We wonder “Where did we come from, in the big scheme of things?” That leads to questions of how our planet began, which in turn leads to the question, “Where did our Solar System come from?” and “Where did the Universe come from?” That last question will be a focus of the last chapter in this book, but the first question is one that some people have been struggling with for a long time.
Evolution of Solar System Models
People in ancient civilizations observed the heavens with care and came to a number of conclusions that were excellent in explaining what they saw in terms of movement of the Sun, Moon, planets and stars. Babylonian astronomer-astrologers (Chaldeans) kept thorough clay-tablet records of eclipse observations covering many centuries, as long ago as 26 February 747 BC. They also knew that the lengths of the seasons are not equal. All early models of the Universe, based on obvious movements of things in the sky appearing to move around Earth, placed Earth at the very center of the Universe.
Hipparchus, a Greek astronomer, geographer, and mathematician that lived between 147 BC and 127 BC used Chaldean records to develop good models for the motion of the Sun, Moon, and planets that predicted positions used by sailors for navigation. He was the first to compile a trigonometric table, which he used in devising solar and lunar theories that could reliably predict solar eclipses. He measured the differences in the length of the seasons through equinox and solstice observations, finding that spring lasted 94.5 days (spring equinox to summer solstice), and summer lasted 92.5 days (from summer solstice to autumn equinox). That was an unexpected result since the prevailing idea was that the Sun moves around the Earth in a circle at a constant speed. Hipparchus’ solution was to place the Earth not at the center of the Sun’s motion, but at some distance from the center— about 1/24 of the radius of the orbit. That model described the apparent motion of the Sun fairly well.
Claudius Ptolemy, Roman astronomer, mathematician and geographer living in Alexandria, Egypt from approx. 87–150 AD established a model of the Universe based on the Greek model that would explain the motions of heavenly bodies well enough to be the standard for many centuries. Ptolemy’s model still assumed that the Earth was the center of not only the solar system, but the entire Universe—a geocentric theory. In Ptolemy’s system, everything orbits the Earth in the order Mercury, Venus, Sun, Mars, Jupiter, Saturn. For accuracy in predicting naked eye positions, it requires at least 80 epicycles, which are smaller orbit paths superimposed on the main orbits. The stars move on a celestial sphere around the planetary spheres.
Christian Church doctrine based on Greek and Roman philosophers required a solid belief in an Earth-centered Universe. The idea of a sun-centered system had been proposed by Aristarchus of Samos around 200 B.C., but arguments of Greek philosopher Aristotle prevailed, when he refuted the Sun-centered system with three questions: (1) If the Earth spun on an axis, why didn’t objects fly off?; (2) If the Earth was moving (around the sun), why didn’t it leave behind the birds flying in the air?; (3) If the Earth was orbiting the sun, why didn’t the stars appear to change their position since they were being viewed from a different perspective (the phenomenon of parallax)? This last phenomena, parallax, does occur, but is much too small to be seen without a telescope due to the extreme distance to stars. [See chapter 4.]
Copernicus recognized that a Sun-centered—heliocentric—model could easily explain certain planet movements that were serious problems for the geocentric system. He organized the five planets that were known at that time in the order that we know they are in today: Mercury, Venus, Earth, Mars, Jupiter, Saturn. The moon orbits around the Earth, he stated, but the stars are distant and don’t revolve around the sun. Since the Earth rotates around its own axis, the stars appear to revolve around the Earth in the opposite direction. Earth moving around the Sun also explained retrograde motions of the planets much more easily than the epicycles of the Ptolemaic model. Alas, Copernicus still thought that the planets move around the Sun in perfect circles, which is not actually the case, so his model still needed to have epicycles—quite a lot of them—to make accurate predictions for the motions of the planets.
“In the center of everything the sun must reside; . . .
there is the place which awaits him where he can give light to all the planets.”
The problems and messiness associated with epicycles would not be overcome until Johannes Kepler (1571–1630 A.D.) came to the rescue. Kepler worked with renowned Danish astronomer, Tycho Brahe in Prague. Kepler was assigned the task by Tycho Brahe to analyze the observations that Tycho had made of Mars. Of all the planets, the predicted position of Mars had the largest errors and therefore posed the greatest problem. Tycho’s data were the best available before the invention of the telescope and the accuracy was good enough for Kepler to show that Mars’ orbit would precisely fit an ellipse. Kepler inherited Tycho’s post as Imperial Mathematician when Tycho died in 1601. In 1605 he announced his first law of planetary motion.
Kepler’s First Law:
T1 = period of planet 1
7.2 Assuming Earth’s period is 1 year and its orbit radius is about 150,000,000 km, using Kepler’s 3rd law,
(a) what is Mars’ orbit radius if it takes 687 days to orbit the Sun and
(b) how long is Jupiter’s year if its distance from the Sun is about 780,000,000 km?
Italian mathematician, physicist, and astronomer, Galileo Galilei (1564–1642 A.D.) was a contemporary of Johannes Kepler and a kindred spirit, to boot. Using the newly invented telescope, Galileo discovered moons of Jupiter, mountains and craters on the Moon, phases of Venus, and sunspots. Some of these observations supported the Copernican heliocentric theory.
[Galileo to Kepler, 1597] ....Like you, I accepted the Copernican position several years ago and discovered from thence the causes of many natural effects which are doubtless inexplicable by the current theories. I have written up many of my reasons and refutations on the subject, but I have not dared until now to bring them into the open, being warned by the fortunes of Copernicus himself, our master, who procured immortal fame among a few but stepped down among the great crowd (for the foolish are numerous), only to be derided and dishonored. I would dare publish my thoughts if there were many like you; but, since there are not, I shall forebear....
Source: Giorgio de Santillana, The Crime of Galileo (1955).
It is interesting that in light of more recent science, both Galileo and his opponents were partly right and partly wrong. Galileo was right in asserting the mobility of the earth and wrong in asserting the immobility of the sun. His opponents were right in asserting the mobility of the sun and wrong in asserting the immobility of the Earth.
If Kepler provided the most accurate descriptions of planet orbits, it was not until the work of Sir Isaac Newton (1643 -1727 A.D.) that the orbit motions would be explained in his theory of universal gravity. Newton made many other discoveries and inventions including:
In 1666, Newton made the breakthrough of imagining that the Earth’s gravity extended to the Moon. Using Kepler’s third law of planetary motion, Newton deduced that there is a force (known as centripetal force) holding the Moon (or any planet) in orbit, and that force depends on distance in a certain way. If the distance is doubled, the force becomes one-fourth as much; if distance is tripled, the force becomes one-ninth as much. In general, if distance increases by a factor of “n,” the force decreases by a factor of 1/n2, a relationship known as the inverse square law. Newton also showed that Kepler’s second law (that the line joining a planet to the sun sweeps out equal areas in equal times) can be explained by the fact that a body moving in an elliptical path and attracted to one focus must indeed be drawn by a force that varies as the inverse square of the distance.
The constant of proportionality G is known as the universal gravitational constant. It is termed a “universal constant” because it is thought to be the same at all places and all times and thus universally characterizes the intrinsic strength of the gravitational force.
• apples falling from trees,
Newton’s one system of laws of nature gave order to most of the known problems of astronomy and terrestrial physics. The work of Galileo, Copernicus, and Kepler was united and transformed into one coherent scientific theory. The new Copernican world-picture finally had a firm physical basis.
Newton’s Laws of Motion
Newton’s First Law of Motion:
Newton’s Second Law of Motion:
How Do Star-Planet Systems Form?
To finish the story that was begun at the beginning of Chapter 3, a nebula — a huge cloud of gas and dust in space — starts to collapse, with gravity pulling the gas and dust together. The explosion of a nearby star (a supernova), may generate shock waves in space which squeeze the cloud and trigger the collapse. Just like a dancer that spins faster as she pulls in her arms, the cloud spins faster and faster as it collapses.
When the center of the cloud gets hot enough, nuclear reactions start occurring and a star, like the Sun, is born. The star not only radiates heat and light but blows its own particles outwards, pushing out remaining gas and dust of the new star system with sort of wind called a stellar wind.
Meanwhile, particles have been colliding and sometimes sticking together in clumps, eventually forming planets and moons. Two main types of planets form: smaller planets of mostly rocky material (e.g. Earth, Venus, Mercury and Mars), and large planets made of icy material and gas (e.g. Jupiter, Saturn, Uranus, and Neptune). Other icy material settles in the outer regions of the disk along with rocky material, where they form a myriad of smaller bodies.
Very small bodies, either icy or rocky are called meteoroids. When a meteoroid falls into Earth’s atmosphere, it interacts with the atmosphere, heats up and leaves a streak of light in the sky that is called a meteor. If it makes it all the way to the ground, the rocky visitor from space is called a meteorite.
Hubble space Telescope image of Comet Shoemaker-Levy 9 fragments before they collided with Jupiter
The worlds come into being as follows: many bodies of all sorts and shapes move from the infinite into a great void; they come together there and produce a single whirl, in which, colliding with one another and revolving in all manner of ways, they begin to separate like to like.
—Greek philosopher (atomist), Leucippus (~480-420 B.C.)