The Moon is about 4x10^8 m from the Earth and has a diameter of about 3.2x10^6 m

Question

The Moon is about 4x10^8 m from the Earth and has a diameter of about 3.2x10^6 m. You want a radar antenna which covers the Mmoon with the central spot of its diffraction pattern. If the radar wavelength is 10 cm, what is the diameter of the radar antenna?
a response to this was made at this link below
http://www.cramster.com/answers-feb-11/physics/diffraction-problem-moon-4x108-earth-diameter_1157580.aspx#post-1159548-comment-55184

but I don't understand where the 1.22*lamda/d came from
I especially don't understand where the 1.22 value came from

Explanation / Answer

diameter of the moon D = L   ........... (1) diffraction by a circular aperture,         sin = (1.22)/d for small angles, sin ˜ therefore, = (1.22)/d    ........... (2) substitute the eq (2) in eq (1), we get              D = L[(1.22)/d]   ............... (3) distance between moon and earth , L = 4*108 m wavelength = 10 cm = 10*10-2 m diameter of the moon , D = 3.2*106 m from eq (3), the diameter of the radar antenna is          d = L[(1.22)/D]    .............. (4) substitute the given data in eq (4), we get         d = (4*108 m)[(1.22)(0.1 m)/(3.2*106 m)]            = 15.25 m

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