Heliocentric, Tidal and Nebular Theory Help (page 2)
The “thought police” of the church held less power in northern Europe than they did in Italy. Proponents of the heliocentric (Sun-centered) theory were taken seriously in places such as Germany, France, Poland, and England.
Nicolaus Copernicus, a Polish astronomer, published a work in the early sixteenth century suggesting that the Sun, not the Earth, must be at the center of the Universe. (Remember that back in the sixteenth century the Earth, the Moon, the Sun, and the planets basically defined the entire Cosmos. No one knew what the stars were, much less how they were distributed throughout space.) The Earth, thought Copernicus, is a planet just like Mercury, Venus, or Mars insofar as its importance in the overall scheme of things. But Copernicus could not prove his theory to the complete satisfaction of the authorities in his part of the world. If the Earth is moving, asked the skeptics, why don’t we feel a constant wind from space? What force could push the Earth? Why should such a force exist?
Another astronomer, Tycho Brahe, was involved with an ongoing meticulous mapping and recording job. He kept careful records of the positions of all the planets over a period of time. Brahe had a German assistant named Johannes Kepler who eventually formulated the three fundamental rules for planetary motion, known as Kepler’s laws . Isaac Newton put it all together and finally changed mainstream thinking. The Earth had lost its exalted position, replaced by the Sun. The heliocentric theory had survived the test of time and had become the conventional wisdom.
Johannes Kepler published his famous rules of planetary motion early in the seventeenth century. They can be stated briefly as follows:
- Each planet follows an elliptical orbit around the Sun, with the Sun at one focus of the ellipse.
- An imaginary line connecting any planet with the Sun sweeps out equal areas in equal periods of time.
- For each planet, the square of its “year” (sidereal period) is directly proportional to the cube (third power) of its average distance from the Sun.
Theoretically, it is possible for a planet’s orbit to be perfectly circular. A circle is an ellipse in which both foci are at the same point. In reality, however, there is always some imperfection, so all planets follow orbits that are slightly oblong.
Kepler did not originally call his rules laws. This label was attached later by others. Kepler came up with his three principles and refined them over a period of several years. The first two rules were finalized in 1609, and the last one came out in 1618. The first two laws are illustrated in Fig. 9-4, and third law is rendered graphically in Fig. 9-5.
The Tidal Theory
According to the tidal theory , the Sun originally had no planets or other satellites. This theory suggests that our Sun formed alone and that the other objects, including the planets and the major asteroids, came later.
Relative Motions Of The Stars
The Milky Way, the spiral-shaped galaxy in which we dwell, is believed to be 100,000 light-years across. A light-year is the distance that light travels in 1 year, approximately 9.5 trillion (9.5 × 10 12 ) km or 5.9 trillion (5.9 × 10 12 ) mi. Our galaxy has roughly 200 billion (2 × 10 11 ) stars, all revolving around the nucleus like an enormous swarm of bees. According to current theories, many of the stars bob up and down, above and below the galactic plane, passing periodically through it. Some stars have highly eccentric orbits around the galactic center.
Although the stars are tiny compared with the space between them, they are in relative motion, and collisions or near misses occur once in a while simply because there are so many stars. On average, however, according to one estimate, an outright collision is an extreme rarity, taking place only about once in every 10 billion (10 10 ) years for a typical spiral galaxy such as ours. This is almost as old as the whole Universe is believed to be! Nevertheless, those people who say that the Sun fell victim to a near catastrophe with another star cannot be discounted completely.
Suppose that another star came close enough to the Sun that it and the other star engaged in a gravitational tug-of-war. What would happen? For one thing, the paths of both stars in the Milky Way would be altered; the two stars would swing around each other. In fact, if they came close enough and the speed was not too great, they would end up in orbit around each other. Suppose, however, that the encounter was extremely close but at high speed so that the two stars did not end up in mutual orbit? They would pull matter from each other and scatter that matter into orbits around either star, where the matter would cool, condense, and form dust, rocky ice chunks, and rocks.
Given time, the particles in orbit around the Sun would coalesce into larger objects because of mutual gravitation. Eventually, several dozen spherical objects, perhaps comparable with the size of our Moon, would be created. These objects would follow all kinds of different orbits because of the chaotic way in which the matter was scattered during the original battle of the stars. The result would be frequent collisions and further coalescing. Computer models can show that the end result would be a few large, massive objects and countless tiny ones. This is, of course, the way we observe the Solar System today.
There are problems with this so-called tidal theory . If this is the way the Solar System formed, the planets would all revolve around the Sun in different planes, and their orbits would be less circular and more elongated than they are (Fig. 9-6). However, the actual state of affairs is far more orderly. The planets all lie in nearly the same plane. With the exceptions of Mercury and the Pluto-Charon system, their orbits are nearly perfect circles. All the planets revolve around the Sun in the same direction. For these reasons, few astronomers today believe that the tidal theory is an accurate representation of what happened. In addition, the fact that such catastrophes in general occur only once every several billion years, in our galaxy at least, is a good reason to doubt that this theory explains how things took place to create our Solar System.
The Nebular Theory
If a star has several times the mass of the Sun, ultimately it will explode in a violent outburst called a supernova . These events leave entrails in space—clouds of gas, dust, and rocks of various sizes—in their vicinity. Such mass of debris can appear either light or dark through a telescope depending on how the light of nearby stars shines on it. The cloud is called a nebula .
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