Human beings are curious by nature and have marveled at the night sky ever since our Homo sapiens ancestors first gazed up into the heavens. What is “up there”? Why do stars shine? How did the Universe begin? Does life exist elsewhere? What is on the other side of the Moon?

Astronomy is one of the oldest sciences, but modern physics and technology, coupled with observations from space, have recently generated a stupendous wave of new knowledge. Most of our earliest questions about the nature of the Universe have now been answered, and many unexpected, intriguing new findings have been made, findings that invite us to be both humble and bold. And one needs not be a professional astronomer or physicist to understand them.

Our intent in writing this book has been to offer to the general reader a summary of current astronomical knowledge, generously illustrated and provided with rigorous but simple explanations, while avoiding mystifying professional jargon.

The 250 “windows” on astronomy in this book do not exhaust the topic, but we hope that they will pique the curiosity of our readers and stimulate them to explore further, by navigating on the World Wide Web or by consulting some of the many fine publications on astronomy, such as those suggested at the end of this book. Most important of all, we hope that they will find renewed wonder in the night sky!

Interested in amateur astronomy? What are the first steps?

Astronomy is not just for professionals – the sky belongs to everyone! Amateur astronomy is a fascinating hobby, running from the simple pleasure of gazing at the night sky, to learning to appreciate the phenomena and mysteries of the Universe, all the way to making quasi professional observations. Perusing books and magazines on astronomy, even studded with spectacular photographs, can never match the emotional impact of engaging directly with the heavens.

Pastime or true passion, here is an activity that is within almost everyone's means. Even the most sophisticated amateur astronomers spend significantly less money on their hobby than do many sportsmen, boating enthusiasts, and golfers. It is important to start out on the right foot, though. If you just rush in and buy a cheap “toy” telescope, you will quickly be disappointed and lose interest. On the other hand, if you acquire the biggest, most expensive instrument on the market, you are likely to find yourself overwhelmed.

The best way to start is to observe the sky with the naked eye from a dark site using a star chart or one of the new handheld devices using GPS technology (such as SkyScout). Once you have learned to find your way in the sky and are familiar with the major constellations and planets, you can move up a step, to binoculars. Standard models, 8 χ 40 or 7 χ 50, are good choices. They are relatively inexpensive, convenient to use, and offer a generous field of view, making it easy to locate objects.

How do refracting and reflecting telescopes differ?

Generally speaking, a telescope† is an instrument that enhances the observation of celestial objects by increasing their apparent size and luminosity. This applies to the entire electromagnetic spectrum, from radio waves to gamma rays, including of course the “optical domain,” which covers visible light and radiation in the infrared and ultraviolet. Optical telescopes can be very diverse, but basically they work just like a photographic camera: they focus the light of a celestial object to form a real image‡ on film or on an electronic detector. In the visual version, an “eyepiece,” which is basically a magnifying glass, is used to observe the image directly.

The terms “refracting” and “reflecting” simply refer to the composition of a telescope's optics. If lenses are used, the instrument is a refracting telescope; if mirrors are used, it is a reflecting telescope.

Which system is better? Well, in general, the larger the telescope – that is, the larger its main mirror or lens – the more light it can collect and the fainter the object it can observe (Q. 205). Reflecting telescopes can be built very large indeed, whereas refracting telescopes have serious size limitations.

What is life?

Are we alone? Does life exist anywhere besides Earth? Modern astronomy can help to answer these fundamental questions, but first we need to know what is meant by life.

The concept is hard to define. It used to be generally accepted that life is organized matter that exhibits seven crucial characteristics: growth, respiration (i.e. exchange of gases), nutrition, excretion, reproduction, reaction to external stimuli, and locomotion (the last is only partly true, since plants and many other organisms are not mobile). Matter in a crystal is organized, and one could argue that a crystal grows by nourishing itself with neighboring atoms, that it reproduces when it branches out, and that it reacts to external stimuli, contracting when subjected to an electrical charge, for example (as in the piezoelectric effect used in quartz watches). Nevertheless, a crystal does not breathe, does not excrete, and does not move. Then what about fire? A fire nourishes itself with combustible material, absorbs oxygen (breathes), grows, moves, can ignite additional fires in new locations (reproduces), excretes heat, and reacts to external stimulations such as wind or a fire extinguisher …

But if it can be argued that fire meets the seven basic criteria for life, living creatures fulfill one additional condition: they can mutate. This allows an organism to adapt to new environments and thus to evolve. Fire cannot evolve into a different form of oxidation, such as rusting, but we know from the fossil records that life transforms itself all the time, resulting in the extraordinary variety of forms around us: from bacteria to watermelons to oysters to elephants.