The tail star and others in this constellation had appeared as novae in the skies between 5 BC, but are no longer visible to today's stargazers. For example, during the period that pioneering Babylonians, Arab astrologers, and Greek astronomers described the stellar constellations, Scorpius (Scorpio to astrologers) still had the whiptail of a scorpion. When astronomers gaze into the night skies, they are observing a mixture of real time, the recent past, and ancient history. This means that light leaving the Andromeda galaxy 2.21 million years ago is just arriving at Earth, unless it was waylaid by reflecting celestial bodies or refracting debris. By comparison, the Milky Way galaxy is estimated to be about 150,000 light-years in diameter, and the distance to the Andromeda galaxy is approximately 2.21 million light-years. The distance from Earth to the next nearest star beyond our sun, Proxima Centauri, is approximately 4.24 light-years. A light-year equals 9.5 trillion kilometers or about 5.9 trillion miles. In diamond, with a rather high refractive index of 2.4, the speed of light is reduced to a relative crawl (125,000 kilometers per second), being about 60 percent less than its maximum speed in a vacuum.īecause of the enormous journeys that light travels in outer space between galaxies (see Figure 1) and within the Milky Way, the expanse between stars is measured not in kilometers, but rather light-years, the distance light would travel in a year. Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index of 1.3 see Figure 2) and 200,000 kilometers per second in glass (refractive index of 1.5).
When light traveling through the air enters a different medium, such as glass or water, the speed and wavelength of light are reduced (see Figure 2), although the frequency remains unaltered. How Fast Is the Speed of Light in Air and Water?
Thus, after light has traveled twice a given distance, the intensity drops by a factor of four. However, the intensity of light (and other electromagnetic radiation) is inversely proportional to the square of the distance traveled. This well-established scientific fact is not a product of the Atomic Age or even the Renaissance, but was originally promoted by the ancient Greek scholar, Euclid, somewhere around 350 BC in his landmark treatise Optica. Light traveling in a uniform substance, or medium, propagates in a straight line at a relatively constant speed, unless it is refracted, reflected, diffracted, or perturbed in some other manner. Not truly a constant, but rather the maximum speed in a vacuum, the speed of light in km, which is almost 300,000 kilometers per second, can be manipulated by changing media or with quantum interference. The speed of light, which scientists have thoroughly examined, is now expressed as a constant value denoted in equations by the symbol c.
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