Renaissance Astronomy in a Nutshell
- The historical understanding of gravity and orbits evolved around explaining the complex motions of the planets amongst the background of stars.
- Ancient cultures tracked the
motion of the Sun, Moon, stars and planets, sometimes with great precision, but usually without a
focus on explanation.
- Learn more by taking Astr 3410 - Archeoastronomy - which satisfies the non-western perspectives area requirement.
Until the Renaissance, the accepted theory of planetary motion placed the
Earth, unmoving in the center of the Solar System.
- Human bias - the lack of sensation that the Earth is moving in this case - supported this view of the Universe.
- The lack of observable stellar parallax further reinforced this viewpoint (although in reality the stars are just really really really far away and parallax is there, but hard to measure).
- Ptolemy (85-165 A.D.) postulated
a complex system of transparent spheres within spheres. Turning
spheres embedded in other spheres (more than 40 in all)
at just the right constant rate
reproduced the complex motions of the planets we see in the night sky.
- Alfonso X, King of Castille, remarked at the end of a set of astronomical tables he helped develop,
- "If the Lord Almighty had consulted me
before embarking upon creation, I should have recommended something simpler."
- This statement reflects the philosophy of "Occam's Razor" -- the simplest explanation
is probably the right explanation, or, conversely, convoluted explanations are usually incorrect.
- Ironically 300 years before Ptolemy the Greek astronomer Aristarchus postulated a Sun centered cosmology, measured the relative sizes of the Earth and Moon, and showed that the Sun was much bigger than the Earth and much farther away than the Moon.
- Unfortunately this view was not widely accepted due to the sensation of a stationary Earth and the lack of stellar parallax.
Copernicus (1473-1543), driven by the accumulating error in the Ptolemaic model and desiring a more simple explanation of planetary motion
postulated that the Sun was at the
center of the Solar System with the planets, including the Earth,
moving about the Sun in circular orbits.
- This model explains planetary motions, particularly retrograde motion, in a much simpler context.
Tycho Brahe (1546-1601) made precise observations of planetary positions over a long
period of time and handed them over to a brilliant (and obsessive compulsive)
mathematician/mystic/astronomer named Johannes Kepler. Kepler (1571-1630) used
Tycho's data to discover
Orbits were in the shape of ellipses with the Sun at one focus.
Planets moved faster when they were closer to the Sun.
A planet's distance from the Sun was mathematically related to the time it took to complete
(Orbital Period (years))2 = (Distance from Sun (A.U.))3
The planets obeyed simple quantitative laws!
Galileo (1564-1642) used the telescope (he didn't invent it!) to
make observations which refuted the old Earth-centered model
and confirmed that the planets orbited the Sun.
- (one A.U. = the average Earth-Sun separation)
Craters on the Moon and spots on the Sun implied that they were not perfect
spheres. The Moon had mountains and valleys just like the Earth.
Venus showed phases as it orbited the Sun, consistent with the Sun-centered solar
system postulated by Copernicus (and inconsistent with an Earth-centered
Jupiter had four large satellites (now called the "Galilean" satellites)
which orbited Jupiter and obeyed Kepler's laws.
- Galileo's heliocentric observations and teachings were in direct
conflict with the church's geo-centric dogma. After being instructed by the Church to cease advertising his heretical idea, he continued to do
so, leading to his trial and confinement by the Church in 1633.
Galileo died under house arrest.
- In 1992 Pope John Paul II acknowledged that the
church had made mistakes in the case of Galileo.
- Related aside: Tycho, Galileo, and Kepler all observed naked-eye supernovae (in 1572 and 1604) and
recognized that, due to the lack of noticeable parallax, these objects were part of a changing celestial sphere. This viewpoint was considered heresy by the church.
But what were the physical principles underlying the systematic behavior
of the planets? Why were orbits ellipses? Why did their
orbital speed depend on their distance from the Sun?
Issac Newton had the insight to provide the answers.
Web pages detailing the history of these characters:
Updated February 17, 2010