Please improve the consistency in the following text:
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.
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 culmination.