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Motion of Stars

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, every 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.

Stars Pivot Around the North Star

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 position of Mars relative to the fixed stars

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

How did astronomers in Kepler's day explain the irregular motions of the wandering stars?

How did they chart observations of the planets from a basic technical standpoint?

Greek & Indian Antecedents