If you look at the sky and forget everything you have learned, passively and actively, about the universe beyond our planet, it would be easy to make a number of wildly mistaken assumptions. Imagine what a young child, naive to astronomy, sees at dawn: The sun appears on one horizon, climbs to a peak as it crosses the sky, and departs as it meets the other horizon. In the night sky, the moon and stars do the same essential thing. By all appearances, the world around us sits still, and everything in the sky revolves around it.
This, in fact, is what most of the serious thinkers of bygone millennia believed. The consensus was that a possibly flat Earth was at the center of the entire universe, and that everything else in the sky, from the sun and moon to the stars and planets, revolved around the Earth. What seems like a quaint and laughable notion today was not only popular in ancient times, but defensible.
What Are the Four Types of Bodies in the Solar System?
In exploring the heliocentric model of the solar system, an overview of the solar system's basic contents is a good starting point. The word "solar" means "pertaining to the sun" (the Latin word for which is "sol"), and the sun, which is merely a star that happens to be comparatively close to Earth, is far and away the most massive object in the system as well as the only body of its type. Because of the gravitational force exerted by the sun's enormous mass, everything else in the solar system revolves around it, directly or as part of another system.
The planet is the second type of solar-system body. There are eight of these, ranging in size from Mercury, the smallest, to Jupiter, the largest. Pluto was formerly considered a planet and was the most distant planet from the sun, but was "demoted" early in the 21st century to a dwarf planet, and as such it is now a small solar-system object (more on this soon).
Moons, or natural satellites, are the third type of body in the solar system. These bodies orbit planets, but because planets orbit the sun, the sun remains at the true center of the path of every moon. Earth has one such natural satellite, which is about one-fourth the diameter of Earth; most of the larger, "gaseous" planets have dozens of moons.
The fourth kind of solar-system body are small objects (or small bodies). These include comets, asteroids, icy regions called the Oort Cloud and the Kuiper Belt, and the mini-system of Pluto and its two satellites (or moons, if you prefer, although this one is tricky since Pluto is no longer considered a planet; its status remains controversial with some organizations calling for its reinstatement as a full planet).
What Is Geocentrism and Heliocentrism?
Purely speaking, geocentrism is the idea that the Earth is the center of some reference system (usually "everything"), whereas heliocentrism is the belief the sun is the center of some reference system (in modern usage, the solar system).
As suggested previously, geocentrism is the outdated and clearly disproven idea that the Earth lies at the very center of creation itself, with the other observed objects in the sky orbiting the Earth at various distances. This notion originated with the Greek scientists Aristotle and Ptolemy well over 2,000 years ago, was embraced by early Christians and the Catholic Church, and only began to be called into serious question in the 16th century, starting with the work of the Polish astronomer Nicolaus Copernicus (1473-1543). Copernicus was not the first to notice that the planets visible to the naked eye – Mercury, Venus, Mars, Jupiter and Saturn – varied in brightness over the years. He was also not the first to observe that they exhibited retrograde motion, in relation to the background stars. This terms describes the way the planets sometimes briefly reverse the direction of their slow trek against the background stars before resuming motion in the usual direction. Geocentrism advocates had well-crafted explanations for these phenomena, but Copernicus understood that a heliocentric model explained them better. Unfortunately, he did not feel comfortable publishing his ideas until he was on his deathbed, fearing reprisals from the Church that held sometimes-violent sway over most of Europe at the time.
It is perhaps easy now to look at a diagram of the solar system as it is firmly understood and see where Copernicus – who even managed to place all of the six planets known in his pre-telescope time in their proper order from closest to the sun to farthest away, including Earth – got his ideas. More difficult to appreciate is the brilliance that inspired these ideas, especially considering that he was challenging a long-standing idea with tremendous ramifications, both scientific and political.
What Is the Heliocentric Theory?
Copernicus is widely considered the primary figure in heliocentric theory, with Galileo Galilei, normally referred to simply as Galileo, often afforded a similar role. But even before Copernicus, a number of historical figures had begun laying the groundwork for the Earth to be displaced from its philosophical central point in the universe.
Dating back to pre-Christian times, Greek mathematicians had worked out a lot of the equations in geometry that govern planetary motion, and orbiting bodies generally. At the time this meant little in terms of astronomy, but Copernicus drew on much of this in formulating a firm heliocentric theory. And in 200 B.C., a Greek named Aristarchus postulated a rotating Earth, but his idea was dismissed because others asserted that if this were true, people and objects would simply fly off the surface into space. (The concept of gravity was a long, long way from being "a thing" in those days.)
In the 10th and 11th centuries, Al-Haitham (also frequently spelled as Al-Haytham), from what is now Iraq, produced a couple of notable ideas. One of these was that the "arm" of the Milky Way Galaxy visible in the night sky, the the spiral-shaped mega-collection of stars in which it is now known the solar system resides, was actually much farther from Earth than was suspected at the time. The other was that the depth of the Earth's atmosphere from the surface to the unofficial boundary of "outer space" was 32 miles, which turned out to be accurate to within a startling 5 percent. Al-Haitham more generally was one of the early proponents of the scientific methods and almost single-handedly developed the field of optics, but is largely forgotten in modern textbooks and science discussions.
Apart from contradicting the relative placement of the objects in the solar system and beyond, heliocentric theory was predicated on challenging other long-standing assumptions in astronomy. One of these was that heavenly bodies travel in circular orbits. They actually travel in elliptical, or oval-shaped, orbits; although some of these happen to be very close to circular at a glance, the difference introduced into calculations concerning gravity and other variables is profound. In addition, ancient scientists assumed that everything in the cosmos, whatever its physical extent, was made of the same basic "stuff." While it's true that everything in the universe is composed of known chemical elements from today's periodic table, anyone who claimed today that stars and planets have a similar composition would raise more than a few eyebrows.
There may be no one heliocentric theory definition, but think of it as a body of knowledge that developed over many centuries and only bore scientific fruit when the weight of the evidence favoring it was too great for even the most staunch opponents in the religious world to refute. As you'll see, this conflict was indeed very dramatic and dangerous to numerous proponents of heliocentric facts.
What Is the Heliocentric Model?
The heliocentric model differs from heliocentric theory in that it allows scientists to create a formal organizational framework that incorporates the sun, the planets and other minor players in the solar system, and places them physically in predictable positions. In other words, rather than merely positing that the sun is at the center of the solar system, it involves testable hypotheses to be created around this central idea.
After Copernicus was gone, other scientists took up the mantle of heliocentrism, or at least modifications of geocentrism. The Dutch astronomer Tycho Brahe (1546-1601), born three years after Copernicus' death, made observations of the heavens that were as painstaking and precise as could be given that telescopes were not yet in humankind's scientific arsenal. Brahe would not concede that the Earth was at the center of the universe but did posit that the other planets revolved around the sun while the sun itself revolved around the Earth. (Terminology side note: "Revolve" usually means "orbit at a distance," whereas "rotate" means "spin around on an axis," like a top. Most astronomical objects do some combination of both.) This was a step in the right direction, one that helpfully did not put Brahe in the cross-hairs of church leaders.
Brahe's contemporary, Galileo (1564-1642), was the man whose work ultimately spelled the demise of scientific geocentrism. In 1610, after he had invented a crude but useful telescope, he discovered moons orbiting Jupiter. If Aristotle had been correct about all things orbiting the Earth, this situation would be impossible. Galileo also used his telescope to observe mountains and volcanoes on the Moon, sunspots, individual stars within the arm of the Milky Way and moon-like phases for Venus. The latter was particularly striking. If one imagines a universe in which Venus is always between the sun and the Earth, it could never appear fully illuminated thanks to basic geometry. It would always appear like a crescent moon of some sort; its fully lit side would always be facing away from Earth and toward the more distant sun. Galileo demonstrated clearly that this was not the case.
For his trouble, Galileo was placed on house arrest by church officials for the last years of his life. While this seems like a rather misguided punishment for someone whose "crime" was greatly advancing the state of human scientific inquiry and knowledge, he at least escaped the death penalty for heresy that had been doled out to other opponents of geocentrism, notably the Italian scientist Giordano Bruno, who was burned at the stake for advocating Copernicus' ideas.
What Is the Importance of Heliocentric?
Clearly, if humankind continued to operate as if the Earth sits at the center of the universe, no meaningful progress could have been made in virtually any field reliant on knowing the gross details of modern astronomy. Sending spacecraft toward planets such as Mars (upon the surface of which humans have landed probes) as well as Jupiter, Saturn, Neptune and Pluto (all of which have hosted close spacecraft fly-bys) using a geocentric model is a thought exercise bordering on the absurd, akin to picturing someone sailing from Los Angeles to Sydney using a hastily scribbled map of California.
Knowing that systems obey key gravitational laws has allowed astronomers studying very distant objects, such as galaxies and supernovae, to better focus their efforts and make more accurate predictions about the motion of heavenly bodies.
- New Mexico State University: The Copernican (Heliocentric, or Sun-Centered) Model of the Solar System
- The University of Rochester: The Copernican Model: A Sun-Centered Solar System
- NASA Solar System Exploration: What Is a Moon?
- NASA Observatory: Planetary Motion: The History of an Idea That Launched the Scientific Revolution
- NASA Science: Small Bodies of the Solar System