关键词: 等日照时间, 等日照方位, 坡地

Abstract: In this paper, the authors study the existing conditions, ranges and distributional laws of the EID and ESA on a slope with an analytical method. EID (Equal Insolation Duration) means that the insloation duration on a slope is not variable according to the data. In the other words, the insolation duration on a slope is equal in every day of a year. ESA (Equal Sunshine Azimuth) is the azimuth of a slope on which EID is existing. The insolation duration of a slope is relative to the sunrise and sunset hour angles ω₁, ω ₂ . ω₁ and ω₂ are composed of the sunrise and sunset hour angies ω_s1 , ω_s2 on the non horizental surface and the sunrise and sunset hour angles - ω₀, ω₀ on the horizental surface. There are many different combined relations. For a slope with given latitude, gradient α and slope azimuth β , the combined relation can vary with the sun declination δ . When one combined relation changes into another one, there are two critical sun declinations δ_c . They meet the following condition: tan²δ_c= sin² cos²φ/(1- sin²βcos²φ) Becaus the sun declination δ vary on the range of (-23.45°,23.45°), the following relation is necessary for δ_c existing: sin ²βcos ²φ≤ sin²23.45°On the other hand, when sin ²β cos ²φ> sin ²23.45°,we can not get the critical sun declination δ_c.

Key words: EID, ESA, slope