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    Sexagesimal conversion
    From: Paul Hirose
    Date: 2017 Jul 27, 21:03 -0700

    In conversions from degrees or hours to sexagesimal, there are some fine
    points which are easy to overlook. For instance, convert 1.99992 to
    sexagesimal at 1 second precision.
    
    1. Obviously, degrees = 1.
    2. Minutes = .99992 * 60 = 59.9952 = 59 whole minutes.
    3. Seconds = .9952 * 60 = 59.71. Rounded to the nearest integer, that's
    60, so the angle will display as 1°59′60″. Oops.
    
    One solution is to convert the angle to the nearest multiple of the
    desired precision. In this case, precision is one second, so one "unit
    of precision" is 1°/3600 = .000277778°. When expressed in that unit, the
    angle is 1.99992 / .000277778 = 7199.71 units. Round to the nearest
    whole unit. (That can be accomplished by adding half a unit, then
    truncating the fractional part.) Thus the rounded angle is 7200 units.
    
    Convert back to degrees: 7200 * .000277778 = 2.00000°. When steps 1
    through 3 above are applied to that value, the result is the correct
    2°00′00″.
    
    Another issue is what happens when the angle rounds up to 360°. Do you
    prefer modulo 360 output? (I.e., zero instead of 360.) Also, bear in
    mind that the same routine may be useful to convert decimal hours to
    DMS, so perhaps the desired modulus can be be passed as a parameter to
    the routine. In that case, a modulus of 0 can be used to signal that no
    modulo adjustment is to be applied.
    
    Remember to adjust date if time rounds up to zero. For instance, the
    same instant of time may display as 23:59:59.9 or 00:00:00 on the next
    day, depending on the precision of the sexagesimal conversion.
    
    That behavior may be seen in my Lunar4 program. If you set the time a
    tenth second before the end of a year, and precision to .01′, the output
    shows the time you entered. But if you reduce precision to 1′, the time
    rounds up to Jan. 1 at 00:00:00. That happens because time is rounded to
    a precision comparable to the angular precision you request.
    
    (Actually, at .01′ precision the seconds display is 59.88, not 59.9,
    because the program automatically sets time precision 15 times greater
    than angle precision. Thus .01′ corresponds to .01 / 15 second, which is
    not a "nice" interval. Nevertheless, time is rounded to that increment
    for display. The roundoff *only* affects the display — for computations
    the program uses what you entered.)
    
    In some cases (altitude and declination, for example) you may have to
    deal with negative angles. The most foolproof method is to process the
    value as if it were positive, then restore the sign on output.
    
    Leap seconds are another thing to consider. If a minute ends in a leap
    second it's proper for seconds to equal or exceed 60. One solution is to
    pass the sexagesimal conversion routine a "step second" parameter. It's
    1 if inside a leap second, 0 otherwise. In the former case, pass decimal
    hours minus one second to the sexagesimal conversion routine, which
    computes seconds in the normal way, then adds the "step second"
    parameter to the computed seconds.
    
    E.g., UTC time is 24.000028 hours (= 23:00:60.1). Subtract 1 second:
    24.000028 - .000279 = 23.99975. Pass that last value to the sexagesimal
    conversion routine, and 1 as the step seconds parameter.
    
    1. Hours = 23.
    2. Minutes = 60 * .99975 = 59.985 = 59 whole minutes.
    3. Seconds = 60 * .985 = 59.1.
    4. Add the step seconds parameter to seconds: 59.1 + 1 = 60.1.
    
    All the operations I've described are implemented in my SofaJpl
    astronomy DLL for Windows. It also includes composite formatting for
    sexagesimal quantities, where a string controls the output format, as in
    the C language printf() function. This is a big convenience in
    applications like Lunar4, where a great quantity of sexagesimals must be
    formatted neatly, the flavor (D.d, DM.m, or DMS.s) and precision being
    unknown until runtime because they're under user control.
    http://home.earthlink.net/~s543t-24dst/SofaJpl_2_0/index.html
    

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