What yearly apparent shift in star positions is used to prove the earth orbits the sun?

Answer 1

First of all ... that's not how Science works at all. A scientist who looks for evidence that

he can use to prove his theory is a very poor scientist, and most likely gets the cold

shoulder from his colleagues.

The 'evidence' comes first, except that it's not called "evidence", because nobody is

trying to prove anything yet. It's called "data", and it's simply a clear, careful, detailed

description of what is seen in nature.

Then the "theory" comes after that. It's an attempt to explain what was seen in nature.

The theory that comes up with the closest fit to what was actually seen is the accepted

theory, until somebody comes up with a theory that fits the observations better, or until

new and better observations show that the theory doesn't really fit so well.

Between the beginning and end of a year, the stars don't shift at all. But they turn completely

all the way around an observer in one day. And if you measure carefully, you observe that

each night, the stars reach the same place about 4 minutes earlier than they did on the

previous night. That's the data. That's what we see in nature. That's what happens,

and there's no debate. Everybody who watches it sees the same thing happening. The

debate starts when different thinkers try to offer explanations for how the whole system

is built, and why we see what we see.

You could explain what you see by saying that we are in the center of a giant globe. All of

the objects in the sky are attached to the inside of the globe, and the globe turns around

us once a day. The sun and the moon are not so firmly attached, though, and they slide

slowly along the inside of the globe. The moon slides all the way around the inside of the

globe every 29 days, and the sun slides all the way around it every 365 days, while the

whole globe turns around once a day.

That's a theory that very nicely explains what we actually see in the sky.

But then somebody comes along and calls our attention to five bodies that also slide along

the inside of the globe, all at different rates. They wander in such puzzling ways that they're

called "wanderers" ... "Planeti" in Italian. They never get too far away from the paths that

the sun and moon follow, which is comforting, but three of the five have this peculiar

habit ... once a year, each one reverses its direction and slides backwards for a while,

then reverses again and resumes sliding along the inside of the globe in the original

direction. Really weird. Maybe they're not even hooked to the inside of the globe like

everything else at all. We start to feel uncomfortable with the theory that explains everything

we see in the sky.

Then somebody comes along, buys his kids the latest thing at Toys Я Us called a "telescope",

steals it one night after they go to bed, looks through it into the sky ... and he sees four new

little tiny bodies that nobody had ever seen before, and they're not attached to the giant

globe at all, they're circling one of the planeti! In a flash, he knows that the accepted theory

is hogwash.

You see where this is going. Long story short ... as new things are observed, we need new

theories to explain what we see. As of today ... the year 2010 ... the theory that best explains

everything we can see and measure is the theory that proposes that the earth orbits the sun,

in the way predicted by Newton's theory of universal gravitation.

As they say ... "It's just a theory".

And that's true. If someone comes along tomorrow with a new observation that the theory

can't explain, and his observation is confirmed by other observers, then the theory is out

the window.

The moral of the story is: If you look for evidence that you can use to prove a theory, then

you're going to ignore evidence that doesn't support it. You'll be a rotten scientist, and

nobody in the business will talk to you or listen to anything you have to say. The theory

is what the scientist offers to explain the data. The earth orbiting the sun is the best

explanation that's been offered to explain everything that people have been seeing in

the sky since we've had eyes to see it and speech to tell other people about what we saw.

Answer 2

Parallax is the yearly apparent shift in star positions used to prove that Earth orbits the Sun. As Earth moves along its orbit, nearby stars appear to shift against the background of more distant stars. This phenomenon is one of the key pieces of evidence for the heliocentric model of the solar system.