An Example of Time Dilation

A spaceship is flying a distance of 5 light hours, for example from Earth to the dwarf planet Pluto. The speed can be regulated with the upper buttons. The applet demonstrates that the clock in the spaceship goes more slowly than the two clocks of the system in which Earth and Pluto are motionless.

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t' ... time indicated by the spaceship clock
t  ... time indicated by the clocks of the Earth-Pluto-system
v  ... speed of the spacecraft relatively to the system of Earth and Pluto
c  ... speed of light


  1. In a simplifying way there was assumed an inertial system in which Earth and Pluto are motionless; especially the motion around the Sun was neglected.
  2. According to an important result of the theory of relativity, an observer in the Earth-Pluto-system would see the spacecraft shortened in the direction of motion. This so-called Lorentz contraction was not taken into consideration in order to make it possible to read off the spaceship's clock.

Physics Applets

© Walter Fendt, November 15, 1997
Last modification: February 14, 2010