Light always moves at the same speed

Everyone measures it at about 300,000 km per second, however fast they move.

Build a clock out of light

Light bounces between two mirrors. Each round trip is one tick.

Now let the clock move past you

From the ground, the light takes a longer, slanted path between the mirrors.

Same speed, longer path: slower ticks

Tiny at everyday speeds, yet GPS satellites must correct for it.

Why moving clocks run slow