Man's physical universe



spent his life making untiring and much more accurate measurements

of the positions of the stars and planets than had before been made.

He designed and built a large observatory equipped with the best

instruments. He rejected Copernicus' system but devised one of his

own, in which the sun revolved about a stationary earth and the

planets, in turn, revolved about the sun.

Johann Kepler (1571-1630), a student of Tycho's who fell heir to his

observations, became a convert to the Copernican system. Kepler's

main contribution was the discovery of three laws which laid the

foundation for the great work of Newton. Kepler's laws may be stated

as follows:

1. The planets travel in paths which are ellipses with the sun at one


2. A planet so moves that an imaginary line drawn from it to the sun

sweeps over equal areas in equal intervals of time.

3. The squares of the times that the planets require to make complete

revolutions about the sun are proportional to the cubes of their

mean distances from the sun.

According to these laws a planet moves fastest when it is nearest the

sun and slowest when farthest from the sun. It remained for Newton

to explain this phenomenon.

Galileo Challenged Authority with His Telescope.

The intellectuals of this time (about 1600) found their teachings

challenged by the concept of the Copernican system and, not being

open-minded, brought pressure to bear

on the not unwilling churchmen for a

protest. Therefore Galileo (1564-1642),

the first of the moderns, met with much


Galileo's great contribution to astronomy

was the construction of the telescope

and its application to the study

of the heavenly bodies. It is quite probable

that he got the idea for it from a

Belgian spectacle-maker, Lippershey, who

had invented combinations of lenses to

magnify distant objects. By applying

his knowledge of optics, he built a tele-

FlG. 2. Galileo. (From the •

r i ir v- u u t

,, , ^, ^ ^ . . J scope for himself which would magnify


Yerkes Observatory, reprinted

t ,. i j i i°jby

permission of the Chicago 33 diameters. He established the validity

University Press.)

of the Copernican theory by his discovery

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