Concepts explained
The Sun You See Is Eight Minutes Old
Flick a switch and the room lights at once, which is why light was long thought infinitely fast. It is not. Light arrives late.
laps the Earth seven and a half times a second. Inside a room there is no way to feel that. Galileo tried: two men on two hills with shuttered lanterns. He got nothing. Human reaction time swallowed it all.
The distance was simply too short. To solve this you had to leave the ground.
Jupiter's moon kept running late
In 1670s Paris a young astronomer spent years logging the moments when a moon of Jupiter slipped behind the planet. The predicted times and the observed ones kept parting company.
The parting had a rhythm. In the season when Earth drew away from Jupiter, the eclipses ran late; when Earth closed in, they ran early.
He went one better. He announced in advance that the next eclipse would run some ten minutes behind — and it did. Not an explanation bolted onto a result, but a call made first and met.
So the sky is entirely past tense
Eight minutes for the Sun; far longer for stars. Light that took centuries or millennia is commonplace. Some star you look up at tonight may have released that light before your country's last dynasty began.
That star may already be gone. We would not know yet. The news is still in transit.
The further you look, the deeper the past you see. A telescope is really an instrument for looking backwards in time. That one property lets astronomy do what no other science can: observe the past directly.
The question that remainsIf every message from the sky is a late message, which moment is the sky we call now?