This year Venus was slim crescent in March when the Earth-Venus was about [tex]L=42*10^6 km[/tex] (the minimum distance) To find its angular diameter we apply the trigonometry [tex]\tan \alpha = D/L[/tex] where D=21000 km is the planet diameter and L is the distance to the planet. For small angles we consider [tex]\tan \alpha \approx \alpha [/tex] Thus the angular diameter of venus when it was slim crescent was [tex]\alpha =D/L =21000/(42*10^6) =2.9*10^{-4} rad[/tex] In degrees this is [tex]\beta =\alpha*180/\pi = 0.0167 degree[/tex] We know that 3600 seconds correspond to 1 degree. Therefore [tex]1''=1/3600 =2.8*10^{-4} degree[/tex] Hence [tex]\beta = 0.0167/2.8*10^{-4} =60.12''
[/tex] Therefore the angular diameter of Venus when it was slim crescent was 60''.
