The first telescopic observation of Mars was by Galileo Galilei in 1610.

And then, he had better telescopic, more fine grain data in which he could figure out that as the Earth is going around faster according to Kepler's Laws than Saturn, then we catch up with it.

Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei.

The sniper rifle itself could be based on a standard rifle (It's hard to define a sniper rifle by the fire modes as some latest designed sniper rifles are semi automatic fire, for example the M110) however, when fitted with a telescopic sight, it becomes a sniper rifle.