A light-year is a unit of measurement used in astronomy to measure vast distances in space. It is defined as the distance that light travels in one year in the vacuum of space. To understand how light-years are measured, let's break it down into steps:

 1. The Speed of Light: The first step is to understand the speed of light. In a vacuum, light travels at a constant speed of approximately 299,792 kilometers per second (or about 186,282 miles per second). This speed is denoted by the symbol "c" and is a fundamental constant in physics.

 2. Distance Calculation: To measure a light-year, we need to calculate the distance that light travels in one year. To do this, we multiply the speed of light by the number of seconds in a year. The number of seconds in a year is obtained by multiplying the number of seconds in a minute (60), minutes in an hour (60), hours in a day (24), and days in a year (365.25, accounting for leap years).

 3. Calculation Example: Let's calculate the distance in kilometers that light travels in one year. The equation would be:
 Distance = Speed of Light × Number of Seconds in a Year
 Distance = 299,792 km/s × (60 seconds/minute) × (60 minutes/hour) × (24 hours/day) × (365.25 days/year)

 4. The result of this calculation is approximately 9.461 trillion kilometers (or about 5.879 trillion miles). This is the distance that light travels in one year.

 5. Usage and References: Light-years are used to measure the vast distances between celestial objects, such as stars, galaxies, and other astronomical phenomena. For example, the nearest star to our solar system, Proxima Centauri, is located about 4.24 light-years away. This means that the light we currently see from Proxima Centauri actually left the star 4.24 years ago.

 References:

- NASA: https://imagine.gsfc.nasa.gov/ask_astro/index.html

- National Optical Astronomy Observatory: https://www.noao.edu/education/peppercorn/pcmain.html

 It is important to note that light-years are a unit of distance, not time. The term "year" in light-year refers to the time it takes light to travel that distance, rather than a measure of time itself.