Nicolaus Copernicus, Polish Mikołaj Kopernik, German Nikolaus Kopernikus, (born February

19, 1473, Toruń, Royal Prussia, Poland—died May 24, 1543, Frauenburg, East Prussia [now
Frombork, Poland]), Polish astronomer who proposed that the planets have the Sun as the fixed
point to which their motions are to be referred; that Earth is a planet which, besides orbiting the Sun
annually, also turns once daily on its own axis; and that very slow long-term changes in the direction
of this axis account for the precession of the equinoxes. This representation of the heavens is usually
called the heliocentric, or “Sun-centred,” system—derived from the Greek helios, meaning “Sun.”
Copernicus’s theory had important consequences for later thinkers of the Scientific Revolution,
including such major figures as Galileo, Kepler, Descartes, and Newton. Copernicus probably hit
upon his main idea sometime between 1508 and 1514, and during those years he wrote a
manuscript usually called the Commentariolus (“Little Commentary”). However, the book that
contains the final version of his theory, De revolutionibus orbium coelestium libri vi (“Six Books
Concerning the Revolutions of the Heavenly Orbs”), did not appear in print until 1543, the year of his
death.

The Copernican Model:

A Sun-Centered Solar System

The Heliocentric System

In a book called On the Revolutions of the Heavenly Bodies (that was published as
Copernicus lay on his deathbed), Copernicus proposed that the Sun, not the Earth, was the center of
the Solar System. Such a model is called a heliocentric system. The ordering of the planets known to
Copernicus in this new system is illustrated in the following figure, which we recognize as the
modern ordering of those planets.

In this new ordering the Earth is just another planet (the third outward from the Sun), and
the Moon is in orbit around the Earth, not the Sun. The stars are distant objects that do not revolve
around the Sun. Instead, the Earth is assumed to rotate once in 24 hours, causing the stars to appear
to revolve around the Earth in the opposite direction.

Retrograde Motion and Varying Brightness of the Planets

The planets in such a system naturally vary in brightness because they are not always the
same distance from the Earth.

The retrograde motion could be explained in terms of geometry and a faster motion for
planets with smaller orbits, as illustrated in the following animation.

Copernicus and the Need for Epicycles

There is a common misconception that the Copernican model did away with the need for
epicycles. This is not true, because Copernicus was able to rid himself of the long-held notion that
the Earth was the center of the Solar system, but he did not question the assumption of uniform
circular motion. Thus, in the Copernican model the Sun was at the center, but the planets still
executed uniform circular motion about it. As we shall see later, the orbits of the planets are not
circles, they are actually ellipses. As a consequence, the Copernican model, with its assumption of
uniform circular motion, still could not explain all the details of planetary motion on the celestial
sphere without epicycles. The difference was that the Copernican system required many fewer
epicycles than the Ptolemaic system because it moved the Sun to the center.

The Copernican Revolution

We noted earlier that 3 incorrect ideas held back the development of modern astronomy
from the time of Aristotle until the 16th and 17th centuries: (1) the assumption that the Earth was
the center of the Universe, (2) the assumption of uniform circular motion in the heavens, and (3) the
assumption that objects in the heavens were made from a perfect, unchanging substance not found
on the Earth.

Copernicus challenged assumption 1, but not assumption 2. We may also note that the
Copernican model implicitly questions the third tenet that the objects in the sky were made of
special unchanging stuff. Since the Earth is just another planet, there will eventually be a natural
progression to the idea that the planets are made from the same stuff that we find on the Earth.

Copernicus was an unlikely revolutionary. It is believed by many that his book was only
published at the end of his life because he feared ridicule and disfavor by his peers and by the
Church, which had elevated the ideas of Aristotle to the level of religious dogma. However, this
reluctant revolutionary set in motion a chain of events that would eventually (long after his lifetime)
produce the greatest revolution in thinking that Western civilization has seen. His ideas remained
rather obscure for about 100 years after his death. But, in the 17th century the work of Kepler,
Galileo, and Newton would build on the heliocentric Universe of Copernicus and produce the
revolution that would sweep away completely the ideas of Aristotle and replace them with the
modern view of astronomy and natural science. This sequence is commonly called the Copernican
Revolution.

Been There, Done That: Aristarchus of Samos

There are many examples throughout history, including in modern times, where a theory, or
a part of a theory, is proposed and doesn't catch on initially but only later bears fruit--and possibly
with later proponent gaining credit that is really deserved by the originator. I think the example of
Aristarchus is a poignant one.

This applies here because the idea of Copernicus was not really new! A sun-centered Solar
System had been proposed as early as about 200 B.C. by Aristarchus of Samos (Samos is an island off
the coast of what is now Turkey). Aristarchus actually proposed that the Earth rotated on in addition
to its orbiting around the sun. Many of Aristarchus' writings were unfortunately lost. More
importantly however, they did not survive long under the weight of Aristotle's influence and the
"common sense" of the time:

If the Earth actually spun on an axis (as required in a heliocentric system to explain the
diurnal motion of the sky), why didn't objects fly off the spinning Earth?

If the Earth was in motion around the sun, why didn't it leave behind the birds flying in the
air?

If the Earth were actually on an orbit around the sun, why wasn't a parallax effect observed?
That is, as illustrated in the adjacent figure, where stars would appear to change their position with
the respect to the other background stars as the Earth moved about its orbit, because of viewing
them from a different perspective (just as viewing an object first with one eye, and then the other,
causes the apparent position of the object to change with respect to the background).

The first two objections were not valid because they represent an inadequate understanding
of the physics of motion that would only be corrected in the 17th century. The third objection is
valid, but failed to account for what we now know to be the enormous distances to the stars. As
illustrated in the following figure, the amount of parallax decreases with distance.
 The parallax effect is there, but it is very small because the stars are so far away that their
parallax can only be observed with very precise instruments. Indeed, the parallax of stars was not
measured conclusively until the year 1838. Thus, the heliocentric idea of Aristarchus was quickly
forgotten and Western thought stagnated for almost 2000 years as it waited for Copernicus to revive
the heliocentric theory.

Note that Copernicus himself originally gave credit to Aristarchus in his heliocentric treatise,
De revolutionibus caelestibus, where he had written, "Philolaus believed in the mobility of the earth,
and some even say that Aristarchus of Samos was of that opinion." Interestingly, this passage was
crossed out shortly before publication, maybe because Copernicus decided his treatise would stand
on its own merit.

Freud's revolution may be viewed as the discovery of a way of locating in the mind objective
entities which can be studied like physical things. ... It is that common challenge rather than a
popular exemplary model, such as mechanics or hydraulics, that shaped Freud's theory.