The gradual loss of Betelgeuse brightness although not a surprise has been reported as a possible sign that the star may explode.
If it does indeed destruct and we see the celestial supernova …when did it actually happen?
Since that star is 642 light years away, I’d say it happened sometime around 1377CE. So we can rule out the “Bethlehem” star.
Everything we see in the sky happened some time in the past, whether 1.25 seconds for anything on the moon to billions of years for distant quasars. But it’s not something to worry about. As far as we’re concerned, they happen when we see them and not a millisecond earlier. There’s occasionally times when it does make a difference, but generally you can just ignore the fact that they happened some time in the past. Astronomers do.
It’s all relative. If it became apparently a supernova to us tomorrow we could say that it actually happened about 640 years ago.
But the photons that left the star then do not experience either time or distance. So from their point of view it just happened, and there is no time.
Another thread, with a somewhat more obscure title, on the topic.
Obscure?!? Metaphorical, maybe, but obscure? 
It’s considered possible that Betelgeuse will explode in a supernova soon, but this “soon” is meant by astronomical standards; it can still be tens of thousands of years away and be considered “soon” by astronomers. So I wouldn’t bet on us getting to see this supernova in our lifetimes.
But then, it’s possible - as others have said - that it occurred already, within the last 640 years, and we simply haven’t found out yet because the photons haven’t reached us.
I don’t think you’ve thought this through.
The “Bethlehem star”, if it happened at all, was something that was seen on Earth circa 0 CE. If it was a supernova, then it must have actually occurred significantly earlier than that.
When they say soon, they mean we will be able to see it soon. Astronomers typically don’t say such and such happened in the distant past and we just saw it today, they say today such and such happened. Of course it happened in the past, that is generally understood.
And for what it’s worth, the best estimate I saw (before these recent observations) was that we’d see Betelgeuse go “sometime in the next 1000 years”. So yes, there’s a pretty good chance that it’s already happened and we’re just waiting to get the news.
Is it really necessary or appropriate to say that an even we see on Betelgeuse today actually happened centuries ago? I thought Einstein’s perspective ended the notion of absolute time.
Yes, it is necessary and appropriate. It’s a common misconception that relativity means things happen when they’re observed. You still take into account light travel time.
Anyone else upon reading the thread title think this was a misplaced and misspelled thread about the POTUS candidate from South Bend, IN?
No.
But it would be beyond cool if it did!
The Crab Nebula, at its height, was visible during the day. 
I suspect that septimus was just pointing out that there is no absolute time.
That statements like “We think the supernova exploded today, but really it happened 20 million years ago” or whatever are laboring under a misconception, since “really” is also just picking a frame of reference.
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You can’t pick a frame of reference in which causality is violated. Since causality travels at c, there is a limit to what you can achieve. You can’t pick a frame of reference that makes the time difference between here and Betelgeuse less than 642 years. You might be able to construct a non-inertial frame that makes it look longer to an observer in that frame.
If an observer at Betelgeuse was watching the Earth as Betelgeuse went nova, he would see the Earth as it was 642 years before information of the nova arrived at the Earth. Since 642 years is the minimum, it makes sense to go with that.
I didn’t claim that there was. I am just suggesting (or rather, supporting what I think septimus was getting at), that there is no absolute time. It is misleading to talk about what time an event “really” happened because all we can ever do is pick a reference frame and say when the event took place in that frame.
I understand why it is convenient for astronomers to usually assume the reference frame of the distant object, watching the light travel X light years away, there’s no issue with this.
It’s only in cases like the OP where it is necessary to point out the distinction between doing this and saying when an event “really” happened.
It becomes more obvious when we examine accelerating reference frames because in that case the apparent order of events, or simultaneity of events, is not even preserved between different observers, and it becomes more obvious that there can be no universal clock.
You can’t pick a frame of reference in which causality is violated, certainly. But that’s a very different statement from “You can’t pick a frame of reference that makes the time difference between here and Betelgeuse less than 642 years”, which I think is incorrect, and that you can pick non accelerating or accelerating reference frames that do make the difference much less than 642 years. And, no, I don’t mean failing to account for the light travel time to observe the events.
I think in any space time diagram there are world lines that, among other things, present limits to how far reference frame choice can be shifted. I think if you create such a diagram you could use it to test how far you could shift the time relationship between here and Betelgeuse.