A clock made of light: up and back is one tick.

Two of them, side by side, tick together.

Send one past at 0.6 times light speed.

To keep up, its light has to zigzag.

Same light speed for everyone: the longer path takes longer.

So it ticks 4 times while ours ticks 5.

The faster it goes, the slower it ticks.

At 0.99 c: 14 ticks for every 100 of ours.

Ride along: its light just goes up and down.

From there, our clock is the slow one.

GPS satellites fly at 3.9 km/s.

Their clocks fall 7 µs behind each day.

That is 2 km of light. GPS corrects for it.

Time depends on how you move.