 The vertical black line is the Earth's path through spacetime and the other two sides of the triangle show the ship's path through spacetime (as in the Minkowski diagram above).As far as the sender is concerned, he transmits these at equal intervals (say, once an hour) according to his own clock; but according to the clock of the twin receiving these signals, they are not being received at equal intervals.After the ship has reached its cruising speed of 0.8"c", each twin would see 1 second pass in the received image of the other twin for every 3 seconds of his own time.