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About these units
Mile (mi)
The mile, equal to 5,280 feet, is a legacy of a long history of measurement stretching back to Roman times. Its ancestor, the Roman "mille passus" (thousand paces), established a rough distance of 1,480 meters, which evolved through medieval and early modern Europe into various regional miles. The English statute mile was standardized in 1593 and survives today in the United States, the United Kingdom (for roadways), and a few other territories. Despite being a non-metric unit, the mile remains deeply embedded in cultural and infrastructural systems. Road networks in the US are laid out with mile markers; athletic events include the famed "one-mile run"; and many people intuitively judge driving distances in miles. Aviation and international travel sometimes reference statute miles, though nautical miles dominate in navigation. Its long history and familiarity keep the mile relevant even in a metric world.
Light Year (ly)
A light year is the distance that light travels in a vacuum in one Julian year, equal to approximately 9.4607 × 10¹⁵ meters. Despite its name, it is a unit of distance, not time. The light year arose from the need to express vast astronomical separations using a concept familiar to laypeople: the finite speed of light. By linking distance to time, it becomes easier to comprehend that observing distant stars is also an act of looking back in time. The light year is especially useful when describing the locations of nearby stars, nebulae, and star-forming regions. For example, Proxima Centauri lies roughly 4.24 light years away. Because light moves at about 300,000 km/s, even the nearest galactic objects are many trillions of kilometers distant, making ordinary units impractical. One of the light year's greatest strengths is how vividly it illustrates the immensity of space. When astronomers say a galaxy is millions of light years away, they convey not only a vast spatial separation but also the staggering temporal depth of the universe. The light year therefore plays a dual role as both a scientific measurement and an educational tool that grounds cosmic distances in a memorable framework.