Sun rise and sun set time in Guide

Pierre Laporte Apr 26, 2001

Last night, at my observation site, I was checking the sun set at the time that GUIDE show me in Quick Info. Even if I had enter the exact location, there was a small but significant discrepancy, in term of few minutes, between what I observe and the time calculated by GUIDE Quick Info. I thing that there is two main reasons for such discrepancy. For one, the atmospheric refraction could explain a part of it. The second is the reference of the part of the solar disk which is used as reference. For example, does GUIDE consider the sun set time when the upper part or the lower part of the disk disappear below the horizon ? Or does it consider just the center ? Is the same rationale is use for calculating the nautic, civil and astronomic twilight ? Does GUIDE take in account the atmospheric refraction and if so, how such correction is made ?

Pierre Laporte

Centre d’Observation Astronomique des Mont-Notre Dame