# Where in the sky is the sun?

Gold Member
Sunlight takes eight minutes to reach Earth. Meanwhile the sun and Earth have moved/rotated.

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?

The approximate angle or distance in sun diameters?

Well that depends on the time of year, and probably slightly on atmospheric condition (refraction is dependent on moisture and angle).

There are equations easily available online which can calculate the position of the sun. You would simply have to find the difference in angle between a point now, and a point 8 minutes from now.

russ_watters
Mentor
That's not what the question asked....

Duplex, given that the earth rotates in 24 hours, how far (in angle) does it travel in 8 min? That is not a difficult calculation...

You would simply have to find the difference in angle between a point now, and a point 8 minutes from now.

how far (in angle) does it travel in 8 min? That is not a difficult calculation...

I'm sorry but I don't see a huge distinction between our two answers. The earth rotates every 24 hours but the elevation angle changes depending on the time of the year. In the winter the change in elevation angle over a full day is much less than in the summer, and peaks at solar noon.

In other words, there are two angles to account for, the azimuthal and elevation angles. simply dividing 360 degrees by 24 hours and using that ratio with 8 minutes will not suffice.

russ_watters
Mentor
Sorry, I read the first 3/4 and missed the last sentence!

However, the OP just said angle, not elevation angle. So I wouldnt go more complicated than a time ratio calc.

Sorry, I read the first 3/4 and missed the last sentence!

It's okay . I might be getting a little defensive today too. You're not the first PF Mentor to give me a hard time today haha.

russ_watters
Mentor
Er..sorry, late edit

Gold Member
Could it be a difference if the Earth rotates or the sun revolves around the Earth? A sort of three-body problem?

1. The sun is stationary in the sky and sends its parallel light as a laser beam towards the Earth.
Eight minutes before sunrise, it emits light that reaches me at sunrise. As the sun and solar direction coincide, the 8-minute light and the zero-time light coincide, i.e. the sun is really where I see it.

No matter the angle where I see the sun in the sky, there is only one sun in which the 8-minute position and the real position are identical.

2. The sun revolves around Earth. The medieval approach.
Don’t we then "see" two suns, the eight minutes old position and the sun's actual position?

I guess this question is somewhat simple, maybe I'm suffering from brain-fag but it was a baffling spin-off from a FTL discussion among some friends a year ago. If we had a measurement instrument for a hypothetical FTL-effect where would we find the sun in the sky.

I don't understand what you are saying.

The sun's position in the sky is always moving relative to an observer on Earth. That's about all I can say.

A.T.
1. The sun is stationary in the sky and sends its parallel light as a laser beam towards the Earth.
Eight minutes before sunrise, it emits light that reaches me at sunrise. As the sun and solar direction coincide, the 8-minute light and the zero-time light coincide, i.e. the sun is really where I see it.
No, you will see two suns. The 8-minute light will be subject to aberration:
http://en.wikipedia.org/wiki/Aberration_of_light
For the zero-time light aberration is zero.

2. The sun revolves around Earth. The medieval approach.
Don’t we then "see" two suns, the eight minutes old position and the sun's actual position?
Yes, here we also see two suns. But in this frame we attribute it to delayed signal from an outdated source position for the 8-minute light.

There is 360 degrees in a full circle. There is 1440 minutes in a day. Work it out.

sophiecentaur
Gold Member
No, you will see two suns. The 8-minute light will be subject to aberration:
http://en.wikipedia.org/wiki/Aberration_of_light
For the zero-time light aberration is zero.

Yes, here we also see two suns. But in this frame we attribute it to delayed signal from an outdated source position for the 8-minute light.

This thread has me confused. What can you possibly be implying when you say that you can see "two suns"? It is either a wind up (never on PF, I'm sure ) or a novel / alternative attempt to consider what is going on. But there is no paradox involved.

The only light that will reach Earth has taken 8 minutes to get here - by which time the Earth has rotated 360X 60/1440 degrees since it was emitted. How can you suggest that you could 'see' the light earlier than that? You could draw a diagram with lines on it to show two paths, possibly, but the one path would represent an impossible journey for the light (the no-time delay path doesn't exist)

Drakkith
Staff Emeritus
I'm not sure why this is so complicated. IF there was this "FTL" light from the sun that reached you, you would see 2 suns, one of them, the FTL light, would be slightly ahead of the normal one. Since FTL doesn't exist this does not happen. To see where the sun "really" is you would predict its location based on the knowledge of how far the Earth is from the Sun and how fast light travels.

sophiecentaur
Gold Member
Doesn't all this boil down to frames of reference?
People are trying to introduce a problem where there isn't one.

The zero-delay sun will be where the sun you see is in 8 minutes time.
Which is probably about a four sun diameters.

The speed of the sun through the galaxy etc doesn't make a difference as the earth goes the same speed.

Gold Member
Thank you guys.
I have considered all your answers, and I can not find anything but the problem arises if I look at the sun's apparent journey across the sky as equivalent to Earth's rotation with a stationary sun.

I think there is a difference. In the first case we have two suns, the illusory one we see with eight-minute delay with our eyes and the real position is slightly ahead in the direction of travel. It's like a jet in a perpendicular motion, where the light reaches us first and then the slow sound is lagging behind.

In the second case, our 24 hour rotation, we see the sun in its real position, where the sunlight that reaches us consists of two components: the old 8-minute light and a hypothetical fresh FTL-light.

I hope my basic problem has been to understand how my academic friends (no physicists) who discussed this subject at a party could have such different views.

Now, it is far past midnight here and it's time to sleep…

A.T.
Duplex, given that the earth rotates in 24 hours, how far (in angle) does it travel in 8 min? That is not a difficult calculation...

...wouldnt go more complicated than a time ratio calc.
There is 360 degrees in a full circle. There is 1440 minutes in a day. Work it out.
So you guys seriously think that the visible position of the sun is about 2° off the actual direction to the sun, because the Earth rotates 360° per 24h?

What if I stand on the north pole (during the summer there)? Same offset? What if I stand on the north pole and start spinning faster than the Earth? Will the offset increase?

sophiecentaur
Gold Member
So you guys seriously think that the visible position of the sun is about 2° off the actual direction to the sun, because the Earth rotates 360° per 24h?

What if I stand on the north pole (during the summer there)? Same offset? What if I stand on the north pole and start spinning faster than the Earth? Will the offset increase?

Wherever you stand, the azimuth error will be the same because the Earth is rigid and rotates at the same rate around its axis all over the surface (forget tides). The actual position in the sky will not be shifted by the same amount but that is only a detail of geometry (not time and c) and doesn't really help in the philosophy of this discussion.

A.T.
the problem arises if I look at the sun's apparent journey across the sky as equivalent to Earth's rotation with a stationary sun.

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.

Last edited:
A.T.
So you guys seriously think that the visible position of the sun is about 2° off the actual direction to the sun, because the Earth rotates 360° per 24h?

What if I stand on the north pole (during the summer there)? Same offset? What if I stand on the north pole and start spinning faster than the Earth? Will the offset increase?

Wherever you stand, the azimuth error will be the same..

- What exactly is "azimuth error" ?
- What is its value?
- If I don't stand still on the pole, but start to rotate will the "azimuth error" be different?

Last edited:
Gold Member
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.

There is an interesting factor.

http://en.wikipedia.org/wiki/Earth's_orbit

“The orbital speed of the Earth around the Sun averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet's diameter (about 12,700 km) in seven minutes”

Seven minutes...

sophiecentaur
Gold Member
- What exactly is "azimuth error" ?
- What is its value?
- If I don't stand still on the pole, but start to rotate will the "azimuth error" be different?
Look up the meaning of Azimuth angle. It relates to angle around the polar axis and is one of the parameters used to specify the position of an astronomical object. It would be better to look at a proper diagram than to have me give some lame verbal description.
The azimuth angle is independent of the latitude of the observer.

Of course if you turn around then the sun will appear to be going round Earth at a different rate. I don't see how that's relevant, though. It's only like assuming the Earth rotates at a different rate.

A.T.
There is an interesting factor.

http://en.wikipedia.org/wiki/Earth's_orbit

“The orbital speed of the Earth around the Sun averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet's diameter (about 12,700 km) in seven minutes”

Seven minutes...

Yes, so while light goes from the Sun to Earth, the Sun moves(from the Earth's perspective) about one Earth diameter (1/109 of the Sun's diameter). As I said, barely visible.

russ_watters
Mentor
Ahhh, carp - you're right. Sorry for that, guys.

sophiecentaur
Gold Member
There are two effects at work (in the very simplest model). There is a 2 degree rotational error due to rotation and a 13(+)k km positional error. The 2 degree error is the larger one by far.
But either way, there is only one visible image of the sun and that is how it looked about 8 minutes ago.

A.T.
Of course if you turn around then the sun will appear to be going round Earth at a different rate. I don't see how that's relevant, though. It's only like assuming the Earth rotates at a different rate.
Exactly. And would that different rotation rate change the angular offset between the visual and actual sun position (seen from the pole)?

Last edited:
A.T.
There is a 2 degree rotational error due to rotation.
Please explain how you arrived at this 2 degree error due to rotation. What if the Sun was further away, so it's light needs 12h to Earth. What would be the rotational error between the visible and actual Sun position?

sophiecentaur
Gold Member
I think we are talking in different frames of reference here, I think. If you are talking of azimuth angle (which is usual) then yes it would.
Imagine if the Earth were neither rotating nor in orbit around the Sun (sky hooks), it would see the Sun in the same position in the sky all the time. Erect a stick, pointing at the Sun and put it on a rail track around the equator. Start the Earth rotating (24hr period) but keep the stick pointing in the same direction (in space) that it was pointing. The Sun will no longer appear to be in line with the stick but will appear to be 2 degrees behind where the stick is pointing (which will still be directly at the 'actual' position of the Sun).

A.T.
Erect a stick, pointing at the Sun and put it on a rail track around the equator. Start the Earth rotating (24hr period) but keep the stick pointing in the same direction (in space) that it was pointing. The Sun will no longer appear to be in line with the stick but will appear to be 2 degrees behind where the stick is pointing (which will still be directly at the 'actual' position of the Sun).
How did you arrive at those 2 degree?

What if the Sun was further away, so its light needs 12h to Earth (which still has a 24hr rotation period). What would be the angle between stick (pointing at the 'actual' position of the Sun) and the visible position of the Sun?

Last edited:
sophiecentaur