To complete last
week’s post, I will offer here a summary of the history of Cosmology, from
the Greeks to the paradigm shift that took place in the sixteenth and
seventeenth centuries.
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- Greek cosmology (with the
exception of Aristarchus of Samos) put the Earth at the center of the universe.
Plato and, above all, Aristotle established the idea that, since the sky
is perfect, the orbits of the planets must be exactly circular, because, for
them, the circumference is the most perfect curve of all.
- The Greek model explained well
the movements of the sun and moon, and therefore made it possible to
predict eclipses, but had a problem with the retrograde movements of the
planets then known (Mercury, Venus, Mars, Jupiter and
Saturn). Three centuries before Christ, Apollonius of Pergamum proposed
that the orbits of these planets are epicycles, circumferences centered on another
circumference (the deferent),
which in turn revolves around a point located near the Earth, but apart from its center (the eccentric).
- Claudius Ptolemy formalized and perfected the
cosmological system devised by the great Greek philosophers and astronomers, and
compiled experimental data. To reconcile the theory with the data, Ptolemy
added the equant to the
epicycles and the deferent: a point around which the planets rotate with
constant angular velocity. To explain his system, Ptolemy wrote a text of
cosmology (called Almagest, its Arabic name) that was used
as the basic text of astronomy for almost one millennium and a half.
- Copernicus found that calculating the orbits o to
adapt to the experimental data is easier if the sun is placed in the center
and the Earth becomes a planet that rotates, like the others, around the
sun. In short, as I said in the previous post, this is a simple change of
the reference system. The underlying physics is the same. In addition, as
Copernicus decided to preserve the circular orbits of Aristotle, his
system, although it makes calculations simpler, still needed epicycles and
deferent, although smaller than those of Ptolemy.
- Trying
to maintain the balance between the two systems, Ptolemaic and Copernican,
Tycho
Brahe proposed an intermediate system, where the five
planets would rotate around the sun, while the sun and the moon (the sun
dragging the planets) would turn around the earth.
- Johannes Kepler, disciple of
Tycho Brahe, made the first really revolutionary proposal,
by stating that the orbits of the planets are not circular, but
elliptical, with the sun in one of the foci. This change
meant the total elimination of epicycles and deferent and the perfect
adaptation of the experimental and calculated data. Kepler discovered two additional
empirical laws that describe the changing speed of the planets as they travel
around these elliptical orbits, and the total time it takes them to travel
around them.
- The
contribution of Isaac Newton consisted in the application
to the movements of the celestial bodies of the new laws of mechanics,
discovered by the French and English medieval researchers and improved by
other researchers, such as Galileo Galilei. In doing so, he theoretically
deduced Kepler’s three experimental laws, which gave scientific backing to
his system.
- The two alternative cosmological systems,
that of Tycho Brahe improved with Kepler elliptical orbits around two
different centers (the sun and the Earth), and that of Copernicus, equally
improved with these elliptical orbits and with a single center (the sun) are totally
equivalent, as they only differ by being based on different
reference systems, something that, as Galileo showed, is
irrelevant. Throughout the seventeenth century the Copernicus system, being
simpler, imposed (a single center is simpler than two), but the proof that
this was the correct reference system (that is, the demonstration that the
Earth really moves around the sun) had to wait until the 19th century. In
1838, the mathematician and astronomer Friedrich Bessel discovered that
the star 61 Cygnus presents a visible parallax, i.e. its apparent movement in the sky describes
a small ellipse throughout the year, a reflection of the ellipse the Earth
travels around the Sun.
Stellar parallax motion from the Earth's movement |
In conclusion: the
true Copernican change in Western cosmology was not made by Copernicus, but by Kepler,
who was the first to dare opposing the Aristotelian dictum that all celestial bodies must travel in perfect
circumferences. But the so-called Newtonian cosmology was not the work of a
single person, but the result of the collaboration of many. As Newton said, if I have seen further,
it is by standing on the shoulders of giants. Who were the
giants? Plato, Aristotle, Apollonius, Hipparchus, Ptolemy, Buridan, Oresme,
Copernicus, Tycho Brahe, Kepler, Galileo, and many others we have no room to name
here.
Manuel Alfonseca
Happy Christmas to everybody
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