Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    Re: New Moon, Perigee, and Solstice
    From: Trevor Kenchington
    Date: 2003 Dec 29, 20:36 +0000

    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.)
    Corrections from the physicists on the list would be appreciated.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

    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