Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Navigation without Leap Seconds
    From: Fred Hebard
    Date: 2008 Apr 15, 19:41 -0400

    Yes, it does.  One gathers elevation information with radar ranging;
    it's the same problem, you're just at a different elevation, so
    there's a larger (!) dip correction.  It was the method proposed by
    Weems, et al, in the delightful book, "Space Navigation Handbook,"
    Navpers92988,  US Govt Printing Office: 1962 0-628762.
    
    
    On Apr 15, 2008, at 7:27 PM, glapook@pacbell.net wrote:
    >
    > Gary LaPook writes:
    >
    > But that doesn't solve the problem. The only reason that CN works on
    > the earth is that the direction of "up" varies with your position on
    > the earth. The altitudes measured on earth (and in aircraft) rely on
    > the direction of "up" for the measurement. The sea horizon used with a
    > marine sextant is where it is due to the local gravitational field
    > which causes water to assume a shape at right angles to "up" and
    > gravitational "down." A bubble sextant uses a bubble to sense "up."
    > Because local "up" changes at a constant rate of one nautical mile per
    > minute of altitude we can find our place on or above the surface of
    > the earth. This relationship does not hold on the way to the moon.
    >
    > gl
    >
    > On Apr 15, 2:12 pm, Fred Hebard  wrote:
    >> I believe they measured altitudes from a limb of the Earth, more-or-
    >> less in the "normal" way.
    >> On Apr 15, 2008, at 4:00 PM, Gary J. LaPook wrote:
    >>
    >>> Gary LaPook wrote:
    >>
    >>> If I remember correctly, the Apollo spacecraft had a sextant on
    >>> board used to mesure angles of celestial bodies in order to compute
    >>> their position in space on the way to the moon, (maybe only as a
    >>> backup.)
    >>
    >>> gl
    >>> Fred Hebard wrote:
    >>
    >>>> So it would have to be sun/moon/planet-star distances. I suppose
    >>>> those are limited by the low degree of parallax of the planets and
    >>>> sun, not to mention one has to know where one is on earth to
    >>>> determine the "position" of other bodies in the solar system,
    >>>> which I guess would be a circular argument. On Apr 15, 2008, at
    >>>> 12:54 PM, Lu Abel wrote:
    >>
    >>>>> Fred: You're right about traditional surveying. But your proposal
    >>>>> is to use star-to-star distances to locate one (if I understand
    >>>>> correctly) in 3-D space relative to some very distant stars.
    >>>>> Imagine a couple of stars several hundreds of light-years away
    >>>>> (that's on the order of 10^20 cm). Suppose I move a few cm closer
    >>>>> to them. By how much would the angle between them change? Not by
    >>>>> much at all. Lu Fred Hebard wrote:
    >>
    >>>>>> Lu, Why billionths of an arcsecond? One arcsecond gets one to
    >>>>>> 1/60th of 100 feet in traditional surveying, or about 50 cm. One-
    >>>>>> thousandth of an arcsecond would drop one to 5 mm. I wonder if
    >>>>>> refraction is a problem here.  Fred On Apr 15, 2008, at 12:33
    >>>>>> PM, Lu Abel wrote:
    >>
    >>>>>>> Fred: In theory, yes; in practice, no. To position oneself
    >>>>>>> using star-star distances would require require measuring
    >>>>>>> angles to billionths of an arc-second. Maybe something an
    >>>>>>> astronomer could do, but not something you or I are going to do
    >>>>>>> with our sextants! BTW, I remember a conversation with a radio-
    >>>>>>> astronomer about 20   years ago where he said that his team had
    >>>>>>> measured the distance between two radiotelescopes on opposite
    >>>>>>> sides of the US to within a cm or so using a technique called
    >>>>>>> long-baseline interferometry. But the whole experiment took
    >>>>>>> them a year or so... Lu Abel Fred Hebard wrote:
    >>
    >>>>>>>> Completely unrelated, but stemming from the same article. The
    >>>>>>>> author states that height can only be known to some few cm or
    >>>>>>>> whatever because of variations in gravity, if I remember
    >>>>>>>> correctly. It would seem that this is due to our tradition of
    >>>>>>>> assuming we are on the surface of a spheroid or ellipsoid when
    >>>>>>>> doing navigation. Confining ourselves to a surface makes the
    >>>>>>>> trig easier, but couldn't one position oneself with greater
    >>>>>>>> accuracy (with feet firmly planted on earth, not on a boat)
    >>>>>>>> using only stars or stars plus the sun, ignoring the earth's
    >>>>>>>> horizon, by measuring star-star distances? Make it a true 3-D
    >>>>>>>> problem. Or would uncertainties in the positions of stars
    >>>>>>>> still hamper ones efforts, especially uncertainty in their
    >>>>>>>> distance from us? Fred Hebard On Apr 14, 2008, at 9:50 PM,
    >>>>>>>> frankr...@HistoricalAtlas.net wrote:
    >>
    >>>>>>>>> The fascinating article which Fred Hebard linked: http://
    >>>>>>>>> www.physicstoday.org/vol-59/iss-3/p10.htmlincludes a
    >>>>>>>>> detailed discussion about the problems of gravitational time
    >>>>>>>>> dilation and extremely accurate clocks. That's the main
    >>>>>>>>> topic, and it's great stuff. The article also mentions leap
    >>>>>>>>> seconds and navigation: "Celestial navigators --that
    >>>>>>>>> vanishing breed-- also like leap seconds. The Global
    >>>>>>>>> Positioning System, however, cannot tolerate time jumps and
    >>>>>>>>> employs a time scale that avoids leap seconds." So here's my
    >>>>>>>>> question: what's the best way of doing celestial navigation
    >>>>>>>>> if leap seconds are dropped from official time-keeping? I
    >>>>>>>>> don't think it should be all that difficult to work around,
    >>>>>>>>> but I'm not sure what the best approach would be. Assume we
    >>>>>>>>> get to a point where the cumulative time difference is, let's
    >>>>>>>>> say, 60 seconds (that shouldn't happen for decades, so this
    >>>>>>>>> is just for the sake of argument). Should we treat the
    >>>>>>>>> difference as a 60 second clock correction before working the
    >>>>>>>>> sights? Or should it be a 15 minute of arc longitude
    >>>>>>>>> correction after working the sights? Or something else
    >>>>>>>>> entirely?? -FER Celestial Navigation Weekend, June 6-8, 2008
    >>>>>>>>> at Mystic Seaport Museum:www.fer3.com/Mystic2008
    > >
    
    
    
    --~--~---------~--~----~------------~-------~--~----~
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com
    -~----------~----~----~----~------~----~------~--~---
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site