Perhaps the most common measure of space is the light-year.
Strangely enough, a light-year can measure both time and space.
Many people believe that time and space are inseparable, simply two sides of the same coin.
It takes time for light to travel. Light’s unvarying speed in space is about 186,000 miles (297,600 km) per second.
That’s 5,800,000,000,000 (5.8 trillion) miles (9.28 trillion km) per year.
So, a light-year, or the distance light travels in a year, is about 5.8 trillion miles (9.28 trillion km).
In light-years, the distance between the star Alpha Centauri and the Sun is about 4.2, which is much easier than saying 23,780,000,000,000 miles, though it means the same thing.
Since the light from Alpha Centauri takes 4.2 years to reach Earth, we are looking at light that is 4.2 years old.
We won’t be able to see the light Alpha Centauri is emitting today until it reaches us 4 years later.
