A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Frank Reed
Date: 2014 Feb 24, 12:20 -0800
Sean, you wrote:
"If the primary means were celestial, how hard would it be to work out an almanac for a distant planet?"
There are various "sci-fi" scenarios you could consider here. Is this a society that has just arrived (within the past few decades) at one of these planets? In this case, they could very quickly generate long-term ephemerides by numerical integration derived from fairly short-term observations. Or is this a society that may potentially have been there for centuries or even millennia? The latter case is more interesting since it could imply that they have gone through some "history" and no longer have even the equivalent of 19th century technology. If that's the case, then they would need to build observatories in order to catalog the positions of the bright stars and create tables that could be used to generate their own nautical almanacs based on the long-term motions of the planets in their solar system. If we assume that they start from scratch having lost nearly all knowledge of science, then you would also have to assume that they would need to develop the equivalent of Newtonian physics (or else be limited to very simple methods like Noon Sun). But if we assume instead that, though reduced technologically, they continue as a literate society and have access to "books" on celestial mechanics and positional astronomy, then none of this is hard.
Sextants would not be too difficult to produce, though accurate ones more so, of course. Constructing good chronometers from scratch might very well turn out to be the biggest problem. Strangely enough, simple radio time signals would be much easier to arrange. Spark-gap transmitters and crystal receivers are relatively easy to mass produce, and radio communication is one of those technological discoveries that would be nearly impossible to lose, once discovered.
You also asked:
"How about refraction corrections?"
Let's imagine an atmosphere that's 80% oxygen and 20% argon but with a surface pressure that's only 25% of sea level air pressure on Earth. This is breathable (oxygen partial pressure same as here on Earth) but the total density is much lower and the refractive index would be somewhat different. These differences would yield refraction tables nearly equivalent to the refraction from a high mountain here on Earth. So no real problem. You could use r = 0.25'*tan(z) for zenith distances less than about 75° and be nearly correct without any further work.
You also wrote:
"I assume dip would be the same."
On a smaller planet, for a given height of eye in feet (they're British imperial interstellar colonists), the dip will be greater though the calculation would be the same. Don't forget that the dip tables include a refraction factor so that would be atmosphere-dependent. The close relationship between dip and distance to the horizon will remain the same (after correcting for that refraction factor) but only if we re-define the nautical mile maintaining sixty per degree of latitude no matter how big the planet is.
If we choose to live on gravitationally "lumpy" objects, like asteroids and moons (maybe in pressurised domes --always popular in sci-fi) or even just unusual terrestrial-szie planets, then you would have to worry about large deviations in the gravitational vertical. In fact, on some oddly shaped moons, celestial navigation would not yield a one-to-one correspondence between points in the heavens and points on the surface. That is, you could measure the same altitudes for stars at multiple places on your asteroid's surface.
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