Celestia Forums

I'm assuming Celestia doesn't re-calculate all the star positions when you move to a different star to extrapolate what you actually would have seen from there at that time. But how much of a difference would that make? Obviously the view from Alpha Centauri is almost identical to the view from here, but what about the view from a star 1,000 light years away? Are the differences still insignificant that far out?

To answer your question, one must determine how far each star moves in 1000 years. Most stars move at such slow speeds that their motion can only be noticed if they're near the viewpoint. While Barnard's Star moves quickly from our point of view, that movement would be difficult to measure from a more distant viewpoint. On the other hand, there might be stars near the distant viewpoint which are moving relatively quickly but we haven't measured their motions very well from here.

In principle, Celestia could include some star motions (e.g. by specifying orbits relative to the galactic center or some other appropriate "barycenter"), but nobody has spent the time to generate appropriate STC catalogs.

selden wrote:In principle, Celestia could include some star motions (e.g. by specifying orbits relative to the galactic center or some other appropriate "barycenter"), but nobody has spent the time to generate appropriate STC catalogs.

Maybe because having them all orbit the Galactic Center would make all their planets invisible due to being beyond the 1 ly limit by thousands of light years?

I know I did a few exercises where I had a fictitious star a few lightyears from Sol orbit Sol, and when the fictitious star was beyond the 1 ly limit, its planets didn't show.

It's a difficult question, but do you have any intuition how big the difference is? Is the view in Celestia from HD 278942 (600+ light years from Earth) a little bit different from what it would actually be, or so far off as to be completely fictional?

It could be almost completely "fictional." The major source of error for the position of these stars in space is the distance. The uncertainty in their parallax can be as much as 25% for many stars. The problem gets worse as you get further from the sun, where smaller and smaller changes in the parallax amount to greater and greater distances. Two stars near each other in the sky that both have a distance of 100 ± 10 pc, you would expect to see represented near each other in Celestia, but their true distances may be 20 pc apart.

It's no fault of Celestia, it's just an effect of the measurement uncertainties in the parallax of these stars.

The 2 Million star database addon has an even worse problem. Most of these stars have distance estimates that aren't even from parallax, but from a spectroscopic guestimate of how far they are (the stars have this spectrum, and stars that have this spectrum are typically a certain brightness, so it must be some distance to have its observed brightness). This method is much less accurate.

The distance uncertainties will overwhelmingly outweigh the position uncertainties due to stellar drift.

Let's look at the example of Betelgeuse. It's parallax is given there as 6.55 ± 0.83 mas. This corresponds to a distance of anywhere between 570 LY and 440 LY. That's an uncertainty of 130 light years! The true position of Betelgeuse could be anywhere along a 130 LY line extending out from Earth. That's a lot of distance and for stars that are near that line, the position of Betelgeuse in their sky is thus hard to guess.

The true uncertainty could be even further, if I'm not mistaken the typical uncertainties given in measurements are 1?, meaning there's a ~68% chance the real position is within the quoted uncertainty. So there's a real chance the true distance is even further off.

Hungry4info wrote:Let's look at the example of Betelgeuse. It's parallax is given there as 6.55 ± 0.83 mas. This corresponds to a distance of anywhere between 570 LY and 440 LY. That's an uncertainty of 130 light years! The true position of Betelgeuse could be anywhere along a 130 LY line extending out from Earth. That's a lot of distance and for stars that are near that line, the position of Betelgeuse in their sky is thus hard to guess.

The true uncertainty could be even further, if I'm not mistaken the typical uncertainties given in measurements are 1?, meaning there's a ~68% chance the real position is within the quoted uncertainty. So there's a real chance the true distance is even further off.

Right! Such crucial facts cannot be emphasized often enough.

Fridger

Thats's interesting, Hungry4info. Thanks for explaining. I had some vague idea that there must be some uncertainty in how far away the stars are, but didn't have a feel for how big it was.

Still, the idea that there's probably a lot of random error in the star positions doesn't really bother me. But there's something that feels inherently wrong about moving my point of view out to Betelgeuse and having the stars and the clock both holding still. I should either be moving back in time or seeing where the stars have moved to in the five hundred or so years since the light we're seeing now from Betelgeuse originated. But I don't have any sense at all of whether that change would be imperceptible, mostly the same except for a few nearby stars, or drastically different.

From what selden says I guess it's probably more like that 2nd option -- mostly the same except for a few nearby stars?