Introduction Antecedents Kepler's New Astronomy On Proportion Recommended Books

From ancient times to the present, the motion of the stars has inspired human observers with a powerful sense of wonder.

From a vantage point in the Northern Hemisphere, each night stars trace out wide circular arcs around the North Star, Polaris, returning to the place where they started over an approximately 24 hour period.

Some stars, however, seem to move at a different rate than the others.  The ancient Greeks called these stars “wanderers” or planetes in their language.

When the movement of these wandering stars is charted over the course of multiple nights relative to the motion of the "fixed stars," they alternately speed up and slow down, tracing out tangled curves and irregular squiggles over time.

The "fixed stars" are so far away that they do not appear to move within a human lifetime, as observed through the instruments of Kepler’s day.

The Sun also moves irregularly with respect to the fixed stars of the celestial sphere.

What causes these irregular motions?

Patterns

Ancient astronomers noticed that although the planets moved irregularly, they always appeared in the same narrow band of the sky. Today, we know that this narrow, 9 degree-wide section of the sky represents the plane of our Solar System. (All of the planetary orbits in our Solar System lie in roughly the same plane).

Because the Earth is tilted on its axis by 23.5 degrees, the plane of the Earth's orbit (and Solar System) is thus tilted with respect to the horizon by 23.5 degrees. (See the illustration).

Ancient observers named the 9 degree-wide region of the night sky where the planets appear the “Zodiac.”

Measurement

How did astronomers chart the movements of the stars without modern equipment?

The North Star acts as reference point because all of the other stars pivot around it (at least in the Northern Hemisphere). Observers could measure the vertical angle (colatitude) between the North Star and other stars.

Observers also imagined a “celestial equator” at the great circle on the celestial sphere 90 degrees from the pole star. This great circle represents the equator of the Earth projected out into space.

To measure longitude, observers made note of the constellations that straddled the 9 degree band of the sky where the wandering stars appeared. The constellations or "fixed stars" are so far away that they do not appear to move, save the movement induced by the daily rotation of the Earth on its axis. Ancient astronomers created a functional coordinate system based on twelve constellations that scientific observers used until the 17th century (and astrologers continue to use).

Ancient astronomers divided up the 360 degree circle of longitude on the Zodiac into 12 sections of 30 degrees each, and named each 30 degree section for the most prominent constellation in it.  Two bright stars, Antares and Aldebaran, lying 180 degrees opposite from each other in the center of their respective constellations anchored the system and served as points of reference. 

During the time of astronomer Johannes Kepler, longitudinal stellar measurements were reckoned as follows: “22 degrees 11 minutes 6 seconds Libra,” indicating the angular distance a star was found beyond a particular constellation.

All the major Eurasian civilizations of the ancient world (India, China and Mesopotamia) shared the Zodiac coordinate system, which endured for centuries.

For more on ancient astronomy, click here.