Light Year
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  Measuring Distance with Light (Years):  


Though it’s called a “light year,” this unit of measurement determines distance, not time. While it can tell us how old a stellar object is, the scientific calculation is based on how far away the object is. Thus again, it’s all about distance.

Speed of Light
Light travels at a speed of 186,000 miles or 300,000 kilometers (km) per second. To figure out how far light travels in a calendar year, simply multiply out for seconds, minutes, hours, and days.

 The result is approximately 9,500,000,000,000 km or 9.5 trillion km.

Formula: 300,000 km • 60 seconds • 60 minutes • 24 hours • 365 days = 9,460,800,000,000 (1 light year)

Distance in Space
In astronomy, distances to stars, planets, and celestial bodies are such that miles and kilometers are no longer practical units of measurement. Even if a light year were written out as 9.5 trillion km, such notation would get problematic fast. Enter the light year.

The light year (and its light increments) enables astronomers to easily describe faraway distances. For example, Alpha Centauri is 4.37 light years away from the Sun (and us). The Sun is approximately 8 light minutes away from Earth. The Moon, which we can travel to, is only 1.3 light seconds (238,897 miles) away from our planet.

  Light Equals Age:
  In addition to measuring distance, light years also tell us about the age of an object. Since light travels at a known speed (9.5 trillion km in one year), we can determine how long it takes for an object’s light to reach us. For Alpha Centauri, we know that it is 4.37 light years away. That mean, it takes 4.37 years for its light to reach us. What we’re seeing is 4.37 years old.

Thus, for a star that’s 1.2 million light years away, it’s taken 1.2 million years for its light to reach us. With this in mind, it’s very possible that what we sometimes look at in the night sky – no longer exists.

All that remains is its twinkle..