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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Mars Tables
From: Bruce Hamilton
Date: 2008 May 28, 18:48 -0700
From: Bruce Hamilton
Date: 2008 May 28, 18:48 -0700
Well, as I suspected between Geoffrey's theodolite and Frank's reasoning, the problem is almost solved. Now we need someone to write up a grant application. Luis could certainly make the Starpilot work for Mars. Can you picture the new key? Welcome to Starpilot, please select your planet. Field trip anyone? frankreed@HistoricalAtlas.net wrote: > Bruce Hamilton, you wrote: > "This is the one place in the world I can ask this question and not get > laughed at, and probably get an answer." > > LOL. I'm definitely laughing, but laughing with you! > > You asked: > "Has any one worked out tables for celestial navigation on Mars?" > > Well, let's see... The stars are in the same positions, so no problem there. > You would have to adjust the annual precession for "Martian" values, and > there is essentially no nutation. The annual aberration would be nearly the > same, though smaller in magnitude (about 16 arcseconds instead of 20, and > somewhat variable) The standard, widely available JPL ephemeride solutions > are accurate enough in 3d space that we could easily compute the SHA and Dec > of the Sun and the navigational planets (Venus, EARTH, Jupiter, and Saturn). > The variability in delta-T that we have on Earth would be much smaller on > Mars. > > Of course, the spherical triangle math doesn't change, so you could use HO > 229, if you want. One small thing to keep in mind: in terms of linear > distances, celestial navigation is twice as accurate on Mars because one > minute of arc difference in a star's altitude corresponds to about 3000 feet > on Mars instead of one nautical mile as on Earth (if you want, you could > define a "Martian nautical mile" with that shorter length). This is a > specific case of the general principle that the accuracy of celestial > navigation (by altitudes from the horizon, or equivalently zenith distances > from the vertical) decreases as the radius of curvature of the surface > increases. So it's less accurate on Earth, more accurate on Mars, and really > accurate (in terms of distance on the ground per minute of arc change in > altitude) on a small spherical asteroid. Similarly, on one planet, if the > local radius of curvature is greater, which implies a flatter portion of the > planet, then the accuracy of celestial goes down. Note that the "surface" > whose curvature is being measured is really the surface of the geoid. If you > have an asteroid shaped something like a diverging lense, concave on both > sides, the geoid would be flat on both sides, and then celestial > observations could not distinguish positions at all (except by small changes > in parallax of objects which are nearby). > > The density of the Martian air is less than 1% of that on the Earth, so > above about ten degrees altitude you could ignore refraction for standard > sextant observations (I say "about" because of the difference in atmospheric > composition). > > Finally, we get to Phobos and Deimos. Lunar distance observations would give > quite accurate time on Mars, though in some latitudes the moons are always > below the horizon. The positions of the Martian moons are known to high > accuracy, but this is a case where ultra-high accuracy would pay back great > benefits. The moons potentially offer the easiest and most accurate method > of finding one's position on Mars. Since many bright stars can be seen in > daylight, this could be done without a sextant. You would photograph Phobos > or Deimos among the background stars, measure its RA and Dec and then from > its known 3d position in space (assuming the Universal Time is already > known), you would draw a ray which would intersect the surface of Mars at > your location. That would give you a very accurate latitude and longitude on > Mars. So we would essentially be using lunar distances to get a position fix > without any need to determine a local vertical... where have I heard that > before??? > > For the foreseeable future, on Mars, it seems to me that "air traffic > control" is probably a better approach to position-finding. In other words, > you emit a signal trackable from orbit, and "they" keep track of your > position and beam it back to you. We've seen some nice examples of this in > the past few days as the Mars Reconnaissance Orbiter has photographed the > Phoenix lander descending under its parachute (SPECTACULAR! Coolest thing > I've seen in months!) and then its landing site complete with ancillary > components. And from that we know the exact position of Phoenix on Mars. > > -FER > > > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---