 At noon the sun's change of altitude is very slow, so determining the exact time that the sun is at its zenith by direct observation is impossible, and therefore it is impossible to obtain an accurate longitude at the moment of Zenith .
<clarity>  At noon the sun's change of altitude is very slow, so determining the exact time that the sun is at its zenith by direct observation is impossible, and therefore it is impossible to obtain an accurate longitude at the moment of Zenith .
 At noon the sun's change of altitude is very slow, so determining the exact time that the sun is at its highest by direct observation is impossible, and therefore it is impossible to obtain an accurate longitude at the moment of Zenith .