RelSim

Special relativity round-trip simulator (WebGL)
Units: c = 1 (light-years / years)
Earth
Asteroid
Earth Observer
Earth time (local)
0.00y
Ship time (observed)
0.00 y
Spaceship Observer
Ship time (local)
0.00y
Earth time (observed)
0.00 y
Observer position
The ghost ship’s Terrell–Penrose distortion is computed for this observer. On Earth the motion is radial (longitudinal stretch); at the midpoint it is transverse (rotational distortion). The clock readings stay Earth-frame references.
Earth trip time: 22.22 y
Ship proper time: 9.69 y
Current t (Earth frame): 0.00 y
Current ship τ: 0.00 y
Ship position: 0.00 ly
Leg: outbound
1.0x
The four clocks show proper times and light-signal observations (signals travel at c).
“Earth time (local)” is Earth’s coordinate time t simultaneous with the ship’s true position.
“Ship time (observed)” from Earth is the ship’s proper time at the retarded (light-delayed) position.
The ghost ship is the same ship as the real one. Its light is Doppler-shifted (dims toward black while receding; runs red›orange›green›blue while approaching) and its shape uses the Terrell–Penrose photographed form by default (uncheck for textbook Lorentz 1/γ). Pick the observer with the radio below: on Earth the distortion is longitudinal; at the midpoint the ship appears rotated (transverse Terrell effect). Clocks stay Earth-frame references. Nothing is clamped — at high speed it can shrink to a sliver or blaze out white.