# Astronomy Understanding how far a Light Year is

A light year is not a measure of time; it is a distance. It is used to measure how far distant objects like the stars are from the Earth. It was defined by the International Astronomical Union as the distance that light travels in a vacuum in one Julian year. A Julian year is 365.25 days. The symbol for the speed of light is “c.”  It is part of Einstein’s famous Theory of Relativity E=MC^2. In words, it is “Energy is equal to mass times the speed of light squared.”

Light travels at the speed of approximately 186,282 miles per second (or about 300,000 kilometers per second).  It is known to be the fastest speed that any object can go. For one Julian year, light travels 186282 miles/ 1 second X 60 seconds/1 minute X 60 minutes/1 hour X 24 hours/1day X 365.25 days/1 year. Everything cancels except for miles in the numerator and year in the denominator, so the distance light travels in one year is 5,878,612,843,200 miles.

In meters, light travels at the speed of 299,792,458 meters per second. This is often abbreviated as 3.0 x 10^8 meters per second.  This makes the light year in kilometers 299,792,458 meters/ 1 second X 60 seconds/ 1 minute X 60 minutes/ 1 hour X 24 hours/ 1 day X 365.25 days / 1 year. Everything cancels except for miles in the numerator and year in the denominator again, making the distance light travels in one year 9,460,730,472,580,800 meters.

The nanosecond is the distance light travels in one billionth of a second. It is used by radar to calculate how far away an object is. Light travels one foot (30 centimeters) in a billionth of a second. Since the radio waves used by radar travel at the speed of light, anything the waves bounce off of that returns to the station is calculated by recording the number of seconds it takes for it to return. This number of seconds is the number of feet when divided by two because the radio waves traveled from the station to the object and then back to the station, doubling the distance.

Measuring the distance to the stars is important in astronomy.  The star that is closest to the earth besides the Sun is Proxima Centauri, which is approximately 3.97 x 10^13 kilometers away.  It is often abbreviated as 4.2 light years away.  It is not visible without using a telescope.  Alpha Centauri is a little farther, about 4.36 light years distant. Barnard’s Star is approximately 5.96 light years away.

The brightest star Sirius is about 8.58 light years from the earth. The brightest star in Canis Minor named Procyon is approximately 11.40 light years away. It is the eighth brightest star in the sky not including the Sun. Groombridge is about 11.62 light years away. The twelfth brightest star not including the Sun named Altair is approximately 16.72 light years from the Earth. One of the farthest known stars from the earth is in the galaxy Abell 1835 IR1916. It is about 13.2 billion light years from the earth.