The speed of light is one of the most fundamental measurements in astronomy. Measured in miles per second or kilometers per second, the speed of light determines distance as well as time. The term light-year refers to the distance light travels in one year. If you look at something that is one light-year away, you are looking into the past; the light entering your eye at that moment actually left the object one year ago, so what you see the object as it existed one year ago.
While defining the speed of light was easy, determining its actual velocity was not, because the true speed of light was vastly greater than could be precisely measured with equipment of the day. With characteristic ambition, Galileo tried to determine the actual velocity of light by stationing an assistant on one hilltop with himself on another. They then flashed light from shuttered lanterns back and forth at each other. Knowing the distance from one hill to the other, and having measured the time between flashing his lantern and seeing his assistant's lantern, Galileo reasoned he could calculate how fast the light traveled to cover the distance. Unfortunately, the true light travel time between the two hills was only a few ten-thousandths of a second. One can imagine a frustrated Galileo standing on a windy hill with a lantern and a timepiece, trying unsuccessfully to measure this vanishingly small interval of time. Sixteenth-century German astronomer Johannes Kepler believed that the stars occupied a thin shell two miles thick, but he placed the distance to this shell only one tenth of a light-year away.
Danish astronomer Olaus Roemer, while observing the eclipse of Jupiter's moons by that planet, discovered that the predicted times for the eclipses were not agreeing with his observations. Roemer assumed the variation was caused by the time it took light to travel from Jupiter to the Earth, and set out to determine light's exact speed. Unlike the small scale of Galileo's hilltops, Roemer's calculations could incorporate the millions of miles that encompassed the known solar system. In 1676 he announced that light traveled 141,000 miles (about 227,000 km) in a single second; a number slightly lower than modern estimates. Roemer's announcement stunned scientists who were not accustomed to such measurements.
In 1785 English astronomer James Bradley observed a slight shift in position of the stars, which he thought might be due to parallax (parallax is the amount of observed shift in an object due to the Earth's motion). Knowing an object's parallax, it is possible to calculate the distance to it; however, this shift was too small to be due to parallax. In 1728 he deduced that the shift was actually due to the combination of the Earth's velocity around the Sun plus the speed of light. He determined that light traveled 10,000 times the rate of the Earth. The Earth covers 29.7 kilometers (18.5 miles) in a single second, resulting in a speed of light of 297,000 kilometers (185,000 miles) per second. In 1838 Friederich Bessel coined the term light-year when he used it to describe the distance between the Earth and the star 61 Cygni.
The first person to successfully measure light using a terrestrial method was the French physicist Armand Fizeau. In 1839, varied Galileo's hilltop method by placing a light source and a spinning toothed disc on one hilltop and setting a mirror on another five miles (8 km) away. Light aimed at the mirror passed through one gap in the disc, and the reflection made it back through another. Based on the speed of the revolution of the wheel, it was possible to determine the time light took to travel 10 miles (16 km). Unlike other calculations up to this point, Fizeau's result was 5% too high.
French astronomer Jéan Foucault, worked independently of Fizeau, but with a similar idea. Working entirely in the confines of his laboratory, Foucault used a rotating mirror to deflect a beam of light, and, while the beam traveled only 65 feet (20 m), he had came to within 1 percent of the actual number by 1862. He also measured the speed of light through water and other media, showing that the speed of light decreases in water. This was compelling evidence that light moved as waves, not particles as some scientists theorized.
American physicist Albert Abraham Michelson adapted Foucault's method beginning in the late 1800s. He reflected light from a rotating eight-sided mirror on Mt. Wilson to a stationary mirror 22 miles (35 km) away on Mt. San Antonio. Calculations based on the distance, surveyed to within accuracy of one centimeter (one-third of an inch), resulted in a speed of 186,243 miles (299,729 km) per second. Using modern methods, the speed of light in a vacuum has been clocked at 186,282.397 miles (299,792.458 km) per second. At that speed, a particle of light could circumnavigate the Earth's equator about seven times in a single second.
The constancy of the speed of light in a vacuum, regardless of the motion of the source generating the light, is one of the fundamental underpinnings of our present understanding of the Universe. Albert Einstein used this fact to derive the essential aspects of his Special Theory of Relativity, and his theoetical results have been proved correct by repeated experiments.
This is the complete article, containing 880 words
(approx. 3 pages at 300 words per page).
