Commander Xillian wrote:And, this brings us to the theory of Zero-Point (Or Universal Stillpoint). Basically, there is a set space in the universe that everything revolves around. Using this theory, one could, theoretically, compute distances accurately, but like all great theories, it would be useless the moment you left your galaxy. But that would never wo-[KLAAANG!]
What the Fuck was That?
Xillian... that makes no sense. It's vaguely amusing, but even though I disagree with Skimmer about just how effective star sightings are for true deep-space navigation to the precision required to find a kilometer-sized object... gah.
Please, you seem like a reasonably clever fellow. You can do better than this.
Coyote wrote:I don't know about this; bear in mind that in stellar navigation we'd be able to track stars in an omnidirectional ball out from the ship, and not be limited to what we can see above a horizon line...
Here's the problem.
In theory, you can determine your exact position,
anywhere, by knowing the angle to three points precisely enough. You know that Star A1 is at angular coordinates (B1 degrees above/below horizon, C1 degrees azimuth), Star A2 is at (B2, C2), and Star A3 is at (B3, C3). You know where those stars are in "absolute" terms (in the galactic frame), and you know where they are relative to you, allowing you to draw lines connecting you to the stars. Since there's only one point in three-dimensional space where the three lines intersect, you know your position.
The problem is: do you know your position
to acceptable precision? Stars are, as I said before, tens of trillions of kilometers apart. An error of one part in a million in measuring the angular position of a nearby star translates into a positioning error of tens of millions of kilometers.
If you use
distant stars for location (such as bright ones that can be seen anywhere in the galaxy), the problem is even worse. Now the distances (and the resulting error in position from a given error in your angular measurement) are ten or a hundred times greater. And you have the added problem of
finding distant references, which requires you to scan the entire sky and identify exactly which stars have the right spectrum to be the landmarks you're looking for.
This is still a solvable problem. Astronomers
do nail down the angular coordinates of distant bodies to extremely high precision... after long observations. As Sea Skimmer so kindly explains, this can be done to solve the problem of figuring out where you are after popping up in the middle of nowhere.
We see the problem solved in the Honorverse in
War of Honor, with the survey ship
Harvest Joy passing through the Talbot terminus of the Manticore wormhole junction. But it's not a trivial problem; it takes them hours to figure out where in the galaxy they are, because they don't know and have to search the surrounding stars to find reference points that tell them their position as I described above. Perhaps Weber was underestimating what the software of his era would be capable of, of course...
In a setting where such constraints apply, where it takes hours to get enough star sightings to know your exact position relative to the galactic frame without taking advantage of
nearby points to sight on (such as the Sun in our own solar system)... it's understandable that you'd use a very visible aiming point (like a star) to place your fleet base. That way, ships aren't at risk of wasting time looking for your base billions of kilometers away from its real location, because if they can get within a few billion kilometers at all it's perfectly obvious where the base is: near the star.
Sea Skimmer wrote:Simon_Jester wrote:Just a guess.
That said, we track the stars on Earth by measuring the angles to the stars, compared to the horizon. In effect we're measuring the plane of the horizon, not the positions of the stars themselves. That tells us where we are... on Earth. We can say "Star 2495 is at azimuth X degrees, elevation Y degrees, so I'm at latitude A, longitude B." In space it doesn't work that way.
Yeah it fucking does. If i have three very distant known stars to sight on, and I measure the elevation and deflection to each one, I have my position in 3-D coordinates.
Yes. To what precision? If I drop your instrument package at a random location in deep space, how precisely can it determine its location within a minute? An hour? A day? At some point it becomes faster to drop out in the vicinity of a star, close enough that it's far and away the brightest thing in the sky, and just say "head to within a hundred million kilometers of that, then fire up the omnidirectional IFF beacon to get a hailing signal from the base."
Early mariners were limited in what they could do because of instrument limitations. However even then, such navigation done all by hand and eye could still be accurate to within 3 nautical miles even though the earth is moving rather fast. 3 miles is already more then good enough for any realistic space navigation given the premise that we already have interseller ships.
They didn't do it by measuring coordinates in a framework that encompassed the whole galaxy, though. They knew
their latitude and longitude on Earth by measuring the position of nearby stars to a precision of tenths or (at best) hundredths of a degree. Drop them in deep space and their instruments become a lot less useful, because they are no longer constrained to the surface of a small sphere. Knowing the angular position of stars to a precision of one part in one to ten thousand doesn't give them their location to within five kilometers anymore; it gives it to within something more like five billion kilometers.
That is what I'm trying to get at. Trying to determine your absolute position "relative to the fixed stars" (or, if you prefer, to the black hole at the galactic core, or to the star that is the capital of the Galactic Empire, or whatever) is a much more difficult challenge, demanding MUCH better instruments, than doing celestial navigation on a planetary surface. Even with those much better instruments, without a known reference point nearby (like a star or planet), nailing down exact positions is difficult.
Fixed points in space? You're whole argument is flawed . A single star is not a fixed point! A star is a rather fast moving object like everything else in space.
Relative to interstellar spacecraft? We're talking about soft-SF FTL drives here. To take an example, the Sun has an orbital speed of around 250 km/s about the galactic core: less than 0.001c. Compared to the speed of an FTL starship... well. To take the Honorverse as a particular example, typical ship speeds are ~1000c. Stellar drift is to ships like that what a 100 meter per year drift would be to sailing ships. It's fast enough to have some effect on navigation over decade or century-long timescales, but for purposes of any given voyage it's well within acceptable margins of error.
So
for purposes of navigation in FTL spacecraft, the stars are "fixed." Not perfectly fixed, but then continents on Earth aren't perfectly fixed either. They just move slowly enough that navigators don't have to care very much.
Furthermore a single star wouldn't work at all, since without taking in other information you have no idea what direction is what.
The star, by itself, IS NOT REQUIRED to be a complete navigation point. It is a beacon for convenience, an aid to fine-detail navigation on the order of millions of kilometers, to go with the coarse navigation that you do by reference to distant stars. You know, detailed enough that it is
helpful to know the exact distance to a specific point, along with knowing the angular position of distant stars.
In deep space, the same purpose would have to be served by an artificial beacon. You could do it, it would work, but it would be clumsy.
In fact long range star tracking is the only space navigation system that will work over a wide area. Low and behold NASA has already created an automatic system to do this for spacecraft. It currently only uses the sun and the planets, since that works fine for movements within the solar system.
How precisely would it give them the position of a spacecraft that was suddenly dropped at a random point in the solar system? To within a million kilometers? Almost certainly. A thousand? Possibly. One kilometer? I would bet a considerable sum against it.
