Original:  However, it is possible to determine the time of zenith for longitude with a useful accuracy by performing a mean time of observation when the sun is on its ascent and descent prior to and following its moment of Zenith .
Revised:  However, it is possible to determine the time of culmination for longitude with a useful accuracy by performing a mean time of observation when the sun is on its ascent and descent prior to and following its moment of Zenith .
Edit intent: style