* @astro.cal
* ? sqrt(Gn*Msun*year^2/au^3)
    6.282940487601840    
* ? 2*pi
    6.283185307179586    
    
T^2=a^3 ???
my favorite planet is Earth: 1^2=1^3 (check)
Earth period = 1 year; semi-major axis = 1 AU