Heliocentric vs. Geocentric Heliocentric

Sun is in the center and the planets travel around the sun

Geocentric

Earth in the middle and the sun, moon and all the planets travel around the earth.

The word Planet The word planet directly translated means

Wanderer

Aristotle’s Picture of the Heavens

He proposed that the heavens were literally composed of 55 concentric, crystalline spheres to which the celestial objects were attached and which rotated at different velocities (but the angular velocity was constant for a given sphere), with the Earth at the center

Retrograde MotionPlanets seemed to move backwards. There for they proposed that planets were attached, not to the concentric spheres themselves, but to circles attached to the concentric spheres, as illustrated in the adjacent diagram. 

Ptolomy: The Algmagest These ideas concerning

uniform circular motion and epicycles were catalogued by Ptolemy in 150 A.D. His book was called the "Almagest" (literally, "The Greatest"), and this picture of the structure of the Solar System has come to be called the "Ptolemaic Universe". 

Ptolomic Explanation of Retrograde Motion In actual models,

the center of the epicycle moved with uniform circular motion, not around the center of the deferent, but around a point that was displaced by some distance from the center of the deferent.

Three Universally “wrong” Ideas: 1. All motion in the heavens is uniform

circular motion.2. The objects in the heavens are made

from perfect material, and cannot change their intrinsic properties (e.g., their brightness).

3. The Earth is at the center of the Universe.

Copernican: Theory of the Universe Copernicus proposed that the Sun, not the

Earth, was the center of the Solar System. Such a model is called a heliocentric system. The ordering of the planets known to Copernicus in this new system is illustrated in the following figure, which we recognize as the modern ordering of those planets.

Copernican: Retrograde Motion The Copernican system by banishing the idea

that the Earth was the center of the Solar System, immediately led to a simple explanation of both the varying brightness of the planets and retrograde motion:

1. The planets in such a system naturally vary in brightness because they are not always the same distance from the Earth.

2. The retrograde motion could be explained in terms of geometry and a faster motion for planets with smaller orbits, as illustrated in the following animation.

Copernicus: Held back by Copernicus was an unlikely revolutionary. It is

believed by many that his book was only published at the end of his life because he feared ridicule and disfavor: by his peers and by the Church, which had elevated the ideas of Aristotle to the level of religious dogma. However, this reluctant revolutionary set in motion a chain of events that would eventually (long after his lifetime) produce the greatest revolution in thinking that Western civilization has seen.

Tycho Brahe’s Model Solar system Brahe proposed a model of the Solar System

that was intermediate between the Ptolemaic and Copernican models (it had the Earth at the center). It proved to be incorrect, but was the most widely accepted model of the Solar System for a time.

Brahe: Changes in the sky He made observations of a supernova (literally:

nova= "new star") in 1572 (we now know that a supernova is an exploding star, not a new star). This was a "star" that appeared suddenly where none had been seen before, and was visible for about 18 months before fading from view.

Since this clearly represented a change in the sky, prevailing opinion held that the supernova was not really a star but some local phenomenon in the atmosphere (remember: the heavens were supposed to be unchanging in the Aristotelian view).

Johannes Kepler: Kepler and Brahe did not get along well. Brahe

apparently mistrusted Kepler, fearing that his bright young assistant might eclipse him as the premiere astronomer of his day. He therefore let Kepler see only part of his voluminous data.

Kepler’s law of Planetary Motion1. All planets have elliptical orbits with the sun at one foci.

2. Planets sweep out equal areas in equal time.

3. The ratio of the period squared of any two planets is equal to the ratio of the semi-major axis cubed for the same two planets.

Views of the solar system

Perihelion Aphelion The point of

nearest approach of the planet to the Sun is termed perihelion;

The point of greatest separation is termed aphelion.

Kepler & The speed of planets  Hence, by Kepler's second law, the planet

moves fastest when it is near perihelion and slowest when it is near aphelion. 

What does Earth Travel the Fastest? When it is in

perihelion

Define Astronomical Unit (AU) The distance from

earth to an object in the solar system.

1 AU= the average distance between the earth and the sun

1 Astronomical Unit = 149 597 871 kilometers

Galileo: The Telescope Galileo did not invent

the telescope (Dutch spectacle makers receive that credit), but he was the first to use the telescope to study the heavens systematically

Galileo & The Sun He observed motion

of the sunspots indicating that the Sun was rotating on an axis. These "blemishes" on

the Sun were contrary to the doctrine of an unchanging perfect substance in the heavens,

Galileo & Venus Galileo used his telescope to show that Venus

went through a complete set of phases, just like the Moon. This observation was among the most important in human history, for it provided the first conclusive observational proof that was consistent with the Copernican system but not the Ptolemaic system.