When Bessell made his observation of the distance in light-years to 61 Cygnus he was only able to do so because he knew the speed of light. This had been observed almost two centuries earlier in 1676 by Ole Roemer, a Danish astronomer.
Up until then it was questioned whether light even had a speed, as many natural philosophers believed its movement between points was instantaneous, or that it was so fast as to make no practical difference. Roemer didn’t set out to enter this debate, his discovery was an accident.
Roemer had been measuring the eclipses of the moons of Jupiter for the purpose of navigation. He found over the course of many years that the eclipses occurred later than predicted when Earth was farthest from Jupiter in its orbit and the eclipses came early when the planets were relatively close.
Roemer figured out that this could be because it took longer for light to reach Earth when the distance is greater and vice versa. Some rough calculations on Roemer’s data by Dutch scientists Christiaan Huygens produced a figure of 210,824 kilometers (131,000 miles) per second for light, not far from the true figure of 299,792 kilometers (186,000 miles) per second.