Chapter 2: Early Astronomy

Astronomy, as depicted by Florinus in 1705.

The Sun, Moon, and the stars all move in predictable ways. Throughout history, cultures all over the world have used these patterns to regulate their lives. For agricultural purposes, they needed to establish calendars and they used the solstices and equinoxes as seasonal indicators. For navigating across land or sea, they needed to map the star patterns or constellations. As early as 2600 B.C., astronomical observations led to the construction of astronomically aligned temples in many parts of the world. These were not observatories in the modern sense, but buildings created around a rich brew of astronomy, religion and mythology.

16th-century representation of the Ptolemy's geocentric model

The birth of science occurred in Greece around 500 B.C. to A.D. 150. Greek philosophers made two extraordinary contributions: they applied logic and reason to understand the universe and they applied the principles of geometry to measure the true sizes and distances of objects in space. The Greeks established that the Earth and Moon were spherical worlds in space and they understood the illumination that leads to the phases of the Moon and eclipses. They used the geometry of lunar and solar eclipses and other observations to estimate the shapes and relative distances of the Moon and Sun. They discovered how to make a direct measurement of Earth's circumference. Aristotle developed a model where the spherical, stationary Earth stood at the center of the universe: a geocentric cosmology. The bold idea of a Sun-centered or heliocentric cosmology was also created at this time, but there was not enough evidence to support it and the geocentric picture was chosen by Ptolemy in his summary of Greek astronomy. After the fall of Greek civilization, astronomy made few advances in Europe for over 1,000 years.

Frontispiece of the "Rudolphine Tables" published by Johannes Kepler in 1627.

Copernicus showed that the complexity and the arbitrariness of Ptolemy's model could be reduced if the Sun were at the center of the planet orbits. The elegance and economy of Copernicus' model were strong points in its favor. Tycho Brahe then produced a new and accurate set of data on the motions of the planets. In Kepler's hands, it was clear that the orbits were better described by ellipses with the Sun at one focus than by circles with the Earth at the center. Kepler also came up with simple relationships that described the orbits in terms of their distance from the Sun. Galileo made the definitive break with ancient physics. His telescope observations showed that the Copernican model was correct and that the planets were worlds like the Earth, with their own moons and their own geology. This led to another new idea called the plurality of worlds. This idea holds that there are many planets, that some of them may have properties like Earth, that there may be planets around other stars, even with life forms. These possibilities are strikingly modern — they also relegate the Earth from the center of creation to just one of many astronomical objects.