The time it takes light to travel from a source to an observer. Light travels at the finite speed of 300,000 kilometers per second. The Sun is so distant that light takes 8.3 minutes to travel from it to Earth. Hence, the Sun has a lookback time of 8.3 minutes; we see it as it was 8.3 minutes ago. The Andromeda Galaxy has a lookback time of 2 million years; we see it as it was 2 million years ago
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