from (1), we can have: vx = x/t

from (3), we can have vy = sqrt(1- vx*vx) = sqrt(1- (x/ t) * (x/t))

considering (2), after a series of transformation:

t=sqrt(x^2 + (y-ct)^2) or t = -sqrt(x^2 + (y-ct)^2)

It is can be inferred that the lowest positive value of t is location, (x,y), dependent. With x = 0, y = ct, t can be 0.