Question

What is light year? express in terms of SI unit.

Answer 1

Hint: The current version of the metric system is the International System of Units. It is the only method of measurement recognised by virtually every country on the planet. It is made up of a logical system of measuring units that starts with seven basic units. Since 2019, all SI units' magnitudes have been specified by stating exact numerical values for seven defining constants when stated in SI units.

Complete answer:
The light-year is a measure of length used to express astronomical distances. It is approximately 9.46 trillion kilometres ($ 9.46\times {{10}^{12}}~km $) or 5.88 trillion miles ($ 5.88\times {{10}^{12}}~mi $) long. A light-year is the distance that light travels in vacuum in one Julian year, according to the International Astronomical Union (365.25 days). The phrase "light-year" is frequently misunderstood as a measure of time since it contains the word "year."
When describing distances to stars and other distances on a galactic scale, the light-year is most commonly employed, especially in non-specialist contexts and popular scientific publications.
Distances between stars in the same general region, such as those belonging to the same spiral arm or globular cluster, are measured in light-years. Galaxies range in size from a few thousand to a few hundred thousand light-years in diameter, and are separated by millions of light-years from neighbouring galaxies and galaxy clusters. The distances between quasars and the Sloan Great Wall are measured in billions of light-years.
The entire distance that light will travel in a year is described as a light year. Because light travels at around 300,000 kilometres per second, we may use dimensional analysis to estimate how many kilometres light travels in a year.
 $ \dfrac{\text{3 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{5}}}\text{ }\!\!~\!\!\text{ km}}{\text{s}}\text{ }\!\!\times\!\!\text{ }\dfrac{\text{60s}}{\text{min}}\text{ }\!\!\times\!\!\text{ }\dfrac{\text{60min}}{\text{hour}}\text{ }\!\!\times\!\!\text{ }\dfrac{\text{24hours}}{\text{day}}\text{ }\!\!\times\!\!\text{ }\dfrac{\text{365days}}{\text{year}} $
Hence we get
 $ \text{1 }\!\!~\!\!\text{ light year }\approx \text{9}\text{.5 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{12}}}\text{ }\!\!~\!\!\text{ km} $

Note:
The metre is the International System of Units' basic unit of length (SI). m is the symbol for the SI unit of mass. The metre was first defined in 1793 as one ten-millionth of the distance in a great circle from the equator to the North Pole, implying that the Earth's circumference is roughly 40000 km. In 1799, a prototype metre bar was used to redefine the metre. The metre was redefined in 1960 in terms of a certain number of wavelengths of a krypton-86 emission line. The current definition was approved in 1983 and somewhat changed in 2002 to make it clear that the metre is a unit of appropriate length.