History of the Space Discovery

Early Views

(Smithsonian Planets, Thomas R. Watters, page 16)
For thousands of years ancient peoples observed the Sun, the Moon, the Visible Planets and the stars. Drawing on their religious and cultural traditions, they often explained what they saw in the heavens by weaving myth and filling the sky with both angry andbenevolent gods as well as with places of wihte light that cuts across the black summer night sky - now known as a Milky Way galaxy - was the heavently Nile River in the land of dead.
Acient peoples also discovered practical applications based upon their observations. They noted the position of the Sun in the various seasons and its effect on crop growth. They saw that the Moon affectes tides. And they saw that the stars fixed in relation to one another, while the planets moved through the stars. Realizing this, they used the stars as pint of reference to guide their travels.

The Ancient Observatory in Beijing, China, was built in 1296. The Chinese led the world in astronomy during the medieval period. They noted two supernovas - exploding stars that become extremely bright for several years and then fade - in 1006 and 1054. these observations helped astronomers in twentieth century to undersand how supernovas happen.

Written records, observatories, monuments, measuring instruments, and other artifacts show that most ancient civilizations not only studied the skies, but did so with considerable accuracy.The Summerians, Babylonians, Egyptians, and Chinese made the first known recorded measurements of the stars and the movement of the planets.
By about the fifth century B.C., Babylonians had identified the region of sky through witch the Sun, Moon, and plantes move as viewed from Earth, and they had named several "planet gods" visible to the human eyes - the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn (the basis of out names for the seven days of the week).
Record of thge Han dynasty, which lasted from 202 B.C. to 220 A.D., show that Chinese astronomers observed everything from eclipses and planetary movements to comets and sunspots.
Studies of an ancient African observatory in Kenya Strongly suggest that an accurate and complex astronomy-based calendar system was developed there by the first millennium B.C. In the Americas, Manuscripts of the Mayans reveal a wealth of astronomical observations including details of lunar and solar eclipses and Venus calendar. And long before the study of astronomy was established in Europe, if flourished in the Arabian empire, wich streched from Spain to india and lasted from the mideighth to the fifteenth century. In fact, many of the measuring instruments that Arab astronomers used were superior to those used by the Greeks.

Greek Contributions

The word planet comes from the Greek Word meaning "wanderers." The Greeks named the region of the sky through which the plantes pass the zodiak and divided in into twelve groups of stars, or constellations. The word zodiak means "circle of animals," and most of the conctellations were given names of animals. The Greeks were also noted for the application of precise geometric measurements to astronomy that led to amazing accuracy in descriptions of the planetary movements. For example, Eratosthenes (276-194 B.C.), librarian at the museum in Alexandria, Egypt, used geometric calculations related to the position of the Sun and concluded that the circumference of Earth was 25,000 miles (40,000 km), which is nearly correct. Other Greeks, including Aristarchus and Hipparchus, also made key contributions.

This drawing of general early conception of the universe is from a book, published in 1614.

Aristarchus In about 270 B.C., Aristarchus of Samos proposed the helicentric model of solar system, which correctly placed the Sun at the center with Earth and the other planets revolving around it. He reashed his conclustion after estimating the relative sizes of Earth, the Moon, and the Sun and discovering that the Sun was much larger than Earth. But the idea was disregarded until nearly 1,900 years later, largely because of the strength of the prevailing geocentric belief, based on Greek Philosophy and religion, that Earthitself was the center of the solar system.
Hipparchus Perhaps the greatest of the astromomers of antiquity was Hipparchus, who died sometime after 127 B.C. He developed the first star catalog, listing some 850 stars, and he calculated the lenght of a year within 6.5 minutes. He also calculated precession, or wobble, of the Earth's axis by comparing his own observations with those made in Alexandria 150 years earlier and in ancient Babylonia. Like other of his time, however, Hipparchus rejected the heliocentric theory of Aristarchus.

The Ptolematic System

Observations of the planets showed that although they move generally toward the east through the constellations, they appear to turn back west at various intervals before resuming their eastward paths. This peculiar backward looping is now called retrograde motion. One explaination of this was proposed by Claudious Ptolemaeus (127-145 A.D.) of Alexandria, generally referred to as Ptolemy. Like most Greek astronomers, Ptolemy believed that celestial bodies moved in circles, which were considered the most perfect geometric shape. He suggested that the planets themselves, while orbiting around centers like wheels, also orbit the Earth in circles. This idea of "epicycles" privided and appoximation to retrograde behavior of the planets.
Basing this studies on the work of Hipparchus, Ptolemy expanded his predecessor's catalog to include 1,022 stars. Like Hipparchus, Ptolemy agreed with geocentric view of the planetary system.

A Thousand Years of Silence

After the fall of Roman Empire in 476 A.D., the important work of the Greek astronomers was mostly lost, forgotten, or ignored. In Europe the Ptolemaic model of the planetary system survived because it supported the philosophy of Aristotle and fit well with the teachings of the powerfull Roman Catholic church. This belief that Earth was the center of the universe would remain unchallenged for more than 1,000 years in the Western world.

Early European Astronomers

The Renaissance in Europe which began in the 1300s, fostered a renewed interestin the ideas and culture of ancient Greece. In this stimulating intellectual atmosphere, some began to rethink the Ptolemaic model of the universe.

Nicolaus Copernicus, considered the founder of modern astronomy, dared to reject the Earth-centered Ptolematic model. He could not prove his theory, but later astronomers like Galileo and Kepler would provide the evidence to support the heliocentric model.

The Copernican Revolution

The Polish astromoner Nicolaus Copernicus, born in 1473, used mathematical proofs to revive the heliocentric model of Aristarchus. As a part of this work, he launched an extensive series of observations and calculations to refine Ptolemy's tables. Copernicus claimed that all the planets including Earth moves repidly through space and believed that this motion accounted for the aooarent motion of the stars and even the retrograde motion of the planets.
Although many resisted the heliocentric theory, astronomers generally accepted Copernicus' highly detailed data. It his great book On the Revolution of the Heavenly Spheres, published in 1543 in only hours before he died, Copernicus showed that the motions of other celestial bodies could be explained by the motions of Earth. This principle became the basis for some of the most important astronomical discoveries of next two centuries.

A page from Copernicus' groundbreaking theatise On the Revolutions of the Heavenly Spheres contains his diagram of the solar system with the Sun at the centerand six planets tracing perfect circles around it. Uranus, Neptune, and Pluto had yet to be identified.

Decades before the telescope was invented, Danish astronomer Tycho Brahe, depicted in a 1598 drawing, made accurate tables of positions of celestial objects.

Tycho Brahe's nose was made of gold and silver. His real nose was cut off in a duel witha a student who was jealous of Brahe's mathematical abilities.

Tycho Brahe

On August 21, 1560, a 14-year-old Danish boy named Tycho Brahe observed a total eclipse of the Sun, occuring just as existing almanacs had predicted. Three years later he observed that jupiter and Saturn were aligned. This time the almanacs and the Copernican tables were days off in their predictions, so the young Tycho decided to preduce more accurate tables. In 1572 he discovered a new star, or nova. Now believed to have been a supernova, a star, that expels much of its mass, it disappeared from sight after 16 months. This discovery conflicted with the prevailing belief that the stars that made up the celestial sphere were fixed and could not change.
Tycho and his assistants continued to amass data, making amazingly accurate observations without the use of a telescope and correcting nearly every known astronomical table. By measuring a comet and showing that its was beyond the Moon, Tycho disproved the belief that comets were hot gasses in Earth's upper athmosphere. As to the heliocentric theory of Copernicus, Tycho agreed that the planets revolved around the Sun, but claimed that the Sun itself was revolved around the fixed Earth.

Johannes Kepler

Born in small German town in 1571, Johannes Kepler received his education through a scholarship program for the sons of poor families. He attended the University of T�bingen, wherehis astronomy teacher believed in the Copernican theory. When Tycho Brahe, impressed by a paper Kepler had written, invited the young student to join his research group, Kepler's genius began to florish. The following year, in 1601, Tycho died and Kepler was appointed to carry on his work, using Tycho's extraordinary accurate data. By 1609 Kepler had announced two critically important findings. In 1618 he announced third.
First he delcared that the orbits of the planets around the Sun are elliptical, rather than perfectly circular. Using Tycho's data about mars and trying to fit various curves to its orbit. Kepler realized in 1609 that the orbit of Mars was ellpise, not a circleor a combination of circles as had been thought. once he mad this discovery, he could explain the orbits of the other planets as ellipses as well.
Kepler's second major finding in 1609 was also based on his observations of Mars. He concluded that a planets moves faster when closer to the sun, ans slower when farther from the Sun, sweeping out equal areas in space in equal periods of time.
Kepler's third discovery was the existence of a mathematical relationship between a planet's distance from the Sun and its orbital period - the length of time it takes to orbit the Sun once.
These three proven observations came to be known as Kepler's laws of planetary motion. They would finally replace the old established beliefs and unquestionably support the findings of Copernicus. They would also providecritical later as a foundation for Isaac Newton's theory of gravitational force.

Galileo

In Italy in 1609, Galileo Galilei built his first telescope without ever having seen one assembled. This telescope had two lenses and magnified objects to three times their size. He continued to study the heavens using telescopes he made by grinding his own lenses. The largest had a lens with a 1.7-inch (4.3-cm) diameter and a magnification factor of 33. In 1610 his pioneering work led to the discovery of four satellites of Jupiter. He also observed sunspots and craters on the Moon.
Galileo's observations that sunspots moved from day to day and thus must be or close to the Sun's surface clashed with the Aristotelian concept of perfection of the Sun. Also contrary to the prevailing view, Galileo agreed with Copernicus that Earth is not the center of the planetary system. His discovery that Venus has phases just like Earth's Moon confirmed that venus orbits the Sun and supported the heliocentric model. But learned people of the time still could not accept this reality. When invited by Galileo to look through his telescope and judge for themselves, professors of philosophy at the University of Padua refused.
Early in the 1500s Martin Luther had challenged the authority of the pope, and by Galileo's time the Protestant Reformations was under way. Europe was being tron by a series ofreligious wars that had begun in 1618. The Roman Catholic Church felt further threatned by scientific studies whose conclusions conflicted with its own teachings. Thus, church authorities forced Galileo to renounce his own theories as errors. His books were banned, he was forbidden to teach the Copernican view, and he was confined to his home from 1633 until his death in 1642.

Galileo, built his first telescope in 1609. Already skeptical of the theories of Aristotle and Ptolemy, he found his suspicions confirmed when he used this new tool to explore the sky. In the painting , Galileo is depicted inviting the chief magistrate of Venice to look through one of his telescopes.

Late in his life, Isaac Newton said he had been "like boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me." His "pretty shells" were ideas that changed the course of modern science.

Isaac Newton

Before the time of Isaac Newton, who was born in 1642, gravity and orbital motion were thought to be unrelated. Newton proposed, however, that the Moon would fly off in straight line if there were no force keeping it from doing do. He recognized that there was a force of gravity constantly pulling the Moon towards the Earth. Taking this discovery one step further, he understood that gravity was a universal phenomenon - between any two bodies that gravity increases with more massive bodies and diminishes as bodies move farther apart.
Newton determined the physical laws that explain Kepler's observations and laws of planetary motion. He demonstrated that the total gravitational attraction of spherical body is exactly the same as if all its mass were concentrated at the center of the sphere. Building on Kepler's discovery that orbits could be elliptical, Newton showed that they could also be circular, parabolic, or hyperbolic. This provided an explaination for the behavior of certain comets that do not follow elliptical paths. Newton also proposed that since any object that has mass is attracted to every other object that has mass, the orbit of each planet is influenced not only by the Sun, but by all the other planets as well. In general the influence of the other planets is very small because of far greater mass of the Sun.

Discovering New Planets

Until the last part of the eighteenth century, only six planets of the solar system was known - Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Using steadily improving instruments, astronomers made increasingly precise observations, checking each finding against Newton's laws. Then, in 1781, William Hershell, a british astronomer, discovered uranus. In 1846, Urbain Leverrier in France and John Couch in England independently concluded that the perturbations, or discturbances of motion, in the orbit of Uranus could not be explained by the presence of other known bodies, and that an eighth planet must exist. Asked by Leverrier to watch a certain area of the sky for an uncharted, the Berlin Observatory found the predicted planet, which was named Neptune.
For the same reasons, another planet beyond Neptune was thought to exist. In 1905 the reasearch began when Percival Lowell and William Pickering predicted it's orbital characteristics and location. A wide-field camera was used , and in 1930 Clyde W. Tombaugh found a new planet within 6^o of where Lowell predicted it would be located. It was named Pluto.
Although the existance of the nine planets of the solar system was now known, the nature of these worlds remained a mystery to be solved.

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