 During these 9 years, the clock of the traveling twin in the screen seems to advance 3 years, so both twins see the image of their sibling aging at a rate only their own rate.Expressed in other way, they would both see the other's clock run at their own clock speed.If they factor out of the calculation the fact that the light-time delay of the transmission is increasing at a rate of 0.8 seconds per second, "both" can work out that the other twin is aging slower, at 60% rate.