Light-year Light-year The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres or 5.88 trillion miles. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time. The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in nonspecialist and popular science publications. The unit most commonly used in professional astrometry is the parsec
Only a Trillion only a trillion. Floyd C. Gale stated that Asimov "[a]lmost certainly had enormous fun writing this collection of articles. He staggers, frightens, amazes and, in two, fiendishly misleads his reader". Following the publication of "Only a Trillion", Asimov compiled several more science essays into the manuscript for another, similar book titled "Only a Light-Year", but that book was never published. Only a Trillion Only a Trillion is a collection of ten science essays and three scientific spoof articles by Isaac Asimov. It was the first collection of science essays published by Asimov. It was first published by Abelard-Schuman in 1957.