History of Astronomy: which is the origins of the pseudo-science astrology as

an attempt to understand, predict and influence events

We have very little in the form of recorded information
on early man's impression of the heavens, mostly some The earliest written records (i.e. history) were
drawings of eclipses, comets, supernovae such as the astronomical observations produced by the Babylonians
Pueblo Petrograph (see below). However, early man was (~1600 B.C.) who recorded positions of planets, times of
clearly frightened/overwhelmed by the sky. One of the eclipses, etc. There is also evidence of interest in
earliest recorded astronomical observations is the Nebra astronomical phenomenon from early Chinese, Central
sky disk from northern Europe dating approximately American and North European cultures such
1,600 BC. This 30 cm bronze disk depicts the Sun, a as Stonehenge, which is a big computer for calculating
lunar crescent and stars (including the Pleiades star the position of planets and the Sun (i.e. when to have
cluster). that big blowout Solstice thing)

The disk is probably a religious symbol as well as a Thus, Astronomy was the 1st science, as it was the first
crude astronomical instrument or calendar. In the thing we recorded observations for.
Western hemisphere, similar understanding of basic
stellar and planetary behavior was developing. For Later in history, 5,000 to 20,000 years ago, humankind
example, Native American culture around the same time begins to organize themselves and develop what we now
were leaving rock drawings, or petroglyphs, of call culture. A greater sense of permanence in your daily
astronomical phenomenon. The clearest example is existences leads to the development of culture, where
found below, a petroglyph which depicts the 1,006 AD people develop narrative stories for cultural unity which
supernova that resulted in the Crab Nebula. we now call myths.

Early man also believed that the heavens held power Most myths maintain supernatural themes, with gods,
over earthy existence (psychology of the unknown) divine and semi-divine figures, but there was usually an
internal logical consistence to the narrative. For
example, myths are often attempts at a rational the Moon were round. And the shadow of the Earth, cast
explanation off events in the everyday world, their goal on the lunar surface during a lunar eclipse, is curved. A
is to teach. Even if we consider some of the stories to be sphere is the simplest shape to explain the Earth's
ridiculous, they were, in some sense, our first scientific shadow (a disk would sometimes display a shadow
theories. They also, usually, follow a particular religion, shaped like a line or oval).
and so this time is characterized by a close marriage of
science and religion.

Eratosthenes used a spherical Earth model, and some

Hellenistic Culture (~500 B.C.):
simple geometry, to calculate its circumference.
Eratosthenes knows that on a special day (the summer
About 1,000 years later, the ancient Greeks inherited
solstice) at noon in the Egyptian city of Syene, a stick
astronomical records from the Babylonians and applied
placed in the ground will cast no shadow (i.e., it is
the data to construct a cosmological framework. Data
parallel to the Sun's rays). A stick in the ground at
was not just used for practical goals, such as navigation,
Alexandria, to the north, will cast a shadow at an angle
but also to think of new experiments, the origin of what
of 7 degrees. Eratosthenes realizes that the ratio of a
we call natural philosophers.
complete circle (360 degrees) to 7 degrees is the same as
the ratio of the circumference of the Earth to the distance
Of the many natural philosophers before the time of from Alexandria to Swenet. Centuries of surveying by
Socrates (the Presocratics) was Thales (~480 B.C.). His Egyptian pharaohs scribes gave him the distance
combination of math and Babylonian data allowed him between the two cities of 4900 stadia, approximately 784
to predict eclipses. kilometers. This resulting in a circumference of 40,320
kilometers, which is amazingly close to the modern
Between the cosmological foundation set by the value of 40,030 kilometers. With this calculation,
Presocratics and the world of Ideas introduced by Plato Eratosthenes becomes the father of geography
was a set of fundamental calculations on the size of the eventually drawing up the first maps of the known world
Earth, Moon, Sun and the distances between the nearby and determining the size of the most fundamental object
planets performed by Eratosthenes and Aristarchus (c. in the Universe, our own planet.
250 BC). Using some simple geometry, these two
natural philosophers were able to, for the first time, Hipparchus (100 B.C.) produced first star catalog and
place some estimate of the size of the cosmos in Earth recorded the names of constellations.
terms.
During the times before the invention of the telescope,
For a long time it was realized that the earth's surface there were only seven objects visible to the ancients, the
was curved by people familiar with the behavior of Sun and the Moon, plus the five planets, Mercury,
incoming and outgoing ships. For it was obvious that as Venus, Mars, Jupiter and Saturn. It was obvious that the
a ship passed over the horizon, the hull disappeared first, planets were not on the celestial sphere since the Moon
then the topmost sailing masts (although one could argue clearly passes in front of the Sun and planets Mercury
this is an effect of refraction in the atmosphere). Ancient and Venus can be seen to transit the Sun (the Sun passes
astronomers could see with their eyes that the Sun and in front of Mars, Jupiter and Saturn). Plato first proposed
that the planets followed perfect circular orbits around Problems for Heliocentric Theory:
the Earth (for the circle is the most perfect shape). Later,
Heraclides (330 B.C.) developed the first Solar System While today we know that the Sun is at the center of the
model, placing the planets in order from the Earth it was solar system, this was not obvious for the technology of
is now called the geocentric solar system model and the the times pre-1500's. In particular, Aristarchus' model
beginning of the geocentric versus heliocentric debate. was ruled out by the philosophers at the time for three
reasons:

1. Earth in orbit around Sun means that the Earth is

in motion. Before the discovery of Newton's law
of motion, it was impossible to imagine motion
without being able to `feel' it. Clearly, no motion
is detected (although trade winds are due to the
Earth's rotation).
2. If the Earth undergoes a circular orbit, then
nearby stars would have a parallax. A parallax is
Note that orbits are perfect circles (for philosophical an apparent shift in the position of nearby stars
reasons = all things in the Heavens are "perfect") relative to distant stars.

Slightly later, Aristarchus (270 B.C.) proposed an

alternative model of the Solar System placing the Sun at
the center with the Earth and the planets in circular orbit
around it. The Moon orbits around the Earth. This model
became known as the heliocentric model.

Of course, if all the stars are implanted on the

Aristarchus was the first to propose a "new" Sun crystal celestial sphere, then there is no parallax.
centered cosmology and one of the primary objections to
the heliocentric model is that the stars display no 3. Lastly, geocentric ideas seem more `natural' to a
parallax (the apparent shift of nearby stars on the sky philosopher. Earth at the center of the Universe is
due to the Earth's motion around the Sun). However, a very ego-centric idea, and has an aesthetic
Aristarchus believed that the stars were very distant and, appeal.
thus, display parallax's that are too small to be seen with
the eye (in fact, the first parallax will not by measured Ptolemy (200 A.D.) was an ancient astronomer,
until 1838 by Friedrich Bessel). The Sun is like the fixed geographer, and mathematician who took the geocentric
stars, states Aristarchus, unmoving on a sphere with the theory of the solar system and gave it a mathematical
Sun at its center. For Aristarchus it was absurd that the foundation (called the "Ptolemaic system"). He did this
"Hearth" of the sky, the Sun, should move and eclipses in order to simultaneously produce a cosmological
are easy to explain by the motion of the Moon around theory based on Aristotle's physics (circular motion, no
the Earth.
voids, geocentric) and one that would provide a objects would not be seen to drop toward the center of
technically accurate description of planetary astronomy. the Earth. Again, if the Earth rotated once every 24
Ptolemy's system is one of the first examples of hours, a body thrown vertically upward should not fall
scientists attempting to "save the phenomena", to back to the same place, as it was seen to do. Ptolemy
develop a combination of perfect circles to match the was able to demonstrate, however, that no contrary
irregular motion of the planets, i.e., using concepts observations had ever been obtained.
asserted by pure reason that match the observed
phenomenon.

Ptolemy wrote a great treatise on the celestial sphere and

the motion of the planets call the Almagest. The
Almagest is divided into 13 books, each of which deals
with certain astronomical concepts pertaining to stars
and to objects in the solar system. It was, no doubt, the
encyclopedic nature of the work that made the Almagest
so useful to later astronomers and that gave the views
contained in it so profound an influence. In essence, it is
a synthesis of the results obtained by Greek astronomy;
it is also the major source of knowledge about the work
of Hipparchus, who made a map of the heavens and
named the constellations.

Ptolemy accepted the following order for celestial

objects in the solar system: Earth (center), Moon,
Mercury, Venus, Sun, Mars, Jupiter, and Saturn.
However, when the detailed observations of the planets
in the skies is examined, the planets undergo motion
which is impossible to explain in the geocentric model, a
backward track for the outer planets. This behavior is
called retrograde motion.

The Christian Aristotelian cosmos, engraving from Peter

Apian's Cosmographia, 1524

In the first book of the Almagest, Ptolemy describes his

geocentric system and gives various arguments to prove
that, in its position at the center of the universe, the The solution to retrograde motion was to use a system of
Earth must be immovable. Not least, he showed that if circles on circles to explain the orbits of the planets
the Earth moved, as some earlier philosophers had called epicycles and deferents. The main orbit is the
suggested, then certain phenomena should in deferent, the smaller orbit is the epicycle. Although only
consequence be observed. In particular, Ptolemy argued one epicycle is shown in the figure below, over 28 were
that since all bodies fall to the center of the universe, the required to explain the actual orbits of the planets.
Earth must be fixed there at the center, otherwise falling
In the Ptolemaic system, deferents were large circles
centered on the Earth, and epicycles were small circles This model, while complicated, was a complete
whose centers moved around the circumferences of the description of the Solar System that explained, and
deferents. The Sun, Moon, and planets moved around predicted, the apparent motions of all the planets. The
the circumference of their own epicycles. In the movable Ptolemic system began the 1st mathematical paradigm or
eccentric, there was one circle; this was centered on a framework for our understanding of Nature.
point displaced from the Earth, with the planet moving
around the circumference. These were mathematically We know from history that the great library at
equivalent schemes. Alexandria burns in 272 AD, destroying a great deal of
the astronomical data for the time. Roman culture
collapses and we enter the Dark Ages. But, the Roman
Catholic Church absorbs Aristotle's scientific methods
and Ptolemy's model into its own doctrine. Thus,
preserving the scientific method and Ptolemy's Solar
System. Unfortunately, the geocentric model was
accepted as doctrine and, therefore, was not subjected to
the scientific method for hundreds of years.

Until ... the Renaissance, where new ideas were more

important than dogma.

Although Ptolemy realized that the planets were much

closer to the Earth than the "fixed" stars, he seems to
have believed in the physical existence of crystalline
spheres, to which the heavenly bodies were said to be
attached. Outside the sphere of the fixed stars, Ptolemy Copernicus (1500's) reinvented the heliocentric theory
proposed other spheres, ending with the Primum Mobile and challenged Church doctrine. Copernicus (c. 1520)
("prime mover"), which provided the motive power for was not the first astronomer to challenge the geocentric
the remaining spheres that constituted his conception of model of Ptolemy, but he was the first to successfully
the universe. His resulting solar system model looked formulate a heliocentric model and publish his model.
like the following, although the planets had as many as He was able to overcome centuries of resistance to the
28 epicycles (not shown) to account for all the details of heliocentric model for a series of political and scientific
their motion.
reasons. Politically, the authority of the Church was from cosmology. However, Copernicus fails to produce
weakening in Northern Europe in the 15th century a mechanically simple scheme for astrologers to cast
allowing more diversity in scientific thinking (although horoscopes or astronomers to produce almanacs, for
the new Protestant faiths were also not quick to embrace ultimately the tables he produces are as complicated as
the heliocentric model). Scientifically, a better Ptolemy's and he did not publish all his results in the
understanding of motion (particularly inertia) was final edition of his work, "On the Revolutions of the
undermining the whole concept of an unmoving Earth. A Heavenly Spheres".
rotating Earth is a much simpler explanation for
the diurnal motion of stars, an Earth that rotates is only
one step away from an Earth that revolves around the
Sun. The heliocentric model had a greater impact than
simply an improvement to solve retrograde motion. By
placing the Sun at the center of the Solar System,
Copernicus forced a change in our worldview
= paradigm shift or science revolution.

However, Copernicus, like Ptolemy, also used circular

orbits and had to resort to epicycles and deferents to
explain retrograde motions. In fact, Copernicus was
forced to use more epicycles than Ptolemy, i.e. a more
complicated system of circles on circles. Thus,
Copernicus' model would have failed our modern criteria
Copernicus began his quest for an improved solar that a scientific model be as simple as possible (Occam's
system model with some basic principles. Foremost was Razor).
the postulate that the Earth was not the center of the
Universe, only the center of local gravity and the Moon.
Second, the postulate that the Sun was the center of the Tycho Brahe (1580's) was astronomy's 1st true observer.
solar system, all planets revolved around the Sun. In this He built the Danish Observatory (using sextant's since
fashion, retrograde motion is not cause by the planets telescopes had not been invented yet) from which he
themselves, but rather by the orbit of the Earth. measured positions of planets and stars to the highest
degree of accuracy for that time period (1st modern
While Copernicus includes a rotating Earth in his database). He showed that the Sun was much farther
heliocentric model, he continues to cling to Aristotle's than the Moon from the Earth, using
celestial motions, i.e. orbits that are perfect circles simple trigonometry of the angle between the Moon and
(rather than their true shape, an ellipse). This forces the Sun at 1st Quarter.
Copernicus to adopt a series of moving sphere's for each
planet to explain longitude motion. While Copernicus
has fewer sphere's, since more of the retrograde motion
is accounted for, his system is still extremely
complicated in a computational sense. It's two greatest
advantages is that it places the inferior planets near the
Sun, naturally explaining their lack of large eastern or
western elongations, and removing any extreme
motions, such as that needed to explain durnal changes.

Copernicus also changes the immovable empyrean

heaven into a fixed sphere of stars, severing theology
 Beyond Tycho Brahe's accomplishments in the
observational arena, he is also remembered for
introducing two compromise solutions to the solar
system model now referred to as the geoheliocentric
models. Brahe was strongly influenced by the idea of
Mercury and Venus revolving around the Sun to explain
The Earth's motion, as a simple matter of dynamics, was the fact that their apparent motion across the sky never
extremely perplexing to the medieval thinker. The size takes them more than a few tens of degrees from the Sun
and mass (called their greatest elongation). The behavior of inner
of the Earth worlds differs from the orbital behavior of the outer
was planets, which can be found at any place on the elliptic
during their orbital cycle.

approximately known since Eratosthenes had measured

the circumference of the Earth (thus, the volume is
known and one could simply multiple the volume with
the mean density of rock to obtain a rough mass
estimate). The force required to move the Earth seemed
impossible to the average medieval natural philosopher.

Brahe had additional reason to question the motion of

Brahe proposed a hybrid solutions to the geocentric
the Earth, for his excellent stellar positional observations
model which preserves the geocentric nature of the Earth
continued to fail to detect any parallax. This lack of
at the center of the Universe, but placed the inner planets
annual parallax implied that the celestial sphere was
(Mercury and Venus) in orbit around the Sun. This
"immeasurably large". Brahe had also attempted to
configuration resolves the problem of Mercury and
measure the size of stars, not understanding that the
Venus lack of large angular distances from the Sun, but
apparent size of a star simply reflects the blurring caused
saves the key criticism of the heliocentric model, that the
by the passage of starlight through the atmosphere.
Earth is in motion. In other works, Brahe's
Brahe's estimate for the size of stars would place them
geoheliocentric model fit the available data but followed
larger than the current day estimate of the size of the
the philosophical intuition of a non-moving Earth.
Earth's orbit. Such "titanic" stars are absurd according to
Brahe's understanding of stars at the time.
Neither successfully predicts the motion of the planets.
The solution will be discovered by a student of Tycho's,
who finally resolves the heliocentric cosmology with the
use of elliptical orbits.
 The formulation of a highly accurate system of
determining the motions of all the planets marks the
beginning of the clockwork Universe concept, and
another paradigm shift in our philosophy of science.

Kepler (1600's) a student of Tycho who used Brahe's

database to formulate the Laws of Planetary Motion
which corrects the problems of epicycles in the
heliocentric theory by using ellipses instead of circles
for orbits of the planets.
Galileo:

Kepler's laws are a mathematical formulation of the

solar system. But, is the solar system `really' composed
of elliptical orbits, or is this just a computational trick
and the `real' solar system is geocentric. Of course, the
answer to questions of this nature is observation.

The pioneer of astronomical observation in a modern

This is a key mathematical formulation because the context is Galileo. Galileo (1620's) developed laws of
reason Copernicus' heliocentric model has to use motion (natural versus forced motion, rest versus
epicycles is due to the fact that he assumed perfectly uniform motion). Then, with a small
circular orbits. With the use of ellipses, the heliocentric refracting telescope (3-inches), destroyed the the idea of
model eliminates the need for epicycles and deferents. a "perfect", geocentric Universe with the following 5
The orbital motion of a planet is completely described discoveries:
by six elements: the semi-major axis, the eccentricity,
the inclination, the longitude of the ascending node, the
argument of the perihelion and the time of the
perihelion.
 spots on the Sun

mountains and "seas" (maria) on the Moon Jupiter has moons (Galilean moons: Io, Europa, Callisto,
Ganymede)

Notice that planets with phases are possible in a

geocentric model. But for a planet to change in apparent
size with its phases, like Venus is impossible if the
planet orbits the same distance from the Earth. And,
lastly, if all bodies orbit around the Earth, then the
moons of Jupiter, which clearly orbit around that planet,
are definitive proof that the geocentric model is wrong.

Milky Way is made of lots of stars

These first three are more of an aesthetic nature. Plato

requires a `perfect' Universe. Spots, craters and a broken
Milky Way are all features of imperfection and at odds
with Plato's ideas on purely philosophical grounds.
However, the laws of motion are as pure as Plato's
celestial sphere, but clearly are not easy to apply in the
world of friction and air currents etc. So these
observations, by themselves, are not fatal to the
geocentric theory. The next two are fatal and can only be
explained by a heliocentric model.
Newton (1680's) developed the law of Universal
Gravitation, laws of accelerated motion, invented
calculus (math tool), the 1st reflecting telescope and
theory of light.

... off to the 18-20th century, with discovery of the outer

Venus has phases planets and where astronomy moves towards discoveries
in stellar and galactic areas, next paradigm shift occurs
in early 1960's with NASA deep space probes