Development of the Modern
View of the Solar System
In the XVI and XVII
centuries, more than 1400 years after Ptolemy, his geocentric model
was finally replaced by a much better (and much simpler!)
one. The essential steps were due mainly to the contributions from the following
- Nicholas Copernicus (Polish, 1473-1543):
Proposed the first modern heliocentric model, motivated by inaccuracies
of the Ptolemaic model and based on aesthetic principles – only
indirectly on evidence.
- Features of the model: It was still
based on circles and epicycles, and was not more accurate than the
Ptolemaic model; However, it allowed the calculation of distances to
planets in AU, and provided the correct explanation for retrograde
Copernicus' model was not widely read or accepted right away, because it is difficult
to change established ideas (although it did spark debates, and eventually condemnation
by the Catholic church), because of its limitations, and because it did not address
some of the better arguments in favor of geocentric models.
Tycho Brahe: The Data
- Tycho Brahe (Danish, 1546-1601):
The greatest pre-telescope astronomer; Became famous after observing
a (super)nova in 1572 and a comet in 1577, and proving that they
were farther than the Moon: Heavens can change! Was given an
island on which to build his observatory.
- His Solar System model: Believed
in a mixed model, with the Sun and Moon orbiting the Earth (no
stellar parallax!); Few people ever believed in it.
- The observations: He made the best and
most systematic naked eye observations up to that time, accurate to
1', kept extensive detailed records of them, ... and hired Johannes
Kepler as his assistant.
Galileo: Observations and
Response to Aristotle
- Galileo Galilei (Italian, 1564-1642): Introduced the concept of inertia, with which he could address
Aristotle's objection to a moving Earth.
- Observations: The first to use telescopes
in astronomy (1609) and publish his results; Saw stars in Milky
Way (so stars can be so distant that they don't show parallax), features
Moon and Sun (so not all heavenly bodies are perfect), four "little
stars" around Jupiter (so another body-and a moving one
at that-can have orbiting moons), phases of Venus (a complete
set); Why is this important?
- Ideas: He supported the Copernican
Model, but was forced to recant. Thought that planets are "worlds,"
not just dots of light.
- Other observations: He also
saw that Saturn sometimes has things sticking out from its sides (like
ears), and looked for stellar parallax in Mizar.
Observation to Model of the Solar System
- Johannes Kepler (German, 1571-1630): Started trying to explain Tycho's data on Mars; Ended up developing
the empirical model we still accept today for the solar system
- Laws of planetary motion: Three
simple laws, 1. Ellipses: All planets
move along ellipses, with the Sun at one focus; 2. Areas: Planets sweep out equal areas around the Sun in equal times (they
move faster when closer to the Sun); 3. Periods: The period2 is proportional
to the distance3.
- [Verification: Helped by 1631
observations of a Mercury transit, and 1655 observations by Giandomenico
Cassini of the Sun.]
- Are there other laws? Is there
a pattern in the spacings or periods among the planets? Not the
way Kepler hoped for, but there are resonances.
- How good are these laws? We
now know that they need small corrections, but they hold for
any planetary system, any moon around a planet.
| After these developments,
by the early 1600s, the motion of planets in the Solar System was correctly
described. The next step will be taken by Isaac Newton in the late 1600s,
when he explained this motion in terms of gravity. Also, since antiquity
the Solar System had essentially been identified with the whole universe.
That began to change too with the observations carried out in this period.
page by luca bombelli <bombelli at olemiss.edu>,
modified 4 sep 2012