If astronauts could travel at v = 0.980c, we on Earth would say it takes (4.20/ 0.980) = 4.29years to reach Alpha Centauri, 4.20 light-years away. The astronauts disagree.
a) How much time passes on the astronauts ‘clocks?
b) What is the distance to Alpha Centauri as measured by the astronauts?