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    Re: History of Twilight
    From: Robert H. van Gent
    Date: 2014 Sep 26, 08:22 +0000

    Hi Frank,
    In Bode's "Erklärung zum Gebrauch der Ephemeriden und der astronomischen Tafeln", 
    page 22, in the 1776 ('first') edition of his _Astronomisches Jahrbuch oder 
    Ephemeriden [etc.]_ (published in 1774) - which you can easily find on Google 
    Books - he wrote:
    "Diese zweyte Dämmerung nennen wir die gemeine oder bürgerliche, weil dies die Zeit 
    ist, da man in Wohnungen, welche nicht gerade gegen den Ort der auf- oder 
    untergehenden Sonne gekehrt sind, Licht anzuzünden genöthiget ist; das 
    Gegentheil findet bey der Morgendämmerung statt."
    He thus clearly links the end of civil twilight with the moment when it necessary 
    to light the lamps in houses for indoor activities.
    Comrie's involvement in the introduction of nautical twilight follows from his preface 
    to the 1937 edition (published in 1936) of  _The Nautical Almanac Abridged 
    for the Use of Seamen_ and the tables found on pp. 164-203. 
    This information is based on notes which I made when I inspected a copy of this 
    publication in a Dutch maritime museum some time ago - I should perhaps have 
    made photocopies of these pages but making photocopies was then somewhat problematic.
    I will look up the relevant page numbers in the _Nautical Almanac and 
    Astronomical Ephemeris_ for the same year which is easier to access for me.
    Thanks for the link to the Kimball article - it has a lot of interesting references.
    Best wishes,
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed
    Sent: maandag 22 september 2014 22:44
    To: Gent, R.H. van (Rob)
    Subject: [NavList] Re: History of Twilight
    Robert, thank you. That confirms what I had found so far and fills in some key details.
    You wrote:
    "Bode named it 'gemeinen Dämmerung' or 'bürgerlichen Dämmerung' and defined it as 
    the moment when the twilight arc (the earth's shadow on the atmosphere) 
    passed through the zenith, thus conveniently marking the time when artificial 
    lighting was needed for indoor activities. Bode adopted Lambert's value of 
    nearly 6.5 degrees"
    I am sure you meant that light was needed for 'outdoor' activities. What you've 
    described here is exactly what's written in H. H. Kimball's article, 
    publiished in 1916. Kimball notes that this process of trying to spot when 
    the twilight arc crosses the zenith was nearly impossible in practice (I've 
    never detected the twilight arc anywhere above 20° altitude!) which was why 
    it made sense to pick some arbitrary altitude below the horizon. He noted 
    that there were various choices in use at the time, but he chose 6°. His 
    choice seems to have been influential; many later publications reference his. 
    You added:
    "Nautical twilight was first introduced in 1936 by Leslie John Comrie"
    That date certainly fits the evidence. Have you been able to find actual 
    documentation on Comrie as the source? I understand that he was director of 
    the almanac office at this time, but did Comrie personally introduce this 
    concept? There was an awful lot going on at this time --the transition to the 
    New Navigation was in full swing, tables with GHA were just being introduced, 
    air almanacs were rapidly expanding-- and though I am confident that you're 
    right that HMNAO was the first almanac office to introduce nautical twilight 
    since the earliest references to nautical twilight around 1940 attribute it 
    to British almanacs, it seems to me that there may be a little more to it.
    You noted that nautical twilight was introduced "as a convenient means of dividing 
    the interval between the end/begin of civil and astronomical twilight in two 
    nearly equal parts. Regular tabulations of the times of nautical twilight 
    first appeared in the 1937 edition of _The Nautical Almanac and Astronomical 
    Ephemeris_ and its abridged version for navigators."
    And for modern celestial navigators and navigation enthusiasts, I think it's 
    important to remember that this is literally all there is to it. There's no 
    big science to it. As far as we know, the end of nautical twilight was not 
    determined by any careful series of observations of the sea horizon trying to 
    determine exactly when the horizon disappears, although there may have been 
    some rough experiments along those lines. That 12° limit is just arbitrary. 
    It splits the difference between civil and astronomical twilight, and that's 
    all it means.
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