The comparison of the statistical characteristics of the results of observations of zenith distances in the night and light time of the day is given. On the basis of experimental observations of zenith distances at night, conducted over several years, it was established that in the mountainous region refractions for the same direction have a difference of 3.3. "When studying the effect of vertical refraction in mountain conditions it was noted that when comparing series the refraction angle is 1.5 "less at night than in the daytime. It is shown that in the case of mutual observations from a signal or from a pointed vertex (tripod) and a flat top (tripod), the assumption of equality of refraction angles at the ends of the line gives an error in excesses of -0.30 m.
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