How do astronomers determine the distance between Earth and Mars at different times in their Orbits?
Using LIght Detection And Ranging (LIDAR), Mars surface points are illuminated. Using reflectors in telescope that flashes light, the faint rebound signals are detected to determine distance.
(To and fro time /2 ) / ( velocity of light ) = distance
This method fails when the space object is a source light that is flashes.
So, parallax method is used for stars.