DK - The Astronomy Book: Big Ideas Simply Explained

In many of the worlds most populated areas today, many of us are barely aware of the night sky. We cannot see it because the blaze of artificial lighting overwhelms the faint and delicate light of the stars. Light pollution on this scale has exploded since the mid-20th century. In past times, the starry patterns of the sky, the phases of the Moon, and the meanderings of the planets were a familiar part of daily experience and a perpetual source of wonder.

Few people fail to be moved the first time they experience a clear sky on a truly dark night, in which the magnificent sweep of the Milky Way arches across the sky. Our ancestors were driven by a mixture of curiosity and awe in their search for order and meaning in the great vault of the sky above their heads. The mystery and grandeur of the heavens were explained by the spiritual and divine. At the same time, however, the orderliness and predictability of repetitive cycles had vital practical applications in marking the passage of time.

Archaeology provides abundant evidence that, even in prehistoric times, astronomical phenomena were a cultural resource for societies around the world. Where there is no written record, we can only speculate as to the knowledge and beliefs early societies held. The oldest astronomical records to survive in written form come from Mesopotamia, the region that was between and around the valleys of the Tigris and Euphrates rivers, in present-day Iraq and neighbouring countries. Clay tablets inscribed with astronomical information date back to about 1600 BCE. Some of the constellations (groupings of stars) we know today have come from Mesopotamian mythology going back even earlier, to before 2000 BCE.

"Philosophy is written in this grand book, the Universe, which stands continually open to our gaze."

Galileo Galilei

The systematic astronomical observations once used for astrology started to become increasingly important as a means of both timekeeping and navigation. Countries had highly practical reasons civil, as well as military to establish national observatories, as the world industrialized and international trade grew. For many centuries, only astronomers had the skills and equipment to preside over the worlds timekeeping. This remained the case until the development of atomic clocks in the mid-20th century.

Human society regulates itself around three natural astronomical clocks: Earths rotation, detectable by the apparent daily march of the stars around the celestial sphere to give us the day; the time our planet takes to make a circuit around the Sun, otherwise known as a year; and the monthly cycle of the Moons phases. The combined motion in space of Earth, the Sun, and the Moon also determines the timing and magnitudes of the oceanic tides, which are of crucial importance to coastal communities and seafarers.

Astronomy played an equally important role in navigation, the stars acting as a framework of reference points visible from anywhere at sea (cloud permitting). In 1675, British King Charles II commissioned an observatory, the Royal Observatory at Greenwich, near London. The instruction to its director, the first Astronomer Royal, John Flamsteed, was to apply himself diligently to making the observations needed for the perfecting of the art of navigation. Astronomy was largely discarded as the foundation of navigation in the 1970s, and replaced by artificial satellites, which created a global positioning system.