Why, when you look at the stars, you see the past

The speed of light is about 300,000 kilometers per second. In a nanosecond, it travels about 30 centimeters: you see the screen you can reach with your hand with a delay of a couple of nanoseconds. On Earth, that doesn't bother anyone - distances are too short for the delay to be noticeable.

In space, distances are different, and the delay becomes the main thing. Moonlight takes about 1.3 seconds to reach you. Sunlight takes 8 minutes 20 seconds: if the Sun went out right now, you wouldn't know for more than eight minutes, and all that time the sky would look completely normal.

Farther out, the gap grows. Sirius, the brightest star in the sky, appears as it was almost nine years ago. Polaris is estimated to be 320 to 430 light-years away - its exact distance is still debated. And the Andromeda Nebula, visible on a dark night without any telescope, sends us light that left 2.5 million years ago. The night sky isn't a snapshot of one moment, but a layer cake of different eras.

It's a beautiful phrase, which is why people repeat it. But it swaps one idea for another: if the light is old, the source must be gone. It's easy to check. The human eye can make out about 9,000 stars across the whole sky, and almost all are within a few thousand light-years. But stars live for millions of years - the most massive and short-lived ones - or billions, like the Sun. The chance that a particular star died in precisely the hundreds of years its light was traveling is tiny.

There are exceptions. Betelgeuse in the constellation Orion is a red supergiant, estimated to be 500 to 700 light-years away. It really will end its life in a supernova explosion, but exactly when is an open question: estimates range from the next few tens of thousands of years to much longer. If the explosion has already happened, we'll only find out centuries later. But that's one star, not the whole sky.