in the case of pseudo-science
many of the phenomena themselves have not been demonstrated to
exist by objective evidence.
Most pseudo-science is harmless, except for the sloppy thinking and
miseducation it encourages. But some pseudo-science poses tangible
threats to you & society. For example:
For instance, we know that penicillin kills certain types
of dangerous bacteria. But no one has ever convincingly been shown to
be able to read someone else's mind.
If credible evidence emerges
for any of the marginal "pseudo" areas, they will then receive
mainstream scientific attention.
Fraudulent or ineffective "alternative" health-care (such
in which the active therapeutic ingredients are diluted to the point
where not a single molecule remains).
False criminal prosecutions (murder, child
abuse, etc.) based on erroneous psychotherapy (e.g.
memories "recovered" by hypnosis).
We will discuss the "UFO phenomenon" as an example of pseudo-science
later in Study Guide 18.
"Cultivated skepticism" is a cornerstone of science.
All good scientists are skeptics. This means that they maintain an
attitude of doubt or of suspended judgement about
Scientists insist on the best possible evidence in support of
new ideas. The adjective that best describes the scientific approach
to evidence is "relentless."
Accepted ideas, no matter how well established, are constantly
tested against new evidence, sifted for quality. All
preconceptions are subject to scrutiny. History shows that truth only
emerges through this kind of skeptical "scrubbing" of ideas and
evidence. The ideas that survive this process are very robust.
Competition over ideas, often intense, is a hallmark of science in
practice. There are strong incentives for scientists to discover new
phenomena or interpretations that countervail the accepted wisdom.
That is how young people make careers in science.
However, scientific doubt isn't frivolous; it must be based on a
balanced evaluation of the quality of the empirical evidence.
Good scientists are scrupulous in their treatment (positive or
negative) of the evidence; their reputations suffer otherwise.
Don't confuse healthy scientific skepticism with the activist
"skeptics" who have emerged over the last couple of decades to
dispute the scientific consensus on evolution, global warming,
vaccination against disease, tobacco-induced illness, etc. These
advocacy groups mostly do not care about the evidence or hold their
judgement in suspension---rather they have a committed belief
in a contrary view.
Skepticism based on facts is not an established element in many
other modes of human thinking. Most people are uncomfortable
with skepticism and instead see virtue in conviction, certainty, and
Russell remarked, and as we can see daily in news reports, this
may be the
"fundamental cause of trouble in the world
Errors occur in science, as in any human endeavor. Historically,
there have been many more wrong ideas in science than right
ones. Understanding the natural world is hard. Every scientist has a
long list (usually private) of mistakes he or she has made. But
because of skepticism and a reliance on real evidence, there is a
strong self-correcting mechanism operating in science: the
errors are identified and discarded.
"He believed in the primacy of doubt, not as
a blemish upon our ability to know
but as the essence of
---- J. Gleick, writing about physicist Richard Feynman.
The Golden Gate Bridge, a premier example
of 20th century
D. Science vs. Technology
Science and technology are symbiotic but distinct
All technology has a societal motivation, whether for good or ill, but
the main motivation for "basic" science is simply curiosity and the
desire to understand.
- Science: Attempt to understand universe, build
a conceptual framework.
Adjectives used: "unapplied," "pure" or "basic"
Examples: principles of gravitation, electromagnetism, biochemistry,
- Technology: Application of basic concepts to
Examples: structural engineering, electrical generators, weapons,
- Scientist: "be curious"
- Technologist: "be useful"
E. Results of Science
The basic result of science
Nature is understandable and operates
according to a small set of universal principles, or "laws"
We may take this for granted today, but it is an astonishing fact.
None of the early scientists who wrestled with trying to understand
nature could have predicted it.
Some other key results
- The laws of nature are the same everywhere in the observed universe
- All matter, including living systems, is organized hierarchically from
a smaller set of subunits
- The structure of molecules & atoms is determined by electromagnetic force
- The Sun is a star
- The Earth is a planet
- Genetic information is transmitted by DNA molecules
- Most communicable diseases are caused by microorganisms
- The universe is very ancient, and its structure and contents have changed
systematically with time
Very few of the important results of science had recognizable
precedents in earlier modes of thought (prior to 1500 AD).
The main exceptions are Greek physics and astronomy, e.g. the
models of the solar system by Ptolemy, ca. 130 AD, or the atomic
theory of Democritus, ca. 420 BC. The Greeks made great progress.
However, because of their inclination to idealism, they disdained
empirical testing of scientific ideas, which circumscribed their
ultimate accomplishments. And those were forgotten or actively
suppressed in the Western world for nearly a thousand years starting
about 400 AD. See Study Guide 6.
How well determined are scientific results?
Some scientific results are known with effective certainty
(e.g. Earth is a planet; only one Sun in the solar system).
But, as noted above, scientific results are subject to revision
based on new evidence. Scientific ideas normally systematically
improve by discarding older, less successful interpretations.
You can picture the progress of science as an inward spiral toward
the truth as better evidence accumulates.
At any given time, some scientific results are much less well
established than others, so there is a huge range of validity
(say from 99.999% certain to less than 50% certain) throughout the
Here is a simple, practical measure of the validity of the most
important scientific ideas:
If our scientific understanding of nature was seriously flawed, then
most of our technology would simply not work. The validity of
ideas like Newton's laws of motion or Maxwell's theory of
electromagnetism is tested continuously in every assembly line,
airplane, and cell phone in the world.
Keep this success in mind when evaluating debates over scientific
results in the public arena. The same analytic procedures and
standards of evidence that produced modern technology are behind the
scientific consensus in areas like biological evolution or global
warming that are disputed by activist groups.
Influence on society
As a new mode of thinking about the natural world, science succeeded
brilliantly. It demonstrated that humans can solve many problems
which at first seemed intractable: e.g. questions like "what causes
bubonic plague?" or "what are the stars?"
Success in science has been a great source of optimism
in human cultures. This was a major wellspring for the Enlightenment
and the political philosophies that culminated in the founding of
Science provides a demonstrably reliable and powerful
understanding of the physical & biological world. Over the past two
centuries it has transformed human societies. It is now the basis of
most of our technology, our wealth, and our collective and individual
well-being. The impact of science on our society is discussed further
in Study Guide 9.
The Moon and Venus at dusk
F. Astronomy as a Science
Astronomy is the study of the physical universe beyond the Earth's
It is the oldest science, nearly universally practiced in
literate & pre-literate societies, and it has been a major stimulus to
other scientific fields from Greek times to modern physics
Relevance to society
- Astronomy investigates ultimate origins (an adjunct or alternative to religion)
- Astronomy provides our basic global perspective of time & space, i.e. a
- Astronomy was fundamental to the historical development of
scientific thinking and the formulation of the first generalized
physical laws (Newton).
- Study of the other planets and our cosmic environment is essential to
assessing the viability of Earth's biosphere and prospects for
long-term human survival.
History of societal influence
- All societies: practical time- and calendar-keeping, navigation
- 5000 BC - 1609 AD: awareness of the Solar System (planets, sun)
- 16th-17th centuries: astronomy leads in the
demolishment of medieval thinking
- 1609: the newly-invented telescope reveals a vast stellar system beyond the planets
- Late 1600's: astronomy leads Newton to his laws of motion, which
initiate the "scientific revolution" and modern technology
- 19th century: speculation about extraterrestrial life
- 20th century: recognition of galaxies ("island universes"), the expanding &
evolving universe, the Big Bang, the extraterrestrial cause of mass
biological extinctions; discovery of other solar systems.
Starfield in center of Milky Way. How many
stars can you count here?
Click for a full-resolution
enlargement. (Image from the Hubble Space
G. ASTRONOMICAL DISTANCES AND AGES
Astronomical time- and distance-scales are tremendously larger
than the "common sense" scales we encounter in everyday life.
Unfortunately, this means that astronomical scales (not to mention
atomic or molecular scales) are utterly non-intuitive for us.
It is important for astronomers to develop a good cosmic
perspective that transcends the perceptual biases of everyday
experience. But it is difficult for anyone to visualize the scales
involved. Because the cosmic range is so enormous, scientists make
regular use of scientific mathematical notation ("powers of
ten"). For a review of this notation,
see Supplement I (PDF file).
Example astronomical scale models
Simple scale models can provide a rough, if never completely adequate,
impression of cosmic scales:
- As an example of the contrast between human perceptions and physical
reality consider the Earth's atmosphere. It seems enormous and
all-encompassing, but it represents only a tiny fraction of the
Earth's diameter. See this
image, showing the atmosphere as a thin blue line on the horizon.
Can you propose a simple scale model for this?
- A sample cosmic time scale:
- The age of the Sun is 5 billion years, a middling age for
an older star. Suppose we wanted to use the run of all the letters in
our textbook to represent this span of time. How many years would we
have to assign to each letter?
We discussed this in the introductory lecture, and by "rough order of
magnitude estimation" we determined that there would be 2000 years
per letter(!) if the entire textbook represented the age of the
Sun. All of recorded human history would fit within the first four
letters of the text! The existence of Homo sapiens on
Earth would extend only a couple of sentences!
The Earth is only about 10% younger than the Sun, so our scale model
graphically illustrates the vast span of time over which the surface,
atmosphere, and biosphere of our planet have been shaped into their
Your first homework assignment was simply to stare at
random pages from the textbook and try to absorb, in the context of
this scale model, the nearly unimaginable period over which the Earth
- The "dripping faucet effect": the very long time scales that
characterize planetary evolution mean that small but persistent
changes can eventually have major consequences. We will
see many examples of these (e.g. polar precession, continental drift,
- Finally, a healthy, fruit-based cosmic distance scale model:
- Use an orange to represent the Sun. This is an
appropriate choice not just because of the color but also because the mean
mass density of the Sun is about the same as an orange (1
In this model, the Earth to Sun distance (defined to be one
"Astronomical Unit") is 25 ft.
[Useful mnemonic: Earth diam : Sun diam : Astron Unit = 1:100:10,000]
- In this model, what is the scale distance to the next nearest
star (Alpha Centauri)?
An additional relevant piece of information that might help
you think about this question is that the Sun is about 10 billion times
brighter than the brightest stars as seen from Earth and that most
of this difference is a distance effect.
Reading for this lecture:
Bennett textbook: Ch 1 and Secs 3.4, 3.5.
Study Guide 1
Optional reading: Alan Cromer: Uncommon Sense; Bertrand Russell:
A History of Western Philosophy; Carl Sagan: The Demon Haunted
Reading for next lecture:
Bennett textbook: Ch 1 and Secs 3.4, 3.5.
Study Guide 2
Supplement I (PDF file) Skim and then refer to
this later as needed.
Optional: Cosmic History: A
Optional: browse the material on the structure &
evolution of the universe in the Bennett textbook Chs
22 and 23
May 2015 by rwo
Text copyright © 1998-2015 Robert W. O'Connell. All
rights reserved. Twilight image of Moon and Venus over Mt. Shasta by
Jane English. These notes are intended for
the private, noncommercial use of students enrolled in Astronomy 1210
at the University of Virginia.