Distances in Astronomy

Ancient Skies – to – Modern Ideas

Study of the objects in the sky and their motions was an important activity for many early societies. Great effort was invested in attempts to predict the motions of celestial objects.

Early Civilizations:  From the dawn of civilization, many societies developed world models to explain the patterns and motions of celestial objects.  Several motivations drove investigation of theses patterns:  Practical applications, such as navigation, and establishing a calendar  Religious, spiritual and mystical reasons  Many societies expanded great effort in constructing elaborate structures to serve as astronomical observatories, such as Stonehenge in rural England.  These world models attempted to explain phenomena visible to everyone:  The daily and annual motions of the Sun across the sky  The motion and phases of the Moon  The daily and annual motions of the stars (i.e., the celestial sphere)  The odd motions of the 5 known planets, or “wanderers”  Solar and lunar eclipses  The planets (Mercury, Venus, Mars, Jupiter and Saturn) appeared only as points of light on the sky, like the stars, but unlike the Sun and Moon. These objects received special attention because they:  moved against the background of stars,  are always located within several degrees of the ecliptic,  periodically undergo retrograde motion with respect to the background stars,  change their observed brightness, unlike the Sun, which remains constant  Many, early societies attained a detailed mathematical understanding of the patterns in the sky, including the Egyptians, Babylonians, Indians, Chinese, Mayans, and Native Americans. Ancient astronomers could predict eclipses, and understood that these were caused by shadows cast by the Earth and Moon.

Greek Astronomy:  The Greeks developed the geocentric model of the Universe circa 450-250 BC. Elements of this model:  Earth was located at the center of the Universe.  The Sun, Moon, planets and the celestial sphere all “circled” the Earth.  Celestial objects moved in perfectly circular orbits at constant speed.  Aristotle (384-322 BC) was the principal developer of this model.  The Greeks also understood that the Earth was a sphere, not a giant flat disk, and correctly estimated the Earth’s size, using basic trigonometry.  In order to adequately explain the odd motions of the planets, the geocentric model used a series of perfect circles, termed epicycles and deferents within an Earth-centered framework.  A few early “astronomers” guessed correctly that Earth was NOT the center of the Universe, notably the Greek philosopher Aristarchus (320-230 BC). He proposed a Sun-centered universe, but could not provide observational support for his ideas.  The Greek astronomer Ptolemy refined the geocentric model, codified in a text called The Almagest – which became the accepted worldview for the next 1500 years (100 AD to ~1650 AD).

The Middle Ages:  During the period from 500 AD to 1500 AD little progress was made amongst European “astronomers”, as Europe languished in the Dark Ages.  Astronomy prospered elsewhere, especially in the Middle East, China, and Central America.  In the Middle East, Arabic astronomers labeled many of the stars in the sky (Betelgeuse for example), and improved on mathematical techniques initially developed by the Greeks.  Chinese astronomers recorded many celestial phenomena, including a “new star” in the heavens which appeared in 1054 AD, then faded away.