Geocentric, Heliocentric....

[geministar]

The standard model of the cosmos has the Earth at the centre.

We see different constellations as the Sun moves through them in a year.

In reality however, the Earth revolves around the Sun; the Earth is not at the centre.

So how come astronomical coordinates are essentially fixed?

Is it that the Earth's movement is small in relation to astronomical distances?

Apologies if this is all too basic, and I've I've just missed some key point in understanding!

[acey]

The standard model of the cosmos has the Earth at the centre.

-No it doesn't.

We see different constellations as the Sun moves through them in a year.

-We see constellations whenever the Sun isn't in the sky. If we switched off the Sun we would see fixed constellations "circling" the Earth every 23 hours and 56 minutes. We see the Sun "circling" the Earth every 24 hours on average. The phase difference is because Earth has 2 rotations - on its axis and around the Sun - and is why we see different constellations at night over the course of a year.

In reality however, the Earth revolves around the Sun; the Earth is not at the centre.

-Correct.

So how come astronomical coordinates are essentially fixed?

-The celestial co-ordinates (RA and Dec) of astronomical objects aren't fixed. The wobble of Earth's axis (precession) means that star atlases were traditionally updated every 50 years or so. SkyAtlas 2000, for example, plots stars in their positions at 2000. They have changed very little since then.

Is it that the Earth's movement is small in relation to astronomical distances?

-Correct. Stars are a very long way off, and although they have their own "proper motions" these are very small - constellations have barely changed shape in thousands of years. Similarly, Earth's motion around the Sun produces "parallax" and "stellar aberration", making stars apparently shift by a tiny amount. The biggest shift in the apparent position of stars is due to precession, and even that is so small that most amateur astronomers never concern themselves with it, and would be content to use an atlas published in 1950. (A further effect that would make a difference between what you see in the sky and what you see in a star atlas is atmospheric refraction - the bending of starlight as it passes through air. Again, it's insignificant except when very high accuracy is required).

-Hope this helps

[ronin]

The standard model of the cosmos has the Earth at the centre.

Not really, no actual centre, just a point of view concerning where you are stood and looking from, something in Andromeda would get the same "view" as we do. They would "see things" as if they were at the centre of the universe, and for us we view from the Earth.

We see different constellations as the Sun moves through them in a year.

Caused by our orbit around the sun and the tilt that the earth has, 23 degrees if I recall right. Also we see the stars/constellations as they appear to us from our location at this time, As we drift around and they drift around all the "constellations" will change.

In reality however, the Earth revolves around the Sun; the Earth is not at the centre.

Not I think particularily relevant but when you think about it that is why we have Summer and Winter constellations. In the Winter the Summer constellations are in the sky during the day so lost in sunlight.

So how come astronomical coordinates are essentially fixed?

They are not, over time they will change, Polaris will change to another star in a few thousand years. Might be a good idea to cease referring to it as Polaris. There is a "fastest" moving star in relation to us. That one will to us change in the short term.

Is it that the Earth's movement is small in relation to astronomical distances?

In out lifetimes yes, we do not move that quick so to us and over a few generations they all appear still. Keeps people occupied remeasuring the RA and Dec of every few years. Suppose Gaia could start supplying new/better data.

All you have missed it the time scale and distances involved. In our terms we are spining around and through the milky way fast, to the milky way we are ambling around very slowly. One of the silliest things I have seen is that Skywatcher now have Altitude to put in for you location. So you are looking at M42, about 1350 light years away, is saying you are at an altitude of 200 meters going to make any difference? Our milky way is a big place, and our milky way is just a small/average galaxy.

[geministar]

Thanks, acey and ronin.  Reckon that's cleared it up.

Sometimes things just don't seem to sit right.

BTW when I said the "standard model" has the Earth at the centre I should have stressed I was referring solely to a coordinate system and not how the whole thing actually is!

Thanks again.

[geministar]

PS:

In a nutshell, what I couldn't get my head round was the idea that the Celestial Sphere is constructed with the Earth at the centre, and the Sun and planets moving along the ecliptic (inclined to the Celestial Equator), yet on the other hand the whole yearly change thing depends on the Earth going round the Sun (so how can it be at the centre??).

A tricky concept, I think.......