Lecture 12

THE NATURE OF THE SOLAR SYSTEM

I. Ancient Astronomy

A. Early Greeks

• The "Golden Age" of early astronomy (600 B.C. and A.D. 150) was centered in Greece.
• The early Greeks used philosophical argument to explain natural phenomena. However, they did rely on observation data as well.
• Many astronomical discoveries have been credited to the Greeks. They held the geocentric (Earth centered) view, believing that Earth was a sphere that stayed motionless at the center of the universe. Orbiting the Earth were the known heavenly bodies (moon, sun, Mercury, Venus, Mars, Jupiter, and Saturn and a hollow transparent sphere called the celestial sphere.
• The geocentric outlook became known as the Ptolemaic system.

B. Ptolemy's model.

• Most of our knowledge of Greek astronomy comes from a 13 volume treatise, Almaget ("the great work"), which was compiled by Ptolemy in A.D.141.
• Ptolemy was believed that the star studded celestial sphere made a daily trip around a motionless Earth.
• He proposed that the sun, moon, and planets made trips of various lengths along individual orbits.
• He explained retrograde motion, that is, if the planets were watched each night, they move slightly eastward among the stars. Periodically, each planet appears to stop, reverse direction for a period of time, and then resume an eastward motion.

C. The birth of modern astronomy.

o        o       Nicolaus Copernicus (1473-1543) from Poland was convinced that Earth is a planet just like the other five known planets. The daily motions of the heavens, he thought, could be better explained by a rotating Earth. He reconstructed the solar system and put the sun at the center of the solar system and the planets orbiting the sun.

o       o       Tycho Brahe (1546-1601) was Danish and born three years after Copernicus death. He measured the locations of heavenly bodies. As a result, he did not believe in the Copernuicus system as presented in previous years. Instead, he believed that the Earth did revolve along an orbit around the sun, the position of a nearby star, when observed from extreme points in Earth's orbit six months apart, should shift with respect to the more distant stars. His idea was correct and became known as stellar parallax.

o       o       Johannes Kepler (1571-1630) derived three laws:

§         §         The path of each planet around the sun is an ellipse with the sun at on focus.

§         §         Each planet revolves so that an imaginary line connecting it to the sun sweeps equal areas in equal intervals of time.

§         §         The orbital periods of the planets and their distances to the sun are proportional.

o       o       Galileo Galilei (1564-1642) was an Italian scientist during the Renaissance. He invented the first telescope and with the telescope made the following discoveries.

§         §         He discovered four satellites orbiting Jupiter. He determined their periods of revolution and dispelled the old idea that the Earth was the only center of motion in the universe.

§         §         The planets are circular disks, not points of light.

§         §         He discovered that Venus has phases like the moon.

§         §         He pointed out that the moon's surface is not a smooth glass sphere. He saw mountains and valleys.

§         §         He discovered that the sun had sunspots and he tracked the movement of these spots.

o       o       Sir Isaac Newton (1643-1727) was born in the year of Galileo's death. He formulated and tested the law of universal gravitation. That is, every body in the universe attract every other body with a force that is directly proportional to their masses and inversely proportional to the square of the distance between them. Thus, the gravitational force decreases with the distance, so that two objects 3 km apart have 9 times less gravitational attraction than if the same objects were 1 km apart.

II. THE PLANETS

A. The planets fall into two categories the terrestrial planets (Mercury, Venus, Earth, and Mars), and the Jovian planets (Jupiter, Saturn, Uranus, and Neptune). Pluto is not included in either category , because it is far away from Earth and its small size make this planet's true nature a mystery.

B. The main difference between the two categories is the size. Jovian planets are huge compared to the terrestrial planets.

C. Origin of the solar system.

• The planets formed at the same time and from the same primordial materials the sun. This material formed a vast cloud of dust and gases called a nebula. The nebular hypothesis suggests that all bodies of the solar system formed from an enormous nebular cloud consisting of hydrogen, helium, and some heavier elements (silicon, aluminum, iron, and calcium) known to exist.
• The heavier elements typical or rock material and gases, about 5 billion years ago began to contact under its own gravitational influence. The contracting of material apparently had some rotational motions.
• This rotational motions created the disk shape of the nebula.

D. The Earth's moon.

• The moon is 3475 km in diameter, about one fourth of the Earth's diameter.
• The most obvious features of the lunar surface are craters. There are craters within craters.
• The highlands are densely pockmarked areas, which make up most of the lunar surface. The back side of the moon is characterized by such topography.
• Maria were impact craters created by asteroids. The craters then became flooded with layer upon layer of very fluid basaltic lava, resembling the Columbia Plateau in northwestern U.S.
• All lunar terrains are mantled with a layer of gray, unconsolidated debris derived from a few billion years of meteoric bombardment. This lunar soil is called lunar regolith, which is composed of igneous rock materials.

E. An overview of the planets. (The Nine Planets)

III. Other Heavenly Bodies

A. Asteroids are relatively small bodies that have been likened to "flying mountains." The largest, Ceres, is about 1000 km in diameter. Most asteroids lie between the orbits of Mars and Jupiter and have period ranging from three to six years.

B. Comets are made of frozen gases that hold together small pieces of rocky and metallic materials. Comets travel very elongated orbits that carry them beyond Pluto. As the comet approaches the sun the solar energy vaporizes its frozen gases, producing a glowing head called the coma. As some comets approach the sun they produce a tail that extends many millions of kilometers.

C. Meteor