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how do telescopes look back in time
No time machine here. Light simply takes time to travel: it needs eight minutes from the Sun, so you always see the Sun as it was.
The farther away, the older the light. The Moon is one second ago, the nearest star four years, and the Andromeda galaxy two and a half million.
A telescope is a light bucket. Webb's 6.5 meter mirror gathers faint, ancient light that your eye, with its tiny pupil, never could.
With it, Webb has seen galaxies whose light left them over thirteen billion years ago, when the universe was under three hundred million years old.
So a telescope shows the past: far means old, and a big mirror collects the faint light. The night sky is a mix of many different dates.
Every point of light you see tonight is a letter from the past, and the farther the sender, the older the letter.