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: Definition of the twilight intervals
    From: Brad Morris
    Date: 2015 Aug 17, 01:06 -0400

    Hello Rob

    I think they are discussing this incorrectly.   The Civil Twilight time figure given in the nautical almanac is an instant in time defined by the sun's orb being a set number of degrees below the observable horizon.  There is no "interval" because the sun is at that angular displacement for just one instant.  Similarly for nautical twighlight and astronomical twilight given in the Nautical Almanac, the times given are for the sun at a particular displacement below the horizon.

    Now what are those displacements?
    -6°,-12° and -18°, for Civil, Nautical and Astronomical respectively.

    But Dutton "Navigation and Piloting" says that the period, which ends at each of those given instants, also has a start.  Article 2810 (in my copy, yours may vary) defines the angular start of that period for ALL THREE as the same value, to wit, the upper limb of the sun is at 0°.

    So per Dutton, we have
    Civil Twilight Period 0° to -6°
    Nautical Twilight Period 0° to -12°
    Astronomical Twilight Period 0° to -18°

    Dutton was "the" book used to teach at the US Naval Academy, when celestial was king.  So if you want to know if the periods are overlapping or not, the definitive answer is that the periods do indeed overlap.


    On Aug 16, 2015 10:32 AM, "Robert H. van Gent" <NoReply_RobertH.vanGent@fer3.com> wrote:

    There is a discussion at the moment at 
    on the question whether civil, nautical and astronomical twilight are sequential (i.e. non-overlapping) intervals as many internet sources claim or that they are overlapping intervals (as is claimed in modern editions of Bowditch's The American Practical Navigator.
    Any thoughts on this matter?
    Rob van Gent ("Astrolynx")

    View and reply to this message

    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