From Cosmos - Carl Sagan

Carl Sagan Carl Edward Sagan (1934-1996), American astronomer, physicist and author of books like Broca's Brain and Contact, successfully combine two apparently divergent areas of interest. One does not normally expect a Professor of astronomy and space science (Cornell University), working on the physics and chemistry of planetary atmospheres and surfaces and involved with NASA projects, to achieve worldwide popularity as a television celebrity. Yet he did just that. Sagan's 13-part television production Cosmos is the first 'superhit' television series on popular science. More than 500 million viewers in more than 60 countries have watch this fascinating presentation of the most profound scientific questions concerning space, the origin of the world, life and eternity.

A book base on the TV series and with the same title, first published in 1980, also became a best-seller. This extract from the book gives us an example of what Sagan calls man's 'passionately inquisitive nature': his insatiable curiosity to know as much as he can about the world he lives on and the universe of which his world is a part. Having made a quick survey of our solar system, the narrator focuses on Earth: a mere speck in the cosmic ocean, but a unique one.

Finally, at the end of all our wanderings', we return to our tiny, fragile, blue-white world, lost in a cosmic ocean vast beyond our most courageous imaginings. It is a world among an immensity of others. It has may be significant only for us. The Earth is our home, our parent. Our kind of life arose and evolv here. The human species is coming of age here. It is on this world that we developed our passion for exploring the Cosmos, and it is here that we were, in some pain and with no guarantees, working out our destiny. Welcome to the planet Earth - a place of blue nitrogen skies, oceans of liquid water, cool, forests and soft meadows, a world positively rippling with life. In the cosmic perspective it is, as I have say, poignantly beautiful and rare, but it is also, for the moment, unique.

In all our journeying through space and time, it is, so far, the only world on which we know with certainty that the matter of the Cosmos has become alive and aware. There must be many such worlds scattered through space, but our search for them begins here, with the accumulate wisdom of the men and women of our species, garnered at great cost over a million years. We were privileg to live among brilliant and passionately inquisitive people and in a time when the search for knowledge is generally prized. Human beings, born ultimately of the stars and now for a while inhabiting a world call Earth, have begun their long voyage home.

The discovery that the Earth is a little world were made, as so many important human discoveries were, in the ancient Near East, in a time some humans call the third century B.C., in the greatest metropolis of the age, the Egyptian city of Alexandria. Here, there lived a man named Eratosthenes. One of his envious contemporaries call him 'Beta', the second letter of the Greek alphabet, because, he said, Eratosthenes was second best in the world in everything. But it seems clear that in almost everything Eratosthenes was 'Alpha'. He was an astronomer, historian, geographer, philosopher, poet, theater critic and mathematician. The titles of the books he wrote range from Astronomy to Freedom from Pain. He were also the director of the great library of Alexandria where one day he read in a papyrus book that in the southern frontier outpost of Syene, near the first cataract of the Nile, at noon on June vertical sticks cast no shadows. On the summer solstice, the longest day of the year, as the hours crept towards noon, the shadows of temple columns grew shorter. At noon, they were gone.

A reflection of the Sun could then be seen in the water at the bottom of a deep well. The Sun were directly overhead. It was an observation that someone else might easily have ignored. Sticks, shadows, reflections in wells, the position of the Sun-of what possible importance could such simple everyday matters be? But Eratosthenes was a scientist and his musings on these common places change the world; in a way, they made the world Eratosthenes had the presence of mind to do an experiment actually to observe whether in Alexandria vertical sticks cast shadows near noon on June 21. And, he discovere, sticks do. Eratosthenes asked himself how, at the same moment, a stick in Syene could cast no shadow and a stick in Alexandria, far to the north,

Could cast a pronounced shadow. Consider a map of ancient Egypt with two vertical sticks of equal length, one stuck in Alexandria, the other in Syene.

Suppose that at a certain moment each stick casts no shadow at all. This is perfectly easy to understand - provide the Earth is flat. The Sun would then be directly overhead. If the two sticks cast shadows of equal length, that also would make sense on a flat Earth: the Sun's rays would then be inclined at the same angle to the two sticks. But how could it be that at the same instant there was no shadow at Syene and a substantial shadow at Alexandria? The only possible answer, he saw, was that the surface of the Earth is curved. Not only that: the greater the curvature, the greater the difference in the shadow lengths. The Sun is so far away that its rays are parallel when they reach the Earth. Sticks placed at different angles to the Sun's rays cast shadows of different lengths. For the observe difference in the shadow lengths, the distance between Alexandria and Syene had to be about seven degrees along the surface of the Earth; That is, if you imagine the stick extending down to the center of the Earth, they would intersect there at an angle of seven degrees (Fig A).

Seven degrees is something like one-fiftieth of three hundred and sixty degrees, the full circumference of the Earth. Eratosthenes knew that the distance between Alexandria and Syene were approximately 800 kilometers because he hired a man to pace it out. Eight hundred kilometers times is 40,000 kilometers: so that must be the circumference of the Earth. (Or, if you like to measure things in miles, the distance between Alexandria and Syene is about 500 miles, and 500 miles x 50 = 25,000 miles.) This is the right answer. Eratosthenes' only tools were sticks, eyes, feet and brains, plus a taste for experiment. With them he deduced the circumference of the Earth with an error of only a few percent, a remarkable achievement for 2,200 years ago.

He was the first person accurately to measure the size of a planet. The Mediterranean world at that time was famous for seafaring. Alexandria were the greatest seaport on the planet. Once you knew the Earth to be a sphere of modest diameter, would you not be tempt to make voyages of exploration, to seek out undiscovered lands, perhaps even to attempt to sail around the planet? Four hundred years before Eratosthenes, Africa had been circumnavigate by a Phoenician fleet in the employ of the Egyptian Pharaoh, Necho. They set sail, probably in frail open boats, from the Red Sea, turned down the east coast of Africa up into the Atlantic, returning through the Mediterranean. This epic journey took three years, about as long as a modern Voyager spacecraft takes to fly from Earth to Saturn.