Ruby 1.8.x, 93
r=$_.to_f
q=0
e=r-1
(p(('*'*(n=1|2*(r*r-e*e)**0.5)).center r+r)
q+=n+n
e-=2)while-r<e
p q/r/r
Run with $ ruby -p piday
Ruby, 93
r=$_.to_f
q=0
e=r-1
(p(('*'*(n=1|2*(r*r-e*e)**0.5)).center r+r)
q+=n+n
e-=2)while-r<e
p q/r/r
Run with $ ruby -p piday
Ruby 1.8.x, 93
r=$_.to_f
q=0
e=r-1
(p(('*'*(n=1|2*(r*r-e*e)**0.5)).center r+r)
q+=n+n
e-=2)while-r<e
p q/r/r
Run with $ ruby -p piday
Ruby, 8293
e=r=getsr=$_.to_f
q=0
e=r-1
(e%2>0&&putsp(('*'*(1|2*n=1|2*(r*r-e*e)**0.5)).center r+r)
q+=n+n
e-=1=2)while e>=-r<e
p q/r/r
Run with $ ruby -p piday
Ruby, 82
e=r=gets.to_f
(e%2>0&&puts(('*'*(1|2*(r*r-e*e)**0.5)).center r+r)
e-=1)while e>=-r
Ruby, 93
r=$_.to_f
q=0
e=r-1
(p(('*'*(n=1|2*(r*r-e*e)**0.5)).center r+r)
q+=n+n
e-=2)while-r<e
p q/r/r
Run with $ ruby -p piday
Ruby, 82
e=r=gets.to_f
(e%2>0&&puts(('*'*(1|2*(r*r-e*e)**0.5)).center r+r)
e-=1)while e>=-r