Part of the show Stars, Cosmology and the Beginning of the Universe
Brian in Singapore asked:
How do scientists check the distances between planets, and how accurate are their methods?
In recent times, the methods have been very accurate because we've sent spacecraft to various planets and can do timing measurements. This is very accurate. In the old days, people had to do incredibly painstaking measurements to see if things were moving a tiny bit. What surprises many people is that Pluto was only discovered in the 1930s, so it's only fairly recently that we've had any idea about the outer reaches of our own solar system.