This is just one of many possible solutions:
when green flag clicked
clear
go to x: (0) y: (0)
point in direction (90 v)
set [pi v] to [3.1415]
set [radius v] to [50]
move (radius) steps
point in direction (0 v)
pen down
forever
repeat (360)
move ((((radius) * (2)) * (pi)) / (360)) steps
turn ccw (1) degrees
end
turn cw (15) degrees
change pen color by (10)
end