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    Re: time divisions
    From: Marcel Tschudin
    Date: 2006 May 10, 17:31 -0500
    May be the books of David A. King, professor in Frankfurt, may help answer such questions. He is a specialist in astrolabs. He published a serie of books on "In Synchrony with the Heavens Studies in Astronomical Timekeeping and Instrumentation in Medieval Islamic Civilization"
    Brill Academic Publishers
    ISBN 900414188X
    ISBN 90 04 14651 2
    ISBN 90 04 12233 8
    ISBN 90 04 11367 3
    Perhaps they can be found in a library.

    On 5/11/06, coralline algae <corallina@gmail.com> wrote:
    My last question was too vague, so ignore it.
    I am interested in the historical aspects of time measurements as it relates to astronomy, mapping and navigation.

    For the first pass, restrict the historical time to late 1500's early 1600's when Tycho Brahe and Johannes Kepler lived. 

    A while back I read a book by J.L. Heilbron,  The sun in the Church: cathedrals as solar observatories.  Although I need to reread the book to refresh my memory, I seem to recall that they could use a beam of light  through a slit and some type of scale on the floor to measure time. 

    By using such an instrument,  endevouring  to measure the time interval of 24 hours or 1 day;  If we use the suns meridian passage, it is discovered that the time interval varies with the seasons, ie. the equation of time.

    Still we persevere and construct a variable rate timer which can count off 24 hours and divide each hour by 60 so we can now count up to 1440 minutes per day,  with the hope that at the start of the next days count we are reasonably close to our previous days full count.

    For the moment, I am ignoring the meaning of a second and focusing on the meaning of a minute during the late 1500's to the early 1600's.

    Assuming we can build a timer with an accuracy of 1 minute from day to day or at least can be corrected in some manner, we have a time instrument that divides the day into 1440 parts and can be used for various scientific studies.

    Now how do we decide to set the length of a day? If we use the interval of the sun's meridian passage on the equinoxes is that a reasonable comprimise?  Otherwise how could we make a decision about the length of a minute or a second and adjust a timepiece to run so that it counts off 1440 minutes or 84600 seconds per day?

    Although I don't use a kamal, crossstaff or astrolabe to measure angles, the way our predecessors accomplished similar tasks is of great interest to me.   Keplers printing of the Rudolphine tables was based on T. Brahe's data  which was accurate to 2 or 3 minutes of arc, all done without the aid of optics.

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