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: New Moon, Perigee, and Solstice
    From: Rodney Myrvaagnes
    Date: 2003 Dec 29, 23:55 -0500

    On Mon, 29 Dec 2003 20:36:32 +0000, Trevor J. Kenchington wrote:
    
    >Rodney,
    >
    >You wrote:
    >
    >>>>A practical interface for this question would be tide tables for a
    >>>>mid-Pacific Island, such as Canton and Enderbury. That should give a
    >>>>good handle on the phase of the tidal bulge as it would be in a
    >>>>uniformly water-covered planet. I don't have such a tide table.
    >>>>
    >>>
    >>>Neither do I but I don't think it would show what you expect. The tides
    >>>of the mid-Pacific are dominated by amphidromic systems just as much as
    >>>those of the North Sea are. They do not resemble a "tidal bulge" on a
    >>>planet that lacked land masses. (I'm not sure that a planet without land
    >>>would have recognizable bulges anyway, unless the ocean was also
    >>>extremely deep and covering a very small solid core.)
    >>>
    >>>
    >> Myabe it wouldn't show the bulge. The bottom is anything but uniform.
    >> But I think the bulge would show in a quite recognizable form in an
    >> ocean over a sperical core, even if the ocean were as shallow as ours.
    >> The friction (hence, the "Q") would be different.
    >
    >
    >I'd not want to be dogmatic on this but my limited understanding is that
    >this is not a matter of friction but of water depth. The wavelength of a
    >semi-diurnal tidal "bulge" would clearly have to extend over 180 degrees
    >of longitude. From tropical to mid-temperate latitudes, that means that
    >the wave would be so long that even if the ocean were a uniform 6000
    >metres deep, the "bulge" would respond as a shallow-water wave. (Even
    >tsunamis do that in the real ocean and their periods are only 15 minutes
    >or so.) Thus, the speed of propagation of the tidal "bulge" would be
    >determined by water depth, not by the rate of rotation of the Earth
    >under the Sun and Moon. The tide generating forces would, therefore, not
    >be able to drag a "bulge" around with them and instead would set up the
    >sort of complex of resonance patterns that we see in the real open oceans.
    >
    >It is not too hard (even for me!) to figure out how deep the ocean would
    >have to be to allow a tidal "bulge" to keep up with the Moon. At the
    >Equator, it would need an ocean nearly as deep as the radius of the
    >planet. (And I don't want to even contemplate the physics of wave
    >propagation when the circuit of the seabed is almost zero and that of
    >the surface is 22,000 miles.)
    >
    
    The depth would certainly have an effect on surface waves as you say,
    but it would affect the amplitude, not the frequency. The solution
    would still be at the frequency of the driving force in the steady
    state.
    
    That is the actual case now, with all the extra complications. It would
    not be less so if the complications were removed.
    
    
    
    
    
    
    
    
    
    Rodney Myrvaagnes                                                              
                          Opinionated old geezer
    
     "It is, of course, quite true that no great amount of skill is required to 
    navigate a ship most of the time, and
    on those less frequent occasions when a higher level of competence is 
    desirable luck may suffice. If that runs out there is always insurance..." 
    __The late Captain Richard Cahill
    
    
    

       
    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