ASTR 1210 (O'Connell) Study Guide


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 astronomical research.

A. What is Science?

My definition


Key feature

The scientific method in practice

Science and "Truth"

Science and "Common Sense"

Science and mathematics

B. Alternative Modes of Thought

It is worthwhile distinguishing science from some other important modes of thinking.

Revelation (religion)




C. Skepticism

"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 scientific ideas.

Skepticism based on facts is not an established element in many other modes of human thinking. In fact, most people are uncomfortable with skepticism and instead see virtue in conviction, certainty, and strong beliefs.

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 they have 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 knowing."

---- J. Gleick, writing about physicist Richard Feynman.

Golden Gate Bridge

The Golden Gate Bridge, a premier example
of 20th century technology

D. Science vs. Technology

Although science and modern technology are often intertwined, they have different goals and value systems; and we need to clarify the distinctions between them. Science and technology are symbiotic but distinct:

All technology has a societal motivation, whether for ultimate good or ill, but the main motivation for "basic" science is simply curiosity and the desire to understand.

Job descriptions:

E. Results of Science

The basic result of science

Some other key results


How well determined are scientific results?

Influence on society

Dusk @ Mt. Shasta

The Moon and Venus at dusk

F. Astronomy as a Science

Astronomy is the study of the physical universe beyond the Earth's atmosphere.

It is the oldest science, nearly universally practiced in literate and pre-literate societies, and it has been a major stimulus to other scientific fields from Greek times to modern physics

Relevance to society

  1. Astronomy investigates ultimate origins (an adjunct or alternative to religion)
  2. Astronomy provides our basic global perspective of time and space, i.e. a cosmic context
  3. Astronomy was fundamental to the historical development of scientific thinking and the formulation of the first generalized physical laws by Newton.
  4. 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

MW Starfield

Starfield near the center of the Milky Way. How many stars can you count here?
Click for a full-resolution enlargement. (Image from the Hubble Space Telescope.)


Astronomical time- and distance-scales are tremendously larger than the "common sense" scales we encounter in everyday life. See the chart above for scales of 107 meters and above. 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

  1. As an example of the contrast between human perceptions and physical reality consider the Earth's atmosphere. It seems enormous and all-encompassing, right? Atmospheric disturbances (rainstorms, tornados, hurricanes, droughts) easily destroy human structures and livelihoods.

      Can you propose a simple scale model for the atmosphere, using, for example, a basketball to represent the Earth?

      Click here for the answer.

      Despite our impression, the atmosphere represents only a tiny fraction (one part in 1600) of the Earth's diameter. See this image taken from a spacecraft, showing the atmosphere as a thin blue line on the horizon. (The dark ridge of snow-capped mountains running diagonally across the Earth's surface is the Himalayas, the world's highest.)

      In fact, you can actually walk out of Earth's atmosphere. Anyone who has been above 26,000 feet altitude (and there are 14 mountains this high) has been beyond the point where the air density has dropped a factor of 2.7---the official "one scale height" definition of the atmosphere's thickness.

  2. A sample cosmic time scale:

    • The age of the Sun is about 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 about 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! (Modern humans have existed only for about 200,000 years, or 0.004% of the age of the Sun.)

        Here is an image of the first two pages of our textbook with the beginning of this time-scale model superposed.

        The Earth is almost the same age as 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 present form.

        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 has evolved.

    • 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, biological evolution).

  3. 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 gram/cc).

        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.

    • Here's the surprising answer. It illustrates how incredibly thinly matter is spread on a cosmic scale by the standards of our everyday, "common-sense" world.

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Last modified October 2023 by rwo

Text copyright © 1998-2023 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.