How to Calculate Simultaneity

In the biggest ideas of the universe, Proxima Centauri is four light years from Earth. That corresponds to an eight-year span of events that could be considered simultaneous with a single event on Earth. In relativity, you can pick reference frames that stretch in any horizontal seeming line as long as it's not tilted more than 45 degrees. Going through the Earth, going through the center of your axes, that would be the reference frame constructed by someone going through the Earth right now.