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    Amplitudes
    From: Jeremy C
    Date: 2010 Jun 1, 19:26 EDT
    George brought up an interesting topic in a previous post about measuring amplitudes.  Over the years I have found amplitudes to be far easier to shoot and reduce by hand than azimuths, but less accurate as well.  I have taken azimuths within an hour or two of a amplitude of the same body and had different compass errors which should not have existed, and in all cases, I tend to believe the azimuth over the amplitude.
     
    Amplitudes are shot when the center of the body is on the celestial horizon.  As George notes, this is not the visible horizon, but some arc-minutes off of the visible horizon depending on the body.  (To be fair, you may shoot the body when the center on the visible horizon, but a correction needs to be applied which increases as you and the body get further from the equator.)  I was never taught, or have I read, to take into account any height of eye when taking the measurement (dip correction). 
     
    I was taught to observe the Amplitude of the sun when the lower limb was 1/3 of the sun's diameter above the visible horizon.  I was also told that stars and planets are observed when 1 sun's diameter above the visible horizon, and the moon when the upper limb is on the visible horizon.
     
    I have shot amplitudes of the sun, moon, and two planets.  Shooting the moon on the celestial horizon is quite the challenge, but it does work.  It is the only body that I prefer to shoot on the visible horizon.  Planets are very difficult because they tend to disappear into the haze of the horizon below about 2 degrees of altitude.  I have never been able to shoot an amplitude of a star because even mighty Sirius disappears before reaching amplitude height.  The sun is the easiest body to observe but you need to watch out for "amplitude clouds" as we called them.  Those were clouds forming as the earth cooled and obscured the sun from view as it decended.
     
    For reduction, I have tended to avoid the tables due to the need to interpolate.  While interpolation is not difficult, it is far more time consuming than a straight calculation.  I find it far easier to just use the formula as even a basic user of any calculator with trig functions can pull it off with little trouble. 
     
    The other major advantage of amplitudes over azimuths is not needing exact time as the only almanac entry is declination which tends to change slowly enough to require little time accuracy.
     
    While I still shoot amplitudes at every opportunity, I rely on azimuths as my preferred method of compass correction at sea.
     
    Jeremy
     
       
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