Filippenko's Book The Cosmos

Section 1.1: Peering through the Universe: A Time Machine

This is an enormously exciting time to study astronomy. Telescopes and the instruments on them are better than ever, providing new clues to a wide variety of phenomena and deepening our understanding of the cosmos.

The Universe is about 14 billion years old. Compared with a 24-hour day scaled to match the age of the Universe, the Earth formed about 8 hours ago, and humans appeared only 2 seconds ago.

Scientists often use a version of the metric system, in which prefixes like “kilo-” for one thousand and “mega-” for one million go with units like meter for distance and second for time, or are found in kilogram for mass. Astronomers also use special units, like the light-year for the distance light travels in one year. By viewing objects that are at different distances from Earth, we look back to different times in the past, because light travels at a finite speed. Thus, the observation of very distant objects we see today provides a glimpse of the Universe as it was a long, long time ago.

Section 1.2: How Do We Study Things We Can't Touch?

Astronomers gain most of their information about the Universe by studying radiation from objects. In addition to taking photographs, we can break down an object’s light into its component colors to make a spectrum, like a rainbow. We study not only the visible part of the spectrum, but also its gamma rays, x-rays, ultraviolet, visible light, infrared, and radio waves.

Section 1.3: Finding Constellations in the Sky

The sky is divided into 88 regions known as constellations. The individual stars in a given constellation are generally at quite different distances from Earth and are not physically associated with each other; they are simply apparent groupings of stars. As the Earth rotates about its axis and revolves around the Sun, different constellations appear in the night sky. It can be fun to look at the sky and know some of the constellations, as well as their related mythology (Sec. 1.3a–d).

Section 1.4: How Do You Take a Tape Measure to the Stars?

Some of the most exciting studies in modern astronomy are related to the determination of distances. We can measure how far away the Moon or a planet is by bouncing radio waves off it and measuring the total amount of time the waves were in flight. It is much more difficult to measure the distances of stars, galaxies, and other objects beyond our Solar System, but later in this book we explain the methods used by astronomers.

Section 1.5: The Value of Astronomy

The study of astronomy has had a major impact on the development of science throughout human history. For example, observations of the changing positions of the Moon and the planets led to an understanding of gravity and the laws governing the motions of bodies. Moreover, astronomy allows us to investigate human origins, including the creation of the chemical elements and the formation of the Sun and Earth. The Universe is a cosmic laboratory to test ideas of science in conditions often not available in laboratories on Earth. Observations of the skies may even help humans avoid or delay extinction, if we are able to discover (and subsequently deflect) large asteroids or comets that are on a collision course with Earth.

Section 1.6: What Is Science?

One of the main principles of science is that results should be reproducible; other scientists should be able to get essentially the same result by repeating the same experiment or observation. Astronomers and other scientists follow the scientific method, which is difficult to define but provides a standard about which scientists agree. In the basic form of the scientific method, a hypothesis passes observational tests to become a theory.

Section 1.7: Why Is Science Far Better Than Pseudoscience?

Many people have beliefs that may seem related to science but either have no present verification or are false; they are based on pseudoscience rather than on authentic science. Astrology (as opposed to astronomy), for example, passes no scientific tests and so is not a science. Furthermore, astrology has been shown not to work.

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