Most of us remember the first stanza of “Twinkle, Twinkle Little Star”:
Twinkle, twinkle, little star,
How I wonder what you are!
Up above the world so high,
Like a diamond in the sky!
But, how high above the world are the stars?
The nearest star to Earth is the sun, which is an average of approximately 92,955,807 miles (149,597,870 kilometers) from us. Dealing with numbers that large is rather cumbersome. So astronomers have given a name to this distance: They call it an “Astronomical Unit” (or “AU” for short). So, by definition, the Earth is (on average) 1 AU from the sun. AU’s are convenient if you’re interested in the distance between planets in our solar system. For example, while the Earth orbits the sun at 1.0 AU, the planet Saturn orbits the sun at 9.53 AU’s. So Saturn orbits 8.53 AU’s farther from the sun than the Earth does.
But when we discuss the distances to other stars we find that the AU numbers start getting rather large. For example, with the exception of the sun, the closest star to Earth is called “Proxima Centauri.” This star is 265,608 AU’s from Earth, which translates into 24,689,794,389,764 miles (or 39,734,372,462,400 kilometers). To avoid dealing with such huge numbers, astronomers use another unit of distance – a “light-year.” Sometimes people get confused by the word “year” and think “light-year” is a measure of time. No, it’s a measure of distance, specifically, the distance light travels in one year – a “light-year.” Light travels about 6 trillion miles (about 9 trillion kilometers) in one year. So a light-year is about 6 trillion miles (9 trillion kilometers). Proxima Centauri is about 4.2 light-years from Earth – that’s a lot more convenient than using 24,689,794,389,764 miles!
Since a light-year is the distance light travels in one year, and since Proxima Centauri is 4.2 light-years from Earth, that means that when you view Proxima Centauri through a telescope, the light you see was generated 4.2 years ago! Indeed, while space is the setting for many science fiction stories about the future, when we look at the stars at night we’re actually viewing light generated in the past.