That depends on where you are in the world. We measure space as a distance, with rulers and meter-sticks (or yardsticks). If time is a direction, just like forward or up, we should be able to measure a distance in time. But we measure time with clocks. What’s going on? Fortunately, we have a very nice way of measuring time with a meter-stick.
The secret is light. Light is a funny thing—it’s very shy. If you’re chasing a beam of light and you speed up to try to catch it, the light will speed up the exact same amount! No matter how fast you go, the light speeds up to stay ahead of you, so that the difference between your speeds is always the same. This is why we say the speed of light is constant: it is always going faster than you by a constant amount. Because light always goes this same speed more than you, you always know its speed (300,000,000 meters per second, in fact—whew!), so you can use its speed to figure out how to measure time.
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