History of Astronomy:

Early Man -> little recorded information on early man's impression of the heavens, some drawings of eclipses, comets, supernovae such as the Pueblo Petrograph. Early man was frightened/overwhelmed by the sky

Early man also believed that the heavens held power over earthy existence (psychology of the unknown) -> origins of astrology as an attempt to understand, predict and influence events

The earliest written records (i.e. history) were astronomical observations - Babylonians (~1600 B.C.) recorded position of planets, times of eclipses, etc. - also evidence from early Chinese, Central American and North European cultures such as Stonehenge, which is a big computer for calculating the position of planets and the Sun (i.e. when to have that big blowout Solstice thing)

-> thus, Astronomy was the 1st science


Hellenistic Culture (~500 B.C.):

The ancient Greeks inherited astronomical records from the Babylonians and applied the data to construct a cosmological framework. Data was not just used for practical goals, such as navigation, but also to think of new experiments = natural philosopher.

Thales (~480 B.C.) used this data to predict eclipses.

Eratosthenes (220 B.C.) - The early Greeks knew the Earth was a sphere based on the shadow of Earth on the Moon during lunar eclipses. Eratosthenes proceeded to use this information to measure circumference of Earth in the following manner; he knew that on a certain date that a stick placed in the ground at Syene cast no shadow. Whereas, a stick at Alexandria has a small shadow. Using simple ratios he showed the following:

Hipparchus (100 B.C.) produced first star catalog and recorded the names of constellations.

Heraclides (330 B.C.) developed the first Solar System model, beginning of the geocentric versus heliocentric debate

Note that orbits are perfect circles (for philosophical reasons = all things in the Heavens are "perfect")

Aristarchus (270 B.C.) developed the heliocentric theory

Problems for heliocentric theory:

Ptolemy (200 A.D.) was the Librarian of Alexandria who resurrected Heraclides geocentric theory and combined with centuries of data on planetary motions -> formulated complete description of the Solar System that explained/predicted the apparent motions. The Ptolemic system began the 1st paradigm or framework for our understanding of Nature

Unfortunately, the Ptolemy framework was extremely complicated in order to explain retrograde motion.

The solution to retrograde motion was to use a system of circles on circles to explain the orbits of the planets called epicycles and deferents. The main orbit is the deferent, the smaller orbit is the epicycle. Although only one epicycle is shown in the figure below, over 28 were required to explain the actual orbits of the planets.

In the Ptolemaic system, deferents were large circles centered on the Earth, and epicycles were small circles whose centers moved around the circumferences of the deferents. The Sun, Moon, and planets moved around the circumference of their own epicycles. In the movable eccentric, there was one circle; this was centered on a point displaced from the Earth, with the planet moving around the circumference. These were mathematically equivalent schemes.

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 proposed other spheres, ending with the primum mobile ("prime mover"), which provided the motive power for the remaining spheres that constituted his conception of the universe. His resulting solar system model looked like the following, although the planets had as many as 28 epicycles to account for all the details of their motion.

This model, while complicated, was a complete description of the Solar System that explained, and predicted, the apparent motions of all the planets. The Ptolemic system began the 1st mathematical paradigm or framework for our understanding of Nature.

Alexandria burns, Roman culture collapses, 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 until the...

The Renaissance, where new ideas were more important than dogma.

Copernicus (1500's) reinvented the heliocentric theory and challenged Church doctrine. The heliocentric model had greater depth than simply an improvement to solve retrograde motion, it also had social and political consequences. For example, the Bible states (Psalm 93, addressing God) "Thou hast fixed the earth immovable and firm", which implies a geocentric model. Also, Joshua commanded the Sun to stand still, not the Earth. Heliocentric theory questioned the authority of the most revered wise men of the ancient world and the outcry over his ideas had little to due with the impersonal concern for truth. 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 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 that a scientific model be as simple as possible (Occam's Razor).

Tycho Brahe (1580's) was astronomy's 1st true observer. He built the Danish Observatory (using sextant's since telescopes had not been invented yet) from which he measured positions of planets and stars to the highest degree of accuracy for that time period (1st modern database). He showed that the Sun was much farther than the Moon from the Earth, using simple trigonometry of the angle between the Moon and the Sun at 1st Quarter.

Tycho's measurements were used to show that there was no detectable parallax with the naked eye, in support of the geocentric theory. So, even though his observations were the best for his time, his result was wrong, a lesson in how science is done.

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.

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.

Galileo (1620's) developed laws of motion (natural versus forced motion, rest versus uniform motion). Then, with a small refracting telescope (3-inches), destroyed the the idea of a "perfect", geocentric Universe with the following 5 discoveries:

spots on the Sun

mountains and "seas" (maria) on the Moon

Milky Way is made of lots of stars

Venus has phases

Jupiter has moons (Galilean moons: Io, Europa, Callisto, Ganymede)

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 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

Solar System Live