Except for rock samples brought back from the Moon by Apollo astronauts, cosmic ray particles that reach the atmosphere, and meteorites that fall to Earth, the only information about objects in space comes to Earth in the form of electromagnetic radiation. How astronomers collect this radiation determines what they learn from it. The most basic collector is the human eye. The retina at the back of the eye is covered with tiny antennae, called rods and cones, that resonate with incoming light. Resonance with visible electromagnetic radiation stimulates nerve endings, which send messages to the brain that are interpreted as visual images. Cones in the retina are sensitive to the colors of the visible spectrum, while the rods are most sensitive to black and white.

Until the early 1600s, astronomers had only their eyes and a collection of geometric devices to observe the universe and measure locations of stellar objects. They concentrated on the movements of planets and transient objects such as comets and meteors. However, when Galileo Galilei used the newly invented telescope to study the Moon, planets, and the Sun, our know-ledge of the universe changed dramatically. He was able to observe moons circling Jupiter, craters on the Moon, phases of

Venus, and spots on the Sun. Note: Galileo did his solar observations by projecting light through his telescope on to a white surface--a technique that is very effective even today. Never look at the Sun directly with a telescope!

Galileo's telescope and all optical telescopes that have been constructed since are collectors of electromagnetic radiation. The objective or front lens of Galileo's telescope was only a few centimeters in diameter. Light rays falling on that lens were bent and concentrated into a narrow beam that emerged through a second lens, entered his eye, and landed on his retina. The lens diameter was much larger than the diameter of the pupil of Galileo's eye, so it collected much more light than Galileo's unaided eye could gather. The telescope's lenses magnified the images of distant objects three times.

Since Galileo's time, many huge telescopes have been constructed. Most have employed big mirrors as the light collector. The bigger the mirror or lens, the more light could be gathered and the fainter the source that the astronomer can detect. The famous 5-meter-diameter Hale Telescope on Mt. Palomar is able to gather 640,000 times the amount of light a typical eye could receive. The amount of light one telescope receives compared to the human eye is its light

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