Light has a finite speed, which sounds like a technical footnote. But if it left a star a thousand away, it reaches your eye a thousand years later. What does that make of what you are seeing right now?
You are seeing the past, always. A star that went out a century ago and sits a thousand light-years off will keep shining for you for centuries, because the light it already sent is still on its way. The same holds close to home: the Sun you see is the Sun of about eight minutes ago.
Everything here follows from one fact: the is finite. It is large, about 300,000 kilometers a second, and it is not endless, so all seeing is late seeing.
At everyday scale the delay is nothing. The face in front of you arrives a billionth of a second behind. At the scale of the solar system it already shows. The Sun you see is the Sun of eight minutes ago, and if it went out we would carry on seeing it for another eight.
At stellar distances the delay becomes the whole point. puts a number on the distance, and the distance is the delay. Light from a star a thousand light-years off set out a thousand years ago, and the star may well be gone. What is in the sky is not a state of affairs but a collection of different moments, each arriving from its own distance.
Hence the use of it. Nobody can travel backwards, and looking far enough does the same work, which is what makes a strong telescope a window on . The price is that the further out you reach, the fainter and more stretched the light, so reading it gets harder rather than easier.
wat. The Sun you see left about eight minutes ago, and a star a thousand light-years off may have died while its light was on the way.
wat wat
It is what makes the early universe observable at all. Nobody can travel back, and looking far enough is the same thing.
How do we know?
Laboratories measure the speed of light to extraordinary precision, and parallax gives stellar distances independently of it. Put the two together and the delay falls out.
What is still unknown
The further you look, the fainter the light and the more it is stretched by the expansion of the universe, so what arrives gets harder to read rather than easier.
Learn something?
Where to next
Is that column of light above the sun actually a beam?light
Why is a rainbow in fog white?light
Why do the colored rings follow your own shadow and nobody else’s?light