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The ancient astronomers and Copernicus made their great contributions to science before the invention of the telescope. They based their conceptions of the universe on astronomical data gathered by naked-eye observers. These observers used sighting and angle-measuring instruments that did not include magnifying lenses of any sort. The classic refracting telescope, consisting of a tube with an eyepiece at one end and a larger objective lens at the other, first appeared around 1609, more than sixty years after Copernicus's death.

The Copernican revolution did not originate in a new set of observations made with a novel scientific instrument. It was an intellectual revolution inspired by changes in the way early modern scientists thought about the structure ofthe universe. In any case, the telescope alone cannot supply crucial evidence for the

Copernican system. A viewer cannot see either a heliocentric or geocentric universe through the eyepiece of a telescope. Observations made with the help of a telescope are like any other sets of observations. Astronomers gather their data and then interpret it within the framework of existing scientific theory and practice. This complex process ends with a majority accepting a given view of the workings of the universe.

Historians divide observational astronomy into three periods. The first period, the era of naked-eye astronomy, dates from the earliest human observation of the skies and ends in 1609. This preoptical period included the work of Ptolemy (second century, a.d.) and Copernicus, two of the greatest figures in the history of astronomy. The second period began with Galileo's use of a telescope in 1609 to study the major heavenly bodies. Telescope makers devised new and more powerful instruments during the following three centuries, when optical telescopes ruled the astronomical sciences. Then, in 1931, Karl Jansky of Bell Telephone Laboratories detected radio signals coming from regions beyond the solar system. Jansky's discovery marks the beginning of the third period of observational astronomy, the era of the radio telescope. A radio telescope is essentially a large antenna, often shaped as a parabolic dish, used to detect, amplify, and analyze radio emissions from celestial sources. It is mounted so that it can be aimed at different portions of the sky.

Telescopes, both optical and radio, play an important part in the search for evidence of intelligent extraterrestrial life. However, astronomers must interpret the raw data collected with their instruments. The difficult process of interpretation yields ambiguous results that fuel scientific debates.

Galileo's telescopic observation of the Moon revealed large circular cavities on the lunar surface. Were these natural cavities of unknown origin, or did the inhabitants ofthe Moon build them? Late nineteenth-century telescopic observers ofMars saw a series of long dark lines on the surface of the planet. Were these lines generated by an observer's visual response to the natural Martian landscape, or were they evidence of a network of canals built by Martian engineers? Late twentieth-century radio telescope operators recorded periodic signals coming from outer space. Were these signals due to physical changes occurring in a distant celestial body, or were they coded messages from extraterrestrial beings in the universe?

In each of these cases, interpretation of the observational data depended upon the state of astronomical knowledge and current ideas about intelligent extraterrestrial life. Scientists can never escape the scientific, philosophical, and social assumptions that influence their best efforts to extract meaning from the observed world. In short, observations do not speak for themselves. Scientists shape their speech for them as they gain knowledge about the physical world.

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