"Of all the celestial objects with which we are acquainted, none make so strong and universal an impression upon our globe as does the Sun. He is that very light, 'the greater light to rule the day,' as stated in the first chapter of the book of Genesis; a vast and fiery orb, kindled by the Almighty on the morn of creation, to cheer the dark abyss, and to pour his radiance upon surrounding worlds. Compared with him, all the other solar bodies are of inconsiderable dimensions; and without him, they would be wrapped in the gloom of interminable night." So wrote Hiram Mattison in his 1864 text book Astronomy for high-school students.
The Sun's supreme and paramount importance to the Earth, in the utter dependence upon it of every living thing, is far too vaguely known and too indefinitely appreciated and understood. Hence, a few basic facts about the Greater Light are in order.
The visible body of the Sun is called the photosphere, or sphere of light. Surrounding the photosphere is the Sun's atmosphere which consists of the chromosphere (or pinkish sphere of color) which extends for several thousand miles above the photosphere, and the corona which extends outward for millions of miles. The corona of the Sun is visible only during a total solar eclipse.
The Sun's dimensions are virtually incomprehensible. Were a railroad—if it were possible—passed through the Sun's center, and should a fast express train start from one side and proceed at the rate of 100 miles per hour continuously to the other side, the time required for such a transit would be nearly one full year. With a diameter of 864,059 miles to be precise, a distance equal to 109 Earths set side by side in contact, or roughly twice the diameter of the Moon's orbit about the Earth, the Sun is an immense ball of extremely hot gasses. The surface temperature alone is in excess of 10,000 degrees Fahrenheit.
More than a century of spectroscopic study has resulted in detailed knowledge of the Sun's composition, of which the most abundant elements are hydrogen and helium constituting 98 percent. The other two percent is made up of heavier elements common in the planets. The volume of the Sun is such that 1,303,600 volumes the size of the Earth would be required to fill it, and its mass is such that given a large enough scale it would take a stack of 332,946 Earths before the balance would tip in favor of the Earth. The almost inconceivable distance to the Sun—93,000,000 miles—would require our high-speed train well over a hundred years to traverse. If there were air to convey a sound from the Sun to the Earth, and a noise could be made loud enough to pass that distance, it would require over 14 years for it to come to us.
The angular diameter of the Sun subtends about one-half degree, or very nearly the same as the Moon, a fact that has important cosmological consequences insofar as eclipses of these bodies are concerned. If a weight trainer's barbell weighed 100 pounds on Earth, the same would weigh well over a ton on the Sun, if it didn't become instantly vaporized in the intense heat. At magnitude -26.8 the solar orb shines with the brilliance of perhaps 500,000 full Moons. The Sun rotates, but not uniformly. Being neither solid nor liquid, but entirely a sphere of gas, its rotation varies, as learned from the observation of the apparent travel of sunspots across its visible face, being most rapid at the equator and the slowest near the poles. At its equator, the average rotational period of the Sun is 24.65 days. Halfway between the equator and the poles, the period is 27.50 days, and near the poles it is 34 days. Light from the Sun traveling 186,262 miles per second takes a little over eight minutes to reach the Earth. The equator of the Sun is inclined a little more than seven degrees to the ecliptic. A cubic foot of the Sun would weigh about 85 pounds if it could be placed on your bathroom scales.
When Galileo first noticed dark specks on the surface of the Sun, he thought they might be openings into the Sun's interior. Actually sunspots are whirlpools of particles that are stirred up by the Sun's intense electric and magnetic fields, which in turn maybe a function of its uneven rotation. They appear dark because they are 30 percent cooler than the rest of the Sun's surface, although they are still thousands of degrees hotter than molten metal. The number of sunspots vary over time, but reach a maximum about every eleven years. Other than the sunspots, the most noticeable detail on the Sun's surface, as seen through a filtered telescope, is the granulations, little islands of heated convection cells averaging 600 miles across that cover the entire face of the Sun. According to Newtonian physics, the Sun comprises 999/1000 of the mass of the Solar System.
In order to get a grasp on the Sun's appearance and visibility over great distances, suppose we board a space ship and go on a journey through the Solar System and out into space directly away from the Sun. To begin with, we recognize that from Earth, the Sun appears about the same size as the Moon. Now, if we were to blast off in our rocket and fly to Mercury, the closest planet to the Sun, the Sun would appear about two to three times larger hanging in the sky over Mercury's baked surface. As we sped over the cloud tops of Venus, the Sun would appear about half again as big as from Earth. If we landed on Mars and saw the Sun from the chilly Martian deserts, the muted solar disk would appear about two-thirds as large as it appears from Earth. As our space ship passed through the asteroids, the Sun would be about a third as large as the Earthian Sun. In the vicinity of Jupiter, the Sun would appear about a fifth as large. As we continued even further outward in the Solar System, the sunlight outside our spaceship's window would appear ever dimmer. By the time we arrived at planet Uranus, the Sun would no longer appear as a discernible disk to the naked eye. At Pluto, the sentinel of the solar system, the Sun would appear as a star although its brilliance would still bedazzling. Once we left the Solar System and headed out into the dark and exceedingly frigid depths of the abysmal cosmos, the Sun would yet remain the brightest object in the sky for a considerable distance. When we reached Alpha Centauri A, the nearest star at a little more than four light years away, we might land on a nearby planet and look in the sky to see the Sun as one of the brightest stars and located just to the left of the group of stars we call Cassiopeia. Finally, as we continued our journey, we would have to travel outward another 45 light years before we reached a point where the Sun would appear as a faint yellowish star barely visible to the naked eye, at about 50 light years out in deep space. Beyond that it would disappear below the threshold of visibility and would require a telescope to see it.
Of all the celestial objects, the Sun is probably the one most taken for granted. Its light and heat is a familiar everyday phenomenon. History is full of evidence that men did not always have our present childlike faith in tomorrow's sunrise or in the return of summer. Chiefs and priests of many cultures offered sacrifices and served obeisance to the Sun god whose anger at mankind supposedly caused it to retreat southward in the fall, punishing the people with bitter weather. Some, no doubt, believed that the return of spring was due solely to their ardent solicitations to the Sun. What would our emotions be if on some evening's news broadcast it was announced: "Ladies and gentlemen, we have just received word that the Sun has gone out"? Such a catastrophe would bring a dawnless morning, the Earth would be doomed to perpetual darkness, in the course of a few hours time summer would turn into winter and perhaps snow would start falling in the darkness and then the lakes and oceans would eventually freeze. Food and fuel would rapidly disappear and civilization would be ended within just a few days at best. Ultimately the atmosphere would condense, liquefy, and freeze to entomb the entire Earth with a thirty-foot casing of solid air at the temperature of deep space, about 400 degrees Fahrenheit below zero.
The Sun is a cleverly designed dynamo that continually furnishes us with light and heat. The Sun's reliability and permanency is remarkable, although it is not known with certainty what mechanism lies within the solar furnace that produces all of its energy. The natural first thought that some kind of fuel was burning had to be ruled out. It is easily calculated that if the Sun were made of coal and had always burned at the observed rate, it would last only 5,000 years. We do know that it is extremely efficient, producing 70,000 horsepower for every square yard of the solar surface. Only one part in two billion of the total radiated energy leaving the Sun actually strikes the Earth. Nevertheless, even if the fraction falling in one week upon an area one mile square could be converted into electricity at the rate of five cents per kilowatt hour, its value would be well over a million dollars. The geyser-like eruptions called prominences that emerge from the Sun can be identified with the stormy sunspots which in turn are associated with the Sun's magnetic field and its coronal halo. All of these may be linked to the Sun's differential rotation. As for its energy production, it is believed that the sunlight we see everyday is made of units of radiant energy called photons which originate in the inferno of the Sun's core. They may take many years slowly wandering up to the surface, then in a little more than eight minutes they speed across the 93,000,000 miles of space to the Earth, if they happen to be headed our way. Depending upon the wavelength or amount of energy a photon has, it may be absorbed by the Earth's atmosphere, reflected back into space, or it may zip down to the Earth's surface to warm a flea or a blade of grass for an instant. Each photon carries only a tiny amount of energy, but trillions of them reach every square yard of the Earth each second. Together they make up the Sun's light and heat. The great pressure and temperature of the Sun's core cause the crowded jumble of atomic particles to smash into each other so violently that they stick together in a process called fusion, which releases the energy. The process is thought to be very similar to that of a continuously exploding hydrogen bomb. The best way to look at the Sun is, don't! Gazing at the Sun much longer than an instant can cause serious, permanent eye injury or even blindness. The damage is imperceptibly caused by the Sun's rays which are focused by the eye onto the retina, in much the same way that a magnifying glass can focus the Sun's rays on a piece of paper and set it afire.
There has been much talk among evolutionary philosophers about the Earth being "just another planet revolving around "just another average star." Yet when the evidence to the contrary is considered, it is clear that neither the Earth nor the Sun are insignificant or typical, and that the Sun is not just another "star" after all, but actually quite unique in the universe and that it ought not be classified as a star. When the Sun is compared to the stars, it truly stands out in its unique suitability as the light- and heat-giver for the Earth as an abode of life. It is a known fact that most stars produce visible light in only small proportions and are most intense in their output of lethal radiations like X-rays and gamma rays.
The Sun is unusual in the life-supporting spectrum of energy that it does provide. Another aspect of the Sun's uniqueness is its singularity. Over two-thirds of the stars are members of star systems containing two or more stars. If the Sun were a member of such a system, continually perturbed by the gravitational interplay of the neighboring stars, life on Earth would be precarious at best, given the drastic variations of tides, light, and heat it would experience. The Sun is unique in yet another way. Compared to most stars, its light and heat is steadfast, constant, and abiding. Many more of the stars are considerably variable in their output of light and heat. Most stars fluctuate greatly in the process of time, with output factors that range from 10 percent to 150,000 percent. Life on Earth could not endure such wild extremes of radiation. Furthermore, the vast majority of stars are smaller, cooler, dimmer, and less massive than the Sun. In addition to the Sun's unique intrinsic suitability to be the Earth's light- and heat-giver, the Earth itself is placed at the optimal distance from such an unusual "star" as our Sun. When seen in the broader context of the cosmos, the Sun can be clearly seen as a grand product of design, with a very special purpose, by an almighty and benevolent Creator who has revealed Himself and declared in His great foundational revelation: "In the beginning God created the heaven and the earth . . . and God made two great lights; the greater light to rule the day, and the lesser light to rule the night: He made the stars also" (Genesis 1:1,16).
References to the Sun fill the Bible. Its importance as a light-giver, symbol, and timetable is inestimable. The divinely ordained purpose of the Sun is well known "to rule the day . . . and to divide the light from the darkness" (Genesis 1:18). Given its importance in the daily affairs of man since the days of old, there is no wonder it was worshipped by many ancient civilizations, and that even in the Bible it is used metaphorically for God.
* J. Timothy Unruh, Back Yard Astronomers, P.O. Box 1034, Rocklin, California 95677. Unruh is an architect, but lectures, researches, and writes on astronomy.