Judging Stellar Distances

When utilizing Parallax, wouldn't it be necessary to physically measure the distance between Earth and at least one star and then to compare it to the angle of light relative to opposing points in Earth's orbit? Otherwise, aren't we beginning with an abstract assumption when we attribute a degree of angle to a particular distance, where we've yet to physically measure and confirm a base?

Are we not also beginning with a similar assumption when we estimate that the period and brightness of a Cepheid directly correlates with a specific distance, where we've also not measured a base? Doesn't this issue in fact persist with regard to all means of measurement of stellar distances? That is, even if we were to agree that the Inverse Square Law is generally true, or always true as the term "law" implies, aren't we actually only measuring the distance in which the photons in question have travelled, rather than the actual distance separating Earth from the stellar object itself? Due to phenomena such as the curvature of space, gravitational lensing, and other matters known and unknown, we can't expect the Inverse Square Law to consistently convey the distance between the Earth and the stellar object in question, can we?

Am I entirely missing something, or do we actually not possess a method of determining distance in deep space? Are the distances which we attribute to stellar objects practically pure guesses with what might as well be an infinite margin for error?

You'd get a much better answer at an astrophysics forum. But as far as I have learned, parallax is simple geometry.

From en.wikipedia.org...
Distance measurement by parallax is a special case of the principle of triangulation, which states that one can solve for all the sides and angles in a network of triangles if, in addition to all the angles in the network, the length of at least one side has been measured. Thus, the careful measurement of the length of one baseline can fix the scale of an entire triangulation network. In parallax, the triangle is extremely long and narrow, and by measuring both its shortest side (the motion of the observer) and the small top angle (always less than 1 arcsecond,[6] leaving the other two close to 90 degrees), the length of the long sides (in practice considered to be equal) can be determined.

Assuming the angle is small (see derivation below), the distance to an object (measured in parsecs) is the reciprocal of the parallax (measured in arcseconds): d (mathrm[pc]) = 1 / p (mathrm[arcsec]). For example, the distance to Proxima Centauri is 1/0.7687=1.3009 parsecs (4.243 ly)



Things are trickier for more distant objects. Again, the best answers you'll find will likely be at one of the astrophysics/astronomy forums, where actual astrophysicists are known to hang out.

a reply to: Greggers
Yes I see now how distance can be deduced by parallax. As we presumably at least roughly know the distance of the triangle's base (diameter of Earth's orbit), and are able to determine the angle of the remaining two sides, we can estimate the number of base lengths needed to complete the triangle, thus determining distance. Science often has a way of overcomplicating such things for the layman. I still didn't understand after reading your Wikipedia excerpt, but was struck with the epiphany as I wrote a reply to that effect. Just needed more time to reflect on what was being said to convert it into something a I could better understand I suppose.

Never the less, I'm still left with the remaining points of my original post which I've yet to grasp. I can't imagine though how distance can could ever be certain giving the curvature of space, gravity and so forth.

a reply to: Navarro

I found this but it's a foreign language to me, my mathematics education didn't go this far
link

As best as I can recall, there is something called the cosmic distance ladder, which has a variety of techniques, with each rung feeding information to the next one up.

A few points that might help:

1) Space is essentially flat.
2) Gravitational lensing is only of concern if it is actually observed between here and whatever distant object one is attempting to measure.
3) There is a difference between how far the light has traveled to get here and how far away the object is NOW, due to the expansion of time/space.

That cosmic distance ladder is heavy stuff. I suggest checking out this forum:
www.astronomyforum.net...

originally posted by: Greggers
3) There is a difference between how far the light has traveled to get here and how far away the object is NOW, due to the expansion of time/space.

Absolutely fascinating, is it not? It would appear impossible to estimate distance without knowing the rate at which a star was moving at the time when it produced the photons which we observe. We can observe it's apparent velocity and direction of travel relative to our position and time, but who's to say whether or not the universe expands consistently and uniformly? We know the expansion is exponential, and estimate that it may one day slow its rate of expansion, potentially leading to a contraction and "Big Crunch."

If we understand that shifts in the rate which the universe expands is possible, then might it not also be possible that such variance does or did exist in localized pockets? As such, a star which was 1000 lightyears distant from Earth at the time which it produced the photons we observe, may have seen a pocket of space between it and Sol contract after those photons had already migrated through that patch, leading observers on Earth to estimate that the star is 1000 light years distant, where in truth it's now only 250.

Of course, radical concepts as that lack evidence to support the theory, but they remain a possibility. After all, might the gaps detected in the CMB be suggestive of a pocket expanding at a more rapid rate than the average, leading to a region containing lower density than the rest? Never the less, a more accepted and common issue is the simple notion of gravity's effect on photons. We know that photons don't always travel a straight line from point A to B. Gravity and perhaps "dark energy" tug, pull and redirect photons as they do with anything else. So the distance which the photon travelled may not be the actual distance between the Earth and the object we observe. The variance may at times be so severe that if we were to estimate it's velocity and direction of travel based on the information presented by those photons, then launch a rocket to intercept that object, our astronauts might end up discovering that they're quite lost in space. That is, if something which disturbed the proton on route to Earth is no longer active, or if its capacity for disturbance is variable, meaning that it flung our astronauts along a path inconsistent with the path which the photons travelled.

Given these things, it seems that we neither know the actual direction of the objects we observe, velocity nor direction of travel, at least not with certainty. After all, as the path of a photon is disturbed, it may radically change directions, leading to our estimation of the placement of its mother object being off by entire degrees. Where that photon is slowed by disturbances, it may be that our estimation of distance is equally flawed. Does this sound accurate to you? It seems a given to me.

I've taken a look at the forum you've recommended. It reminds me of a physics forum in which I was once banned "for peddling psuedo-science," then shortly later invited back by another moderator and academic who thought my theory valid and worth consideration. I never returned, and the first thing I saw in the forum you recommended was a sticky warning against posting "new theories" and "psuedo-science." As you see above, I like to imagine, see what people think, and to learn about the concepts proposed both in favor of and as a rebuttle to what I've imagined. I'd love the opportunity to learn what they know, but the environment seems awefully oppressive.

originally posted by: Navarro
When utilizing Parallax, wouldn't it be necessary to physically measure the distance between Earth and at least one star and then to compare it to the angle of light relative to opposing points in Earth's orbit? Otherwise, aren't we beginning with an abstract assumption when we attribute a degree of angle to a particular distance, where we've yet to physically measure and confirm a base?

Are we not also beginning with a similar assumption when we estimate that the period and brightness of a Cepheid directly correlates with a specific distance, where we've also not measured a base? Doesn't this issue in fact persist with regard to all means of measurement of stellar distances? That is, even if we were to agree that the Inverse Square Law is generally true, or always true as the term "law" implies, aren't we actually only measuring the distance in which the photons in question have travelled, rather than the actual distance separating Earth from the stellar object itself? Due to phenomena such as the curvature of space, gravitational lensing, and other matters known and unknown, we can't expect the Inverse Square Law to consistently convey the distance between the Earth and the stellar object in question, can we?

Am I entirely missing something, or do we actually not possess a method of determining distance in deep space? Are the distances which we attribute to stellar objects practically pure guesses with what might as well be an infinite margin for error?

We use " the standard candle" method to calibrate our measurements.

All but flawless.

A type 1-A supernova always produces the exact same amount of energy, every time one happens we use this to check our previous guesstimates of distance.

We are pretty damned accurate.

a reply to: Navarro

You really don't have to consider any of that because the stars we see and measure are in our OWN galaxy and relatively close. In relative terms they are not moving at vast rates unlike other Galaxies.

a reply to: Navarro

You are WAY overestimating the amout of curvature of space, bending of light, the velocity of stars and the rate of expansion in the local neighborhood (within 5,000,000 light years). All of the effects you maention are negligable (if they are even measurable) at that scale.

Hope this helps.

originally posted by: Navarro

I've taken a look at the forum you've recommended. It reminds me of a physics forum in which I was once banned "for peddling psuedo-science," then shortly later invited back by another moderator and academic who thought my theory valid and worth consideration. I never returned, and the first thing I saw in the forum you recommended was a sticky warning against posting "new theories" and "psuedo-science." As you see above, I like to imagine, see what people think, and to learn about the concepts proposed both in favor of and as a rebuttle to what I've imagined. I'd love the opportunity to learn what they know, but the environment seems awefully oppressive.

If you want a degreed astrophysicist to answer your questions, you'll have to play by his rules. Unless you act like you know more than you do, you won't have any problems there.