What Is the Real-Time Position of the Sun in the Sky?

[sophiecentaur]

I do not have the vaguest idea what you are attempting to say but it sounds like utter nonsense. The only answer that matters: The sun is exactly where you see it in the sky, right now, end of story. It does not make sense to ask where it is *now*, because now is now and the only contact you have with the sun's light and therefore apparent position is through that 8 minute gap. Where it is *now* is really where it will appear 8 minutes in the future, defined by the distance to the sun and the speed of light - we can try to predict where it will appear, and we'll probably get it right, but then again the Earth might get knocked out of it's orbit and flung into interstellar space by a random passing relativistic black hole between now and then. Who knows.

"Where it is now" is really a meaningless concept. Where and when are concepts which are totally dependent upon the observer - where he is and how fast he's going. Relativity is alive and well and you can't get away from it, I'm afraid. There is no graph paper grid or master clock at work in space.

 

[A.T.]

sophiecentaur would you confirm if I understand you correctly:

So for 12h light travel time we would get an angle of 180° between the visible sun position and the stick that points to the 'actual' sun position. Correct?

That means, when we would see the Sun directly overhead, the stick would point directly into the ground, indicating that the sun is 'actually' on the other side of the planet. Is that what you are saying?

If you agree with the above, then please explain the following: If the sun is on the other side of the planet how can we receive its light directly from above?

What happens if we suddenly stop the Earth's rotation in that very moment, with the visible Sun directly above us and the stick pointing into the ground? Would the visible Sun suddenly disappear?

Of course not. It would appear to set in the normal manner (as the last twelve hours worth of light arrived at us from the positions it had occupied during the last twelve hours)

The light that reaches the planet can only have occupied positions on the line between the sun and planet. But we are on the sun-opposite side of the now non-rotating planet. How can the light on this line reach us in the next few hours, so we can see the sun set in the normal manner?

 

[sophiecentaur]

sophiecentaur would you confirm if I understand you correctly:

If you agree with the above, then please explain the following: If the sun is on the other side of the planet how can we receive it's light directly from above?

Because we are receiving the light that it emitted when it was under out feet - i.e. twelve hours ago. It sent out that burst of light when we were on the other side of the Earth and arrived on the day side just in time to see it arrive from overhead.

You are not responding to my laser statements. Do you see them as being totally irrelevant?

 

[A.T.]

If the sun is on the other side of the planet how can we receive its light directly from above?

Because we are receiving the light that it emitted when it was under out feet - i.e. twelve hours ago.

No, it is now under our feet (the stick points into the ground). How can we receive its light directly from above? How did the light get to the sun-opposite side of the planet? Did it make a U-turn somewhere?

 

[sophiecentaur]

Thank you for that post, A.T.. It has straightened me out. It's never bad being not right if you learn something on the way.

No, it is now under our feet (the stick points into the ground). How can we receive its light directly from above? How did the light get to the sun-opposite side of the planet? Did it make a U-turn somewhere?

You have got to be right about that, of course. The shadow of the Earth is always in the same place in our simple model and we just enter and exit it as we rotate. We can't make the Sun rise or set any earlier or later angle.

I have just cooked us mushroom and bacon omelettes and enjoyed them. During the process, I have been doing some thinking. (Brain food I think) You are making more and more sense in what you say.

Talking of shadows, I have another model to consider. Imagine there is a massive planet which is placed, statically, between the Sun and Earth. No amount of rotating the Earth can make light from the Sun get to the Earth so their two apparent relative positions can't be changing.

I was having a problem separating those observations that are affected by c and those that aren't. My laser experiment prediction is clearly right but the situation 'the other way round' is not as simple to arrange. Two (stationary) lamps at different distances would be seen to flash at the same time and in the same direction only if they were to lie on the 'spacetime c spiral' but they would still have to lie along a straight radial line from Earth. The lamps would need to flash at different points in the Earth's rotation time in order that the flashes should appear to be at the same time. Any other timings would make their flashes appear at different places in the sky even if they did lie on a radial line. That's the department of the bleedin' obvious to me now.
BUTTTTT, of course, stars do not flash. They are on all the time so, at any observation instant, you will see them lined up. All the angles will look right. It's just that the light that arrives is from different times in their history.

Looking at the positions of moving objects in the sky will not tell you their 'current' positions, because of the transit time of light from them and the Earth's rotation won't affect where you see them - just the apparent position at which they 'do something, like blowing up.

I'm really glad you didn't go off in disgust! Cheers

 

[Drakkith]

Perhaps we are confusing a "pulse" of light with a "stream" of light.

 

[sophiecentaur]

Exactly. Streams of light will appear to come from the 'correct' relative directions - moving across the sky at the same rate. If you have pulses (events), measurements of their relative positions can be distorted according to the relative time delays and the rotation speed.

 

[Drakkith]

Now does this account ONLY for the rotation of the Earth, or does it include it's orbital motion as well?

 

[sophiecentaur]

The latter part of this thread has only dealt with a stationary situation with a rotating Earth - to get the basics sorted out. If you are considering an orbiting Earth and include the 'equation of time' factor (due to the elliptical orbit) then the time of Noon will vary throughout the year, in any case.
But I'm not sure exactly what you are getting at.?

 

[Drakkith]

If we had a non-rotating asteroid, would it's orbital motion mean that the sun is very very slightly ahead of itself since the light that is hitting it was emitted when it was thousands of miles back in it's orbit?

 

[sophiecentaur]

Not sure what you mean - you have put it a funny way. But its apparent (from Earth) orbital position would need to be corrected for the transit time. When it is nearest the Sun, it could be going very fast and the 8 minute delay (at that distance) could correspond to a very large positional error.
I don't know what you mean about its non-rotation. Do you mean the apparent position of the Sun for an observer on the asteroid would be affected because of its speed and the delay?

 

[Drakkith]

Not sure what you mean - you have put it a funny way. But its apparent (from Earth) orbital position would need to be corrected for the transit time. When it is nearest the Sun, it could be going very fast and the 8 minute delay (at that distance) could correspond to a very large positional error.
I don't know what you mean about its non-rotation. Do you mean the apparent position of the Sun for an observer on the asteroid would be affected because of its speed and the delay?

Nevermind, I think I am just confusing myself lol.

 

[sophiecentaur]

Talking of confusion, you should read some of the rubbish I wrote early on.
No. Don't

 

[D H]

Suppose that the sun emitted two types of light, the 8-minute light and a spooky “zero-time light”. Where in the sky would we see/measure the second spooky sun - the real position of the sun?

In short, "Suppose we allow something to violate the laws of physics. What do the laws of physics say would happen?"

This question cannot be answered in terms of relativity because in terms of relativity it is a nonsense question. It can be, and has been, answered in terms of a Newtonian universe. Here is the answer:

The only thing that causes an offset between the visible and real direction, is the component of relative velocity (between source & detector) that is perpendicular to the real direction. The dominating effect here is the orbital velocity of the Earth, but the offset from it is very small probably not visible with the naked eye. The rotation of the Earth plays a tiny role, and that only if you are not one of the poles, so you have some tangential velocity from that rotation.

 

[A.T.]

If we had a non-rotating asteroid, would it's orbital motion mean that the sun is very very slightly ahead of itself since the light that is hitting it was emitted when it was thousands of miles back in it's orbit?

Yes, there would be an offset due to relative motion perpendicular to the real line of sight. The reason depends on the reference frame. In the frame of the source where the detector moves you blame aberration of light. In the detector frame where the source moves you blame signal delay.

 

[A.T.]

In short, "Suppose we allow something to violate the laws of physics. What do the laws of physics say would happen?"

This question cannot be answered in terms of relativity because in terms of relativity it is a nonsense question.

Don't be so harsh. He just asks about the visual impression it would create, and is not trying to draw any further consequences from FTL. It doesn't violate relativity to know where something actually is if it moves in a predictable manner.

 

[Duplex]

In short, "Suppose we allow something to violate the laws of physics. What do the laws of physics say would happen?"

My intention was not to violate the laws of physics.
I am a law-abiding person.

But I see what you mean.

My visual metaphor FTL was perhaps a bit unnecessary and confusing, because my question was just about the Sun's real-time position, not hypothetical FTL effects.