# How can we tell?

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#### science_man

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According to one rule in the general theory of relativity, "It is impossible to detect the motion of the Earth, or any other heavenly body, relative to ether assumed to be stading perfectly still in the universe. Consequently, It is impossible to know if any heavenly body is truly standing still or moving in the universe."<br /><br /> Given this, how do we know that we travel around the sun? What proof do we have that it cant be just turning on and off? Of if the sun was 1 sided and was rotating around it's axis producing day and night? How do we know that mars is acctually traveling around the sun. According to Einstein's theory of relativity, it is impossible to know. So how can we make these conclusions?<br />

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#### doubletruncation

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<font color="yellow">It is impossible to detect the motion of the Earth, or any other heavenly body, relative to ether</font><br /><br />The key phrase here is relative to *ether*. There is no problem measuring motion with respect to another object (e.g. the sun) - you can tell that you're moving at 60 mph relative to the ground when you're in a car. I don't understand how the sun turning on and off or rotating around it's axis would give rise to day and night (why would the sun rise and set?). I think the other possibility would be that the Sun is going around the Earth. However, the observed motions of the planets on the sky relative to the stars makes a lot of sense when you say they're all going around the Sun. Nowadays of course we have much better proof than just that given that we can actually measure the distances to those planets at different times in their orbits and the 3-dimensional geometry then tells you that we're all going around the Sun. <div class="Discussion_UserSignature"> </div>

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#### CalliArcale

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It is possible to describe the apparent motion of the planets and the Sun from a geocentric perspective (something which relativity says is perfectly legitimate to do, because no frame of reference is inherently superior to another). But the math is horrendous. <img src="/images/icons/wink.gif" /> The heliocentric representation is vastly simpler, to the point where there's really no sense in using a geocentric model, except as an intellectual curiosity. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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#### doubletruncation

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Agreed. In practice Earth-bound astronomers do end up transforming everything back to Earth-centric coordinates (local azimuth and elevation, or RA/dec which is Earth-centric but rotates with the sky) so you know which way to point your telescope - but this is the last step, no one calculates an orbit working in these coordinates, that would just be silly <img src="/images/icons/smile.gif" />. <div class="Discussion_UserSignature"> </div>

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#### dragon04

##### Guest
Very simple answer. The Earth's axis is tilted. We experience seasons. <br /><br />Seeing that our axis does not change significantly year by year, we can use that as a frame of reference that implies one of two things.<br /><br />Either the Earth revolves around the Sun, or the Sun revolves around the Earth. We have ample evidence that (for the sake of simplicity) that the other 8 "planets" have calculable "orbital periods" that describe the amount of time it takes each one to complete one orbit around the Sun.<br /><br />The discovery of extrasolar planets support this. One way that we discover bodies orbiting other stars is by the transit method.<br /><br />A large planet in orbit around any other observed star dims the apparent brightness of a star on a predictable timeframe.<br /><br />In several cases, more than one large planet does so in reference to the star being observed.<br /><br />It's an observable, repetetive and consistent phenomenon outside our own solar system. Unless one would consider our solar system unique, one can extrapolate the fact that what happens "there" also happens "here".<br /><br /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

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#### enigma10

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To answer the bigger questions, you must find the only point that may be standing still,i.e. the center of the universe., then assuming it is not drifting, stop all movement toward or away from that point. You should now be standing still. <br /><br /> Another proposal may be to send out an object in every direction away from you. Make sure no other forces can interact with those objects, and make sure they can stop a certain distance travelled uniformal to all of its sibling objects. <br /><br /> If this can be done, based on the distance variation not associated to known outside forces, should give an idea of which direction you are moving in the universe. <div class="Discussion_UserSignature"> <em>"<font color="#333399">An organism at war with itself is a doomed organism." - Carl Sagan</font></em> </div>

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#### Saiph

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actually, the answer is typically simpler still.<br /><br />GR says you can't differentiate motion (refering to uniform motion) between two bodies. It doesn't say you are unable to differentiate <i>acceleration</i> between two bodies.<br /><br />The earth is in an orbit about the sun, and feels an acceleration, if not a change in speed, than a change in direction of motion (it's turning).<br /><br />Now, GR <i>can</i> be used to construct a system that says everything else is moving and accelerating in such a way as to only make us think we're the one's orbiting...but that's far, far more complicated of a system, so we apply Occam's Razor, and settle for earth orbiting. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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