"The Sun is the only star known to grow vegetables." This pithy quote from Professor Phil Scherrer of Stanford University beautifully summarizes the fascination and the uniqueness of the star nestling on our doorstep. As stars go, the Sun is not something to write home about except for the fact that we would have no home without it. The energy provided by the Sun sets the conditions for the formation and development of life on Earth, life that eventually led to you, me, and the other 7 billion humans who inhabit this water-laden ball of dirt circling some 150 million kilometers from the star at the center of our solar system. Whether there is life on other balls of dirt circling other stars is, as yet, unknown. That there are planets orbiting other stars is now a matter of fact, with some 342 planets having been discovered to date lying in 289 distinct planetary systems. The planets discovered so far are more like Saturn and Jupiter in our own solar system—massive planets, probably gaseous, where life, if present, is most certainly very different from anything we have encountered on Earth. Until we can determine whether life exists or has existed on other planets in other star systems, we must assume that the Sun is a unique star that has the enviable characteristic of being able to grow vegetables.

While growing vegetables is clearly one way that influences how we humans view the Sun, throughout human history there has been a special reverence, both practical and mystical, for this daily visitor to our skies.

Cultures around the globe have revered the Sun as a divine being and utilized the predictability of its annual wanderings through the stars to mark special times of year, relating its behavior to changes in the natural world around them. Celtic henges, Egyptian sun temples, aboriginal starlore, Easter Island Moai, Native American medicine wheels, and many other ancient astronomical structures around the world mark out special points in the Sun’s apparent path across the sky (e.g., solstices, equinoxes, and eclipses) and tell the tale of the universal importance that the Sun’s patterns of motion had on the everyday life of our ancestors.

THE HELIOCENTRIC UNIVERSE

That the Sun is at the center of our solar system is an undisputed fact as we move into the twenty-first century. However, until the publication of De Revolutionibus Orbium Coelestium [Concerning the Revolutions of the Heavenly Spheres] by Polish astronomer Nicolaus Copernicus in 1543, a geocentric model of the solar system had dominated our ideas since the time of Aristotle (384–322 BC). The Earth-centered universe was supported by observation (at least, to the best of the abilities of the pre-telescope astronomers). Additionally, a model developed by Ptolemy (CA. AD 100–170) in which the planets wheeled around on numerous interlocking circular paths, known as epicycles, provided the backbone of the geocentric worldview. Such a complex mechanism of circles upon circles was a result of the dictates of Aristotle and his teacher Plato (CA. 428–348 BC), which stated that the Earth was at the center of the universe and that everything in the heavens was perfect and therefore unchanging, moving at constant speed in perfect circles. To match his detailed observations, Ptolemy had to resort to a complicated overlap of circular pathways.While the Earth-centered solar system dominated, a number of other "theoretical ideas" were developed by the ancient Greeks, most notably Aristarchos of Samos (310–CA. 230 BC), who proposed the idea that the Sun lay at the center of the solar system and was seven times larger than the Earth (the true value is closer to 100 times). The dominance of Aristotle and Plato meant that these ideas lay essentially dormant for almost 1,800 years, until what is known as the Copernican revolution. It is not clear what drove Nicolaus Copernicus to consider a Sun-centered solar system, and, in fact, he was reluctant to publish his work—the first print of his book appeared on the day he died. It is often said that Copernicus did not like the complexity of the Ptolemy epicycle model and that this led him to the "simpler" picture of the planets revolving around the Sun. However, Copernicus was still constrained by the Aristotelian doctrines of circular and constant motion, and to make his Sun-centered model fit the observations, he also had to resort to epicycles. In the end, the Copernican model required more epicycles than Ptolemy’s Earth-centered model (there is a debate over exactly how many each model required).

The Copernican ideas developed into a revolution of thought at the hands of German mathematician and astronomer Johannes Kepler (1571–1630), who devoutly believed in the Sun-centered universe proposed by THE SUN Copernicus. After decades of struggling to understand the motion of the planets around the Sun, Kepler eventually made a revolutionary breakthrough: he threw out the Aristotelian constraints of circular orbits and constant velocities. Kepler found that by allowing the planets to move in elliptical orbits, with the Sun at one focus of the ellipse, and the speed to vary with distance from the Sun, all the data could be explained. Kepler’s three laws of planetary motion state the following: The orbit of each planet about the Sun is an ellipse with the Sun at one focus. As a planet moves around its orbit, it sweeps out equal areas in equal times. This is the equivalent to allowing the speed to change with distance from the Sun.

More distant planets orbit the Sun at slower average speeds such that the square of the orbital period, p (in years), equals the cube of the average distance from the Sun, a (in astronomical units): p2 = a3.These laws, based entirely on observations, eventually paved the way for Sir Isaac Newton’s theory of gravitation