Light-year (ly)
The light-year is a unit of astronomical distance equal to the distance that light travels in one Julian year - approximately 9.461 trillion kilometers or 5.879 trillion miles. Despite its name, the light-year measures distance, not time. It is the most commonly used unit for expressing distances between stars and galaxies in popular science, news reporting, and science fiction.
Definition
One light-year equals exactly 9,460,730,472,580,800 meters (approximately 9.461 trillion km or 5.879 trillion miles). It is defined as the distance light travels in one Julian year (365.25 days) in a vacuum. One light-year also equals approximately 63,241 astronomical units or 0.3066 parsecs. Professional astronomers generally prefer parsecs over light-years, but the light-year remains dominant in public communication because it is more intuitive.
History
The concept of using light's travel time to express astronomical distances emerged in the 19th century. German astronomer Friedrich Bessel made the first reliable measurement of stellar parallax in 1838, calculating the distance to the star 61 Cygni. The term "light-year" became common in popular astronomy writing by the mid-1800s. The International Astronomical Union (IAU) formally defined the light-year in 1984 based on the Julian year of exactly 365.25 days and the speed of light at exactly 299,792,458 meters per second, giving a precise value of 9,460,730,472,580,800 meters.
Common Uses
Popular astronomy uses light-years to express the distances to stars, nebulae, and galaxies. The nearest star to the Sun, Proxima Centauri, is about 4.24 light-years away. The Milky Way galaxy is roughly 100,000 light-years in diameter. The Andromeda Galaxy is approximately 2.5 million light-years from Earth. Science fiction frequently uses light-years to convey the vastness of space. News coverage of astronomical discoveries almost always uses light-years rather than parsecs or kilometers when communicating with the public.
Did You Know? Facts About Light-year
- Light from the Sun takes about 8 minutes 20 seconds to reach Earth - the Sun is roughly 0.0000158 light-years away.
- The farthest object visible to the naked eye is the Andromeda Galaxy at about 2.5 million light-years - the light you see left before Homo sapiens existed.
- The observable universe has a radius of about 46.5 billion light-years, even though the universe is only 13.8 billion years old, due to the expansion of space.
- If you could drive a car at highway speed (100 km/h) non-stop, it would take about 10.8 million years to travel one light-year.
- The Voyager 1 spacecraft, the most distant human-made object, has traveled about 0.0025 light-years from Earth since its launch in 1977.