Where is the Sun?

g__day (Matthew) · #1 11-07-2009, 05:14 PM

Science thought for the day.

A simple experiment - ask someone to point to the Sun. Now ask them to point not where they see the Sun, but exactly where the Sun actually it is. It's easy to do - just advance your aim four Sun diameters (or two degrees) ahead of where it appears in the sky.

The maths is fairly simple. The Sun is about 8 light minutes away from Earth and about 30 arc minutes wide and the Earth rotates at 15 degrees an hour (360 degrees / 24 or 15 arc miniutes a minute if you like). So in the eight minutes it takes light from the Sun to reach the Earth the Sun's true position is actually 15 / (60 / 8) = 2 arc minutes or four Sun diameters ahead of where you see it.

This should give folk an easy way to confirm a gravitation field re-shapes or radiates (shudder) at the speed of light; as a sensitive gravitometer orbiting the Earth would show the centre of mass for the Sun to be where the Sun appears visually - not where it is absolutely.

Science can challenge what seems fundametally apparent - cool hey!

bojan · #2 12-07-2009, 07:01 AM

I would like to see this sensitive gravitometer and how it works in orbit.

BTW, isn't the Sun actually the "source" of that pull, and Earth is rotating around it's own axis?
When you point your finger in the direction of the Sun (and keep it pointed in that direction for a while), your finger is actually tracking the Sun.. (compensating the rotation of earth).
So, the Sun's direction in the sky at any moment is exactly where it appears to be... and not ahead of it's image.

Starkler (Geoff) · #3 12-07-2009, 01:54 PM

Quote:

Originally Posted by g__day

The maths is fairly simple. The Sun is about 8 light minutes away from Earth and about 30 arc minutes wide and the Earth rotates at 15 degrees an hour (360 degrees / 24 or 15 arc miniutes a minute if you like). So in the eight minutes it takes light from the Sun to reach the Earth the Sun's true position is actually 15 / (60 / 8) = 2 arc minutes or four Sun diameters ahead of where you see it.

2 degrees, not 2 arc minutes.

I think Bojan is right. If you consider the Sun to be fixed in position and the earth rotating in relation to it, at any given time the observer is looking directly at where it is. But wait! Is this any different to the earth being a stationary object and the Sun orbiting the earth every 24 hrs? My brain hurts.

g__day (Matthew) · #4 13-07-2009, 09:56 AM

The Earth’s rotation has little to do with the centre of the Solar system’s gravitational well? It’s basically a point under the Sun’s surface (largely thanks to the influence of Jupiter) as the Sun contains over 98% of the Solar Systems mass.

The Sun’s position (not direction) is ahead of where you see it – from Earth you see where it was eight minutes ago, and given the Earth’s rotation you are always 2 degrees behind where it is.

There are no fixed positions in space. So for convenience imagine rather than the Earth moving about 5,000 km (around the Sun) in the eight minutes it takes for light to make its journey and rotating 2 degrees whilst it does so, imagine the Earth is your fixed point in space and the Sun was in relative motion. Would you envisage it actually was where you see it?

A thought experiment for you. If every ten seconds the Earth moved a light year from our Sun, would you expect a lag in where you saw the Sun or no change?

bojan · #5 13-07-2009, 10:12 AM

Mathew, I am no satisfied with your explanation, it does not make any sense to me. And I am sure there are others who do not know whom to believe.
Since we are not here to spread or discuss beliefs, could you please elaborate this idea in more detail?

Is this idea of yours based on GR? How do you intend to measure (or detect.. this gravitometer is extremely intriguing to me.. please explain?) the position of the centre of Sun - Earth system?

And, does this knowledge (where the the Sun "actually" is) makes any sense? Because, information is not information before we receive it. So, if information about Sun's position is spreading from Sun with c, it (theoretically) can disapear in the very next moment.. and according to what you are saying, its position is 2 degrees away, while in this proposed reality it does not exist any more.

sheeny (Al) · #6 13-07-2009, 12:00 PM

I'm with Bojan on this.

The earth's rotation on its axis does not significantly effect the sun gravitation field (other than the miniscule/negligible amount of spacial frame dragging), nor the apparent position of the sun.

Yes it takes 8 minutes for the light from the sun to reach earth, and yes the sun is moving in space. The Earth also has a component of its motion that is exactly the same as the motion of the sun in space - so for the relative positions of the earth to the sun in the reference frame of the solar system, the sun appears exactly where it is/was 8 minutes ago.

What your experiment is really demonstrating (if you reverse the direction of offset), is where the sun appeared in the sky (relative to the rotating reference frame of the earth) when the light left the sun.

Al.

xelasnave · #7 13-07-2009, 12:11 PM

The lag spoken about here is from the Earths rotation I expect but when looking at the Sun (with protective eye gear of course) in which direction thru space is it going..that is what is its movement / direction as it orbits the central massive black hole or general center of the Milky Way as we will observe.. I think its North but I dont know where I got that idea from really...
alex

erick (Eric) · #8 13-07-2009, 02:40 PM

Err... so what is the answer? Forgetting gravity and Einstein (may I?) and that the solar system is moving through space (orbiting the galactic centre), it makes sense to my simple brain that the Sun ("absolutely") is located "8 minutes of earth rotation" ahead of where its image appears.

Of course, it doesn't really matter given we are all down here, some eight minutes away from the Sun.

IanT · #9 13-07-2009, 03:11 PM

My head hurts. I think (I think!) that since we are gravitationally attached to the sun, the position of the sun relative to earth remains the same but it appears to change because of earths rotation (I'm sure there will be some uber-astrophysical reason for some other minor variations). However, I think that the 8 minute lag merely reminds us of what the sun looked like 8 minutes ago rather than where the sun was 8 minutes ago. Does that make sense??
Ian

g__day (Matthew) · #10 17-07-2009, 12:50 PM

Yes

Forget frame dragging or the movement of our Sun in the Solar system, or the bary centre of our solar system, or our solar systems movement around galactic centre or that centre moving with respect to the local group or the Virgo super cluster etc.

If you like imagine the Sun and Earth are fixed in space, there was no such thing as gravity and accept the Earth is rotating. Realise it takes light takes 8 minutes to reach the Earth. The observed position of the Sun will always be off by the transmission delay versus the rotational speed of the Earth. If a solar day was 32 minutes long in this scenario the Sun's observed position must always be 90 degrees away from its true position.

Bojan - a space based LIGO system should be able to detect and anomoaly between GR and the observed position of the Sun. If the Gravity field of the Sun is not centred at the observed location of the Sun, then you have a delta between the speed of light and how a gravitational field radiates.