# Thread: Is the sun where it seems?

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## Is the sun where it seems?

If the light from the sun doesn't hit the earth instantaniously, doesn't that imply that the sun isn't where I see it right now?

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Yes. In the 7 minutes the light took to reach you, the sun will have moved about 1.75 degrees across the sky (provided I did my math right)

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That means, technically you CAN look directly at the sun. However, you should never look at where the sun was a few minutes ago.

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I'll let you try that one first

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I think 1.75 degrees in the sky would still be too close to the bulk of the sun to make looking at it uncomfortable.

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Originally Posted by frenat
I think 1.75 degrees in the sky would still be too close to the bulk of the sun to make looking at it uncomfortable.
Is 1.75 degrees enough to occult the disc of the sun with your hand? That might be a fix.

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There's an error in that thinking as it's not the sun that moves across the sky, so the 7 light minutes distance don't come into play here. The sun is exactly at the spot where you see it.

8. you are still seeing a 7 minute old image, and the sun appears to move through the sky (we all know that it isn't actually moving through the sky), so you are seeing where the sun was in the sky 7 minutes ago.

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Aim a perfectly straight flying rocket at the center of the image of the sun we see. Launch it and it will eventually crash into the center of the sun.

You are seeing the light that was emitted 7 minutes ago and went in a straight line to earth. Trace the light ray back and you will get exactly the spot you were seeing.

All this is neglecting earth revolution around the sun, I'm only disputing that earth rotation makes any difference.

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Aim a perfectly straight flying rocket at the center of the image of the sun we see.
A what?

I'm only disputing that earth rotation makes any difference.
But it is the difference. If the Earth were in a stationary position relative to the Sun, and the Sun was stationary with respect to the CBR (yeah, yeah, don't bite guys), then the light we get would be where the Sun is.
This is not the case though.

Aside from the fact that the Sun is orbiting a center of gravity nearby itself (Jupiter is the main culprit for this), the Earth is orbiting the Sun, and it is rotating! When we see the Sun, we are seeing the Sun 8 minutes into the past. In the 8 minutes it has taken for that light to reach you, the Sun has moved (extremely slightly), the earth's orbit has changed (also very slightly), and the Earth has rotated 8 minutes (which is the major factor in the calculation, the others do not really contribute).

TinFoilHat's calculations may be correct for where he is, but further north and south of the equator the change in incidence will be less. But nonetheless, you are not seeing exactly where the Sun is now.

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Originally Posted by freddo
Aside from the fact that the Sun is orbiting a center of gravity nearby itself (Jupiter is the main culprit for this), ...
Just curious, wouldn't that center of gravity you speak of here be within the Sun's diameter rather than nearby?

12. Originally Posted by freddo
TinFoilHat's calculations may be correct for where he is, but further north and south of the equator the change in incidence will be less. But nonetheless, you are not seeing exactly where the Sun is now.
Not exactly, of course (nothing is ever exact), but TinFoilHat's calculation is way off, as Andreas is pointing out. The main reason that the sun seems to move 1.75 degrees across the sky in 7 minutes (the light from the sun actually takes a little longer than that, but nevermind that) is because the Earth is rotating. That rotation has almost no effect on the time delay. The Sun's position in the sky actually only changes one degree in a whole day! Nowhere near the 1.75 degrees in 7 minutes.

Point to the sun. Now point to the place where you thought it was 7 minutes ago, 1.75 degrees away. Was the sun actually there seven mintues ago? No, it was not, as it takes the sun more than a day to go that far.

Or, look at it another way. What if the light took 6 hours to get here. Then, the light would leave the sun when it was directly overhead but we wouldn't see it until about sundown. But if we were to point to the sun at sundown, we would still be pointing almost exactly towards the actual sun. It would not be ninety degrees away!
Originally Posted by beskeptical
Just curious, wouldn't that center of gravity you speak of here be within the Sun's diameter rather than nearby?
No, it's outside.

13. I'd just like to say that I once stared at the sun for fifteen seconds. There was a splodge of green in the centre of my vision for a couple of hours afterwards, but at my last opticians checkup, he gave my eyes a perfect bill of health.

14. Originally Posted by Glom
I'd just like to say that I once stared at the sun for fifteen seconds. There was a splodge of green in the centre of my vision for a couple of hours afterwards, but at my last opticians checkup, he gave my eyes a perfect bill of health.
BABB Disclaimer: don't try this at home! Your mileage may vary. 8)

15. But back to the topic at hand: the real problem with the original question is, what do we mean by "now"? As Einstein showed, there is no such thing as universal simultaneity. Different observers in different frames may see the same events in a different sequence.

It's perfectly valid to say that the Sun is, now, right where it appears to be. After all, no time has elapsed for the photons between their creation in the photosphere and their absorption by our eye. If they left and arrived at the same time, then the Sun must be exactly where we see it to be.

Which raises a sort of 21st-centurn Zeno's Paradox. If those photons arrived the instant they left, then they must not have moved in the meantime (since there was no meantime). Therefore the Sun must be attached to my eye -- no, to my retina!

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Ok, I'll admit it: my math was right, but my conceptualizing of the problem was completely off. I was analyzing the problem as if the sun goes around the earth, which it of course doesn't. If we regard the sun as a stationary reference point, the time it takes for the light to go from the sun to the earth doesn't matter, and the sun is exactly where it appears to be.

17. Originally Posted by TinFoilHat
Ok, I'll admit it: my math was right, but my conceptualizing of the problem was completely off. I was analyzing the problem as if the sun goes around the earth, which it of course doesn't. If we regard the sun as a stationary reference point, the time it takes for the light to go from the sun to the earth doesn't matter, and the sun is exactly where it appears to be.
I think you were doing what I tend to do alot and visualise from the point of view with reference to a fixed point ie the ground. In which case it appears that the sun has moved because you will pointing at the sky using the ground to 'measure' the angle. A little lateral thinking and I have (just about) managed to correct myself...

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Originally Posted by Donnie B.
the Sun must be attached to my eye -- no, to my retina!
Since the photons are actual physical particles of the Sun then that's true.

19. Originally Posted by SAMU
Originally Posted by Donnie B.
the Sun must be attached to my eye -- no, to my retina!
Since the photons are actual physical particles of the Sun then that's true.
Oh, the pain... the pain! :wink:

20. Originally Posted by TinFoilHat
Ok, I'll admit it: my math was right, but my conceptualizing of the problem was completely off. I was analyzing the problem as if the sun goes around the earth, which it of course doesn't. If we regard the sun as a stationary reference point, the time it takes for the light to go from the sun to the earth doesn't matter, and the sun is exactly where it appears to be.
Again, not exactly. The sun still moves in the sky--or, if you want to look at it from a different perspective, the earth does. It's just not 1.75 degrees in 7 minutes. BTW, light from the Sun takes more than 8 minutes to reach the earth.

Some stars themselves seem to shift their positions, depending upon which side of the earth's orbit we are on (which direction we are headed).

21. Sun's position in the sky actually only changes one degree in a whole day!
Doesn't the Earth rotate through 360 degrees in a 24 hour period? That is a guarter degree every minute (If I did the math right). If the sun only moved (apparent motion) 1 degree a day, Wouldn't it be in the sky a lot longer?

If I point directly at where the sun appears now, in 8 minutes, wouldn't the sun appear to have moved 2 degrees away from where I am pointing? Say I pointed a long rod at the sun at sunrise, (call it 6 AM for simplicity). I set the rod at 6AM and it is pointing directly at the sun. I come back at noon, 6 hours later, is it still pointing directly at the sun? I hope not.

22. Originally Posted by Musashi
Sun's position in the sky actually only changes one degree in a whole day!
Doesn't the Earth rotate through 360 degrees in a 24 hour period? That is a guarter degree every minute (If I did the math right). If the sun only moved (apparent motion) 1 degree a day, Wouldn't it be in the sky a lot longer?

I believe that it means the sun itself does not move, only our perspective on it. In other words, if you look where the sun is now, and check at the exact same time and in the exact same place in 24 hours, it will have moved 1 degree. Someone correct me if I'm wrong.

23. That would make sense, but would it impact the fact that the sun's position in the sky is different after 7 or 8 minutes and so, by looking at the sun I am seeing where is was 7 or 8 minutes ago?

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The Earth doesn't really rotate 360 degrees in a 24 hour period though. Since the Earth is orbiting the sun as it rotates, that means that the time it takes an object (i.e. the Sun) to rotate 360 degrees in the sky is different than the time it takes for the Earth to rotate 360 degrees on its axis.

It's a very small amount on Earth, but planets like Mercury, which have different rotational periods find the effect much more pronounced.

25. Aye, makes sense (again!). I was really trying to keep it as simple as possible. I don't know all the factors and I wouldnt be able to quantify them, so I have to leave them out of my posts :wink:

26. Originally Posted by Musashi
That would make sense, but would it impact the fact that the sun's position in the sky is different after 7 or 8 minutes and so, by looking at the sun I am seeing where is was 7 or 8 minutes ago?
Kilopi has this right. You are seeing the sun as it was eight minutes ago, but it's apparent motion in the sky during eight minutes is almost entirely because the Earth is rotating under it. It's a typical case of confusion about when a geocentric reference frame is and is not appropriate for calculations (though let's not get into that here! ), I'm afraid. You could do these calculations with the Earth as a fixed reference frame, but you'd have to use general relativity to get it to come out right, and the answer would come out the same as for the much easier calculations assuming a rotating Earth.

You can set this up yourself! Get a coin to represent the Sun, a smaller coin to represent the Earth, and set them on a piece of paper. Start drawing a line to represent the light coming from the Sun (move your pencil at a constant speed). While you're doing that, rotate the Earth (feel free to exaggerate the speed of rotation). Eventually, the light reaches the Earth, and you're standing right below it. If you look along the path the light just travelled (which is the direction in which you'll see the Sun, you'll notice that it's the same direction as the Sun is right at that moment.

27. Ah! Satori! I get it now. Thank you Grey. I can even see where my thinking was wrong. Thanks again Grey and Kilopi both.

28. Originally Posted by Musashi
Thanks again Grey and Kilopi both.
You're welcome. But I gotta admit I didn't even think about it until Andreas pointed it out.

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