Question

which astronomer spent 20 years plotting the positions of the planets

Answer 1

That was Tycho Brahe, and I thought it was actually more years than that.

Answer 2

Answer

Tycho Brahe

Explanation

Tycho Brahe who was born on 1546. He was an astronomer.Tycho Brahe spent 20 years in plotting the positions of the planets and he was among those astronomers who helped in giving the  heliocentric model of the universe..