Abstract:For the given latitude and solar declination,for no polar day or polar night,there are a pair of slope azimuth which are supplementary with each other on the slope surfaces whether their slope azimuths inclined to east or to west. These two slope azimuths are called the general critical slope azimuths. They can be expressed as follows:β1 =±arcsin(sinδ/cosφ),β2 =±(π-arcsin(sinδ/cosφ)).Between and beyond the two general critical slope azimuths, the match relations between sunrise and sunset hour angles of slope surfaces are different with each other. The match relation is ω1 =-ω0,ω2=ωs2for slope azimuth β between |β1 |and|β2|,or ω1 =ωs1,ω2=ω0 for slope azimuth β between -|β1 |and -|β2|. Beyond the two general critical slope azimuths, the match relations between sunrise and sunset hour angles of slope surfaces may alter with slope gradient and slope azimuth. There is a reference lope gradient α0 (where α0=acos(sinφ/cosδ)).Divided by α0,the match relations between sunrise and sunset hour angles of slope surfaces beyond the two general critical slope azimuths may be different.For the given slope gradient,there may be a slope azimuth where the no-horizental surface is polar day or polar night,so the the match relations between sunrise and sunset hour angles of slope surfaces may alter with slope azimuth. For the given slope azimuth , there is a reference latitude φ0(whereφ0=acos (|cosδ/cosβ|)). Divided byφ0,the match relations between sunrise and sunset hour angles of slope surfaces beyond the two general critical slope azimuths may be different.There may be one or two slope gradients where the no-horizental surface is polar day or polar night,so the match relations between sunrise and sunset hour angles of slope surfaces may alter with slope gradient.