No doubt this question has been asked ad-nauseum. How does SRT define simultaneousness? Therefore how does SRT define relative simultaneousness? Depending on the answers I would like to ask how relative can simultaneousness [not tick rates] get and how would you calculate it? My reasoning: What I am trying to consider here is that we can readily accept that tick rates are relative to velocity. But do we consider how this relative tick rate effects an objects "NOW location rest frame" When we deem an object at Rest we also deem it to be existing it in it's own Now which according to SRT is relative. So when we look at another object from our perspective we deem the now of that other object as simultaneous with our Rest frame. How ever that Now of our rest frame extrapolated onto our moving object is not the NOW [being it's past or future] of the object but the Now of the RF.So when we swap Rest frames where does our new Rest Frame object exist in its own Now location? This location I assume can only be arrived at by knowing the time [past and future]separation between our objects I understand my question may be a little confusing but maybe someone out there can get the gist of what I am endeavoring to explore here. However first thing to do is clearly define what relative time is regards to the future and the past of both our objects I feel.