Imagine a spaceship traveling at 95% of the speed of light to a planet 9.5 light-years away. A stationary observer on
Earth would measure the journey time as distance divided by speed, or 9.5/0.95 = 10 years. The spaceship crewmembers, on the other hand, experience time dilation and thus perceive the trip as taking only 3.12 years. (The math here is a little more complicated, but we'll get to it later.) In other words, between leaving Earth and reaching their destination, the crewmembers age a little over three years, while 10 years have passed for people back on Earth.