Preface

Education is what survives when what has been learned has been forgotten.

This Web-based course, or "webbook", is a college-level, semester-long, non-calculational introduction to astronomy (it may also be suitable for a high school course). It was initiated while I was a teaching affiliate in the Physics Department at Emory University in Atlanta, Georgia. I taught the class for many years in a traditional manner using a blackboard, models, slides, transparencies, and video, but was frustrated by many aspects of it. If I attempted to cover all of the material I wanted to discuss, I had to proceed very quickly, resulting in much information only "in the air" rather than written on the blackboard. Images I drew were necessarily schematic and even then were difficult for the students to reproduce in their notes. Many of the concepts I was presenting involved motion or other changes with time, and sequential images and/or arrows on a blackboard are not as descriptive as a real animation. When I used slides and videos, they were not generally accessible to the students for review. The common result was that the students' record of the course material was often poor, and their understanding suffered.

Also, I found that the students I was teaching were changing. They had more difficulty absorbing and integrating information, especially when presented textually. They were becoming less focused and more easily bored. A traditional "chalk talk" could easily lose their attention.

I was therefore pleased when I began to find many digital images of astronomical phenomena appearing on the World-Wide Web. (Astronomy Picture of the Day has been a favorite place to discover new and interesting images of the cosmos.) These photographs could be easily integrated together with text, animated images, video, and sound to form the basis for class presentations, as well as a record that the students could use later for studying. In addition to addressing the issues described above, it becomes possible to spend more time on areas of difficulty (Just-in-Time Teaching) or with in-depth inquiry-based activities.

The order of presentation of the material differs from all of the astronomy textbooks I have seen, though it is largely inspired by the early editions of Discovering the Universe, by William J. Kaufmann III, in which a comparative approach is emphasized. It is certainly also influenced by the fact that I am a physicist first and an astronomer second.

As is common in introductory astronomy textbooks, the Foundations section discuss early astronomy and astronomical technology. This results in an almost complete overview of basic physics, but other sources usually leave important pieces of physics out, waiting until absolutely necessary to introduce them to explain astronomical phenomena. I found this approach to be very fragmentary, and I believe a more complete, self-consistent introduction of physics provides a better foundation for understanding our universe. Therefore, if a physical principle is used to understand astronomy multiple times, or if it closely fits in with the overview, I have included it in the Foundations. When the principle shows up again, it provides an opportunity for comparison and review, which enhance comprehension. Only if a physical principle is relatively isolated, conceptually, is it introduced later.

I have also long been puzzled by the lack of discussion of the concept of energy in introductory astronomy textbooks, the conservation thereof being so important in physics and astronomy. The term is commonly used, but never fully defined, except in a few instances, such as in special relativity. I have therefore provided a complete though basic treatment of the subject. Other conservation laws (mass, charge, angular momentum, etc.) are also mentioned.

After Foundations I have taken yet another deviation from the "standard", by placing the Stars section before the Planetary Systems section. This is for two reasons, the primary one being that the formation and death of stars are necessary precedents to the formation of planetary systems, and so I feel it helps to have an understanding of the former process first. In addition, with stars there is a greater opportunity to apply or reapply much of the physics learned in earlier chapters, and so reinforce it sooner.

Not surprisingly by now, the Planetary Systems chapters have their own uncommon order, beginning with a general consideration of what we know about observed planetary systems, followed by a discussion of the smallest objects (dust, comets, asteroids, and icy asteroids), proceeding to the smaller terrestrial worlds (Mercury and the Moon), the Jovian Moons, and then the larger terrestrial worlds (Earth, Venus, and Mars). The Jovian worlds round out the section. In this order I can progressively discuss cratering, the existence of water and atmospheres, etc., and apply Kaufmann's comparative approach.

The course ends in the usual way with a Galaxies and the Universe section. I haven't seen much uniformity in the way that the Universe is discussed, but I've organized it with an initial presentation on its structure as we understand it now, followed by its past and future evolution.

I have always made these web presentations available to the entire Internet, and it has been gratifying to receive so many positive responses from students, teachers, and others who have discovered them and learned from them. Although I am no longer at Emory, I am pleased to be able to continue to develop and publish these educational materials.

I encourage questions, comments, suggestions, and corrections.