What is a light year? How long is it? Why you might not be seeing stars in real time.

Humans have discovered powerful telescopes to gaze at the beautiful night sky, only to realize that distant objects in space are things of the past. The light of these distant objects takes a certain amount of time to reach Earth.

Take the Sun for example, it is the closest star to Earth and its light takes about 8.3 minutes to reach Earth and its inhabitants. This also means humans always see the sun as it was 8.3 minutes ago each time they look up.

This characteristic applies to stars, galaxies and any other illuminating celestial objects in space. Depending on their distance from Earth, the light exuded by these space objects takes equivalent time to reach humans.

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What is a light year?

While a year is a measure of time, a light year or light-year is a measure of distance. A light year is the distance light travels in one Earth year, according to NASA. 

The International Astronomical Union defines a light year as the distance traveled by light in one Julian year or 365.25 days in vacuum.

According to NASA, light travels in the interstellar space at 186,000 miles (300,000 kilometers) per second and 5.88 trillion miles (9.46 trillion kilometers) per year.

Earth is approximately 320 light years from the North Star, Polaris which means that it takes 320 years for the Polaris' light to reach Earth. 

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How fast is a light year in mph?

There’s no answer to this question. A light year is a measure of distance while mph (miles per hour) is a measure of speed. So, a light year cannot be measured in mph.

Here's what we know:

Light travels at a constant speed of 670,616,629 mph which means that light covers a speed of 670 million miles in an hour.

Light travels a distance of 5.88 trillion miles in one Earth year, according to NASA.