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Light-travel time calculator

From the Sun: 8 min 19 s · From the Moon: 1.28 s · From Proxima Centauri: 4.246 years

How light-travel time works

The speed of light in vacuum is a fixed 299,792,458 m/s. Distance divided by that speed gives you the lag. Sunlight reaches Earth in 8 minutes 19 seconds, which means the disc you see in the sky is the Sun as it was minutes ago. The Moon's reflected light arrives in 1.28 seconds.

Why it matters

Light-travel time is also a look back in time. When you observe Andromeda you see it as it was 2.5 million years ago. When NASA's Deep Space Network sends a command to a Mars rover, the round trip can take 20 minutes — which is why rovers run autonomously, not joystick-style.