<p>We express when events are going to happen (or have happened) in <strong><em>our</em> observed time frame.</strong> </p>

<p>For example, we say <a href="http://en.wikipedia.org/wiki/SN_185">SN 185</a> went supernova in the year AD 185; but it didn't. The star actually <em>physically</em> exploded about 9,100 years earlier. </p>

<p>When we say the sun will set tonight at 8:07 PM ET, that is when we observe the event. But our <em>actual</em> position relative to the tangent from the sun will have already passed about 8 minutes 20 seconds earlier (plus several minutes due to <a href="http://en.wikipedia.org/wiki/Atmospheric_refraction">atmospheric refraction</a>).</p>

<p>So when you say Betelgeuse is predicted to explode sometime in the next million years, that's how many more Earth years are expected to pass until we <strong><em>see</em></strong> it happen&hellip; from <em>here.</em> From Betelgeuse's point of view, it would have happened about 642 years earlier&hellip; or it may have happened already.</p>

CC BY-SA 3.0
2014-05-19T17:29:32.187Z
2014-06-26T15:21:10.833Z
2014-06-26T15:21:10.833Z
Robert Cartaino
<p>I was scrolling idly through <a href="http://en.m.wikipedia.org/wiki/Orion_%28constellation%29">the Wikipedia article on Orion</a> when I read:</p>

<blockquote>
  <p>Orion will still be recognizable long after most of the other constellations—composed of relatively nearby stars—have distorted into new configurations, with the exception of a few of its stars eventually exploding as supernovae, for example Betelgeuse, which is predicted to explode sometime in the next million years.</p>
</blockquote>

<p>Given that Betelgeuse is only 640 light-years away from Earth, the question of whether we're talking about Betelgeuse going supernova in the next million years, or about <em>news of the supernova reaching us</em> in the next million years, is irrelevant, since 640 years is hardly noticeable when you're talking about ten thousand centuries. </p>

<p>But what about a star further away? Say, something in the Andromeda Galaxy, 2.5 million light-years from us? If I read an article stating that a star in the Andromeda Galaxy was going nova in a million years, would that mean:</p>

<p>a) That we think the star will go nova one million years from now, and its light will reach us in 3.5 million years; or<br>
b) That we believe the star went nova 1.5 million years ago, and its light will reach us in one million years </p>

<p>In other words, are we making estimates in our time, or in celestial-body-local time?</p>

CC BY-SA 3.0
2014-12-21T04:06:57.393Z
It actually makes a lot of sense to measure (and quote) in our local time, from where observations are made, because you can only give predictions based on measurable data (information) we have about them. And since information about them travels towards us at the fastest with the speed of light in vacuum (for actual light on average a tiny fraction slower, depending on the medium density, but that doesn't slow down, say, gravitational waves or information about their remote-local time frame events such as transits), we can only quantify probability of events in our own local time frame, too.
TildalWave
CC BY-SA 3.0
2014-12-16T14:02:52.480Z
We have an equivalent question on Space: http://space.stackexchange.com/q/5875/38
Rory Alsop
CC BY-SA 3.0
2014-05-19T13:57:55.737Z
Probably our time.
Cheeku
<distances>
Do we make predictions in our time, or local time?
hairboat
# Do we make predictions in our time, or local time?

(Asked by: username_0 on 2014-05-18)

I was scrolling idly through [the Wikipedia article on Orion](http://en.m.wikipedia.org/wiki/Orion_%28constellation%29) when I read:

> Orion will still be recognizable long after most of the other constellations—composed of relatively nearby stars—have distorted into new configurations, with the exception of a few of its stars eventually exploding as supernovae, for example Betelgeuse, which is predicted to explode sometime in the next million years.

Given that Betelgeuse is only 640 light-years away from Earth, the question of whether we're talking about Betelgeuse going supernova in the next million years, or about *news of the supernova reaching us* in the next million years, is irrelevant, since 640 years is hardly noticeable when you're talking about ten thousand centuries.

But what about a star further away? Say, something in the Andromeda Galaxy, 2.5 million light-years from us? If I read an article stating that a star in the Andromeda Galaxy was going nova in a million years, would that mean:

a) That we think the star will go nova one million years from now, and its light will reach us in 3.5 million years; or  
b) That we believe the star went nova 1.5 million years ago, and its light will reach us in one million years

In other words, are we making estimates in our time, or in celestial-body-local time?

**Tags:** distances

> **Comments:**
> **username_3**: Probably our time.  
> **username_2**: We have an equivalent question on Space: http://space.stackexchange.com/q/5875/38  
> **username_1**: It actually makes a lot of sense to measure (and quote) in our local time, from where observations are made, because you can only give predictions based on measurable data (information) we have about them. And since information about them travels towards us at the fastest with the speed of light in vacuum (for actual light on average a tiny fraction slower, depending on the medium density, but that doesn't slow down, say, gravitational waves or information about their remote-local time frame events such as transits), we can only quantify probability of events in our own local time frame, too.  

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## 1 answers

### Answer by username_4 on 2014-05-19. Score: 8

We express when events are going to happen (or have happened) in ***our* observed time frame.**

For example, we say [SN 185](http://en.wikipedia.org/wiki/SN_185) went supernova in the year AD 185; but it didn't. The star actually *physically* exploded about 9,100 years earlier.

When we say the sun will set tonight at 8:07 PM ET, that is when we observe the event. But our *actual* position relative to the tangent from the sun will have already passed about 8 minutes 20 seconds earlier (plus several minutes due to [atmospheric refraction](http://en.wikipedia.org/wiki/Atmospheric_refraction)).

So when you say Betelgeuse is predicted to explode sometime in the next million years, that's how many more Earth years are expected to pass until we ***see*** it happen… from *here.* From Betelgeuse's point of view, it would have happened about 642 years earlier… or it may have happened already.


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