8 light minutes to the sun, so where is it now?

Or ignore the fact, for the time being, that the light waves propagate at all. Imagine that they are a stick stretching from the sun to the earth, and photons are ants crawling along the stick. It doesn't matter how long it takes the ants themselves to reach the earth, the stick always indicates the direction of the sun.

I don't think so. In my opinion, you would have sun beams radiating from the sun into every direction. Because the Earth rotates, the beams that arrive now hit the observer under a different angle than the beams that will arrive in 8 minutes. But this is true no matter what the speed of light is. Anyway, at every given time, an observer pointing at the sun will point in the actual direction of the sun. The direction of the observer's pointing will change only to offset the Earth's changing orientation.


I believe there is such a difference only because the Earth propagates around the sun during those 8 minutes. If you account for that, you get a mismatch of (360° * 8 minutes) / (365 days), which computes as something like 0.005°.

If you ignore that, and only account for the Earth's changing orientation, there will be no difference between the sun's actual position and its apparent position. At least that's how I see it.

-- Thomas

Very good illustration, patiodog! Wish I had come up with it myself!

Even revolution (though I've a history of being wrong hereabouts) shouldn't make a difference, in light of what Thomas is talking about. The incident angle of sunlight striking the earth is still indicative of the direction of the sun.

Still disregarding atmospheric effects, of course...

Patiodog --

I think revolution is different. If you assume the Earth weren't rotating, you would see not where the sun is, but where it apparently was 8 minutes ago. Unlike the previous example, the mismatch doesn't break down this time if you look at it from the sun's perspective. The sun, too, sees not where the Earth is, but where it was 8 minutes ago, so a photon from the sun fired at where the earth appears to be will miss by those 0.005 degrees.

I could be just as wrong as you though.

PatioDog, we perceive the direction of the Sun by the ants which arive at our location, not by the stick (which we cannot see). So it still seems to me that the Sun is not *actually* where it appears to be.

Here's how I see this:

Let's picture a point in time when the Sun is directly overhead. We'll call that TimeReference-A. In eight minutes the Sun will not be directly over head any more. It'll be some distance further along its apparent path across the sky. We'll call that TimeReference-B.

Putting aside for the moment how far the sun has apparently moved, we can still say that there is *some* change in apparent location between TR-A and TR-B.

We also know that the Sun was *actually* at location TR-B when we saw it at TR-A, Due to the travel time of light from Sun to Earth. Are we in agreement on this?

This being the case, if we were to fire a rocket (at infinite speed) directly to where we see the Sun at any given point in time. It will miss the Sun because we do not perceive it as being where it actually is.

Right? Or am I missing something here.

Thanks,

The mistake I think I was making was in imagining that the photon I see was actually aimed at my location eight minutes ago when it left the sun, but this is not the case. In a way, I (in my position on the earth) have moved to intercept a photon that was not aimed at me eight minutes previously. The photons are moving out in a straight line from the sun. We just happen to get in the way. (Perhaps this would be more obvious if the sun did not radiate light in all directions.)

Rosborne and patiodog seem like they are in agreement. I am pretty sure they are both correct.

The difference is that Rosborne is using the perspective of someone standing on a rotating Earth (that is about an axis). Patio is using the perspective of someone on the Sun (for the sake of this discussion not rotating).

The stick with ants is a good axample of photons going in a straight line. However assuming a person on the Earth could see the ants (but not the stick) these ants would "appear" to be going in the same circular motion that the sun "appears" to go.

Rosbornes post about the "apparent" deflection of the photons is both ingeneous and correct. Remember that light did leave the sun 8 minutes ago and the Earth did rotate about its axis 2 degrees during this time. If this is not clear think about what someone on Earth watching the ants would see as the Earth rotated under them.

This by the way is very quickly reaching into the ideas of General Relativity...

Rosborne, I believe the rocket at infinite speed *would* hit the Sun (neglecting the theory of relativity(which says this is impossible anyway)).

Your picture of the deflecting photons is correct. But you should go the additional step. The photon reaching eye is "apparently" coming from where the sun is "at that moment". So your infinitly fast rocket would be aimed in the correct direction.

Does this make sense?

P.S. If we are thinking correctly, the two models (Person on Earth centered, and Sun centered) should give the same results. The sun-centered model offers a straight path.

Any theory that doesn't meet this test has a problem.

Hi Ebrown,

Sorry, I disagree. I think that the photon reaching the eye is *apparently* coming from where the sun *was*, not *is*.

The photon reaching your eye is from the Sun 8 minutes ago, not the sun now.

Right?

Hi Rosborne,

I am not quite sure what we are disagreeing about. Here is how I see it. (Assuming the photon leaves the sun when the sun is directly overhead),

In Thomases point of view (which is perfectly correct). The photon travels in a straight line from the sun to the Earth. The fact that the Earth is rotating does not chenge this straight line.

When the photon reaches your eye, you be able to trace the path of the photon along a straight line to the sun. Of course the Earth will have rotated in this time and the Sun will know longer be directly overhead. But the straight line still exists and and still points directly to the Sun. Your inifinitely fast rocket can fly directly along this stright line back to the sun.

With your point of view, the photon started traveling along this line (think of the ants here). But as the photon (ant) travels toward you, you are spinning underneath it. The photon will appear to take a circular path for the same reason the sun does. (Of course this will be added to its path toward you).

As you said previously the photon's path will be curved (becase you are spinning under it).

When the photon hits your eye, it will appear to have curved 2 degrees (the same as the Sun). Your eye always assumes that light travels in a straight line.

You are right the the photon is eight minutes old. It was in the old location when it started. However it has moved 2 degrees in these eight minutes. It's new position corresponds to the new position of the Sun.

So the photon reaching the eye is certainly *apparently* coming from where the sun "is". It's path has been deflected 2 degrees from where the sun was 8 minutes ago. Precisely the same amount for precisely the same reason.

It was your thought experiment that convinces me that this is the correct analysis.

It also matches up with Thomases point of view, which as I suggested above, is a criteria for any "correct" theory.

Ebrown, bear with me here, this could be one of those cases in which Email is just tough to communicate with and we're actually taliking about the same thing, but not recognizing it yet.

For the sake of argument, let's increase the values here to make a more compelling visual example. Instead of an 8 minute travel time to the Sun, lets say that the Sun was 12 hours away at light speed (I'm splitting a 24 hour day just for convenience, and assuming equatorial and equinox values just to eliminate variations from the analogy).

And now let's say that we're watching sunrise in the East. But the light we are seeing left the sun 12 hours ago, so we know where the sun must really be: setting in the west.

Your first sentence: "Here is how I see it. (Assuming the photon leaves the sun when the sun is directly overhead)," isn't specific. It doesn't say whether the Sun is *actually* overhead, or is simply *perceived* overhead. And that difference in meaning matters a lot to the rest of the argument.

The fact is that we *perceive* the Sun as being in a different location that in *actually* is due to the travel time of light. If this were not true, then the same thing would not apply to other stars, and it does. Many of the stars we see at night do not even exist any more, nor are they *where* we see them if we were to shoot an "infinitely" fast rocket at them. All we are seeing are the echoes of where they were sometime in the past. The same applies to the Sun, only it's 8 minutes away, instead of 80 million years away.

The discussion turned out to be trickier than I had anticipated. Very fun

Thanks everyone

Best Regards,

The poet's eye, in a fine frenzy rolling,
Doth glance from heaven to earth, from earth to heaven;
And as imagination bodies forth
The forms of things unknown, the poet's pen
Turns them to shapes, and gives to airy nothing
A local habitation and a name.

Rosborne, I think you are getting to hung up with comparing the sun to the horizon. But, a picture is worth a thousand words, so here goes (I hope this comes out OK).

Here is a picture of Thomas' model.



The photon that is emited at point A is the one that leaves the sun at the right time and the right place. This photon doesn't count because you never see it. By the time it get's to you you have already moved 8 minutes worth.

The photon B is 8 minutes old by the time it reaches you. But this is the one you see. Note that it travels in a straight line. Note that the straight line gives travelled by photon 'B' leads directly to where the Sun is at the time you see it at point D. The photon will "appear" to be coming from the place the Sun currently is.

The amount of time it takes for the photon to travel during this time is irrelevent. The 8 minute old photon at point D is travelling in the same direction as the new photon at point B. If the B photion travelled infinitely fast, it would be travelling in the same direction as the D photon when it reached your eye.

This is the other model.



As the Sun rotates the photon apparently travels in a curved path (staying in line with the Sun). When this photon reaches you it will appear to have come from the "latest" position of the sun. This will correspond to where the sun is "currently". If you shoot your infinitely fast rocket in a direction corresponding to the latest position of the photon (as shown) it will follow the line I have drawn directly to the "current" position of the Sun.

Again the 8 minute old photon about to hit your eye is travelling in the same direction as the photon that is currently leaving the sun. If this "new" photon travelled infinitely fast along a straight line, it would have the same direction as the old photon when it reached your eye.


I hope this is clear... Note that both of these models give the same result.

Ah Ha! I get it You're right. The Sun isn't moving, it's only our angle of perception which is changing. And that information is transmitted to us almost instantly, as Thomas said.

So even though we do see the Sun as it was 8 minutes ago, its position doesn't change, only its "apparent" position relative to our perception changes.

Why didn't I see this before. It's so obvious now. Thanks everyone

For me, this ranks up there with Satt's thread on Abuzz regarding the 11 year solar cycle and the Sun's movement through the barycenter of the solar system: http://boston.abuzz.com/interaction/s.209484/discussion

I learned something there also.

Best Regards,

The amount of time it takes to see the light: 44 hours, 37 minutes
Cool puzzle!


BTW, what if the sun *is* moving sideways, relative to the center of the galaxy, at say 10 times the speed of the Earth's movement around the sun? Seems like the sun rays would be drifting, bent as the sun itself travels.

Would it be like two cars speeding along the freeway, but the image you see of the other car is 8 minutes old? Aiming your infinitely fast rocket, would you have to lead ahead, in the direction of the sun/Earth's motion through the galaxy or universe?

Tweaky.

I just flap away and keep my mouth shut.

http://www.pbase.com/image/18222271

Great. Now Codeborg wants to bring relativity into it. I'm not sure that makes any difference, though, as long as our path around the sun is determined strictly by out relationship with the sun. The earth is obviously in motion around the sun, but to an observer on the moon who disregards the sun, it's position would appear to be quite fixed. I don't think the sun's motion re: galactic central point (or anything else) matters unless it has an effect on the earths rotation and orbit -- and I think this effect is probably negligible. And doesn't this take us to "any point can be considered an inertial reference point"?

(but, again, i've been wrong before.)