On what terrestrial date were the two principal stars of the Alpha Centauri system last known to be closest to each other in space?
According to the orbit recently published by Belgian astronomer Dimitri Pourbaix, α1 and α2 Centauri were closest in space (at periastron) in early August 1955, and they will be again in June 2035. But the orbit of this famous double star is highly inclined to our view, so we don’t see the components closest on these dates.
In a telescope the stars are 10.0" apart in October 2005 and closing. After passing through a minimum separation of 4.0" in late 2015, they will widen to 10.4" in mid-2029 before closing to just 1.7" in November 2037.
All the above dates refer to the Alpha Centauri system as we see it. That’s customary in astronomy, because the light travel time from most objects is poorly known (if at all). In the case of this nearest star system, however, you can confidently subtract 4 years 5 months from any date if you want to find out when the event actually took place there.
— Roger W. Sinnott