"The Sun and Moon both subtend a solid angle of 5.11×10-6 sr in the sky of Earth, or about 1/2,500,000th of the celestial sphere." 

 Could this be wrong? Using the formula provided an answer of 5.98×10-5 is given. 

 Another web based article appears to be correct -  

 "The sun and moon are both seen from Earth at a fractional area of 0.001% of the celestial hemisphere or 5.98×10-5 steradian [1]."