7 angled at C, ADB is the semicircle described on AB as diameter, AEB the circular arc described with centre C and radius CA= CB.
Make d bf=pc, cg=pc. From f, with the radiusfa, draw the circular arc ah; from g, with the radius ga, / B
draw the circular arc ak.
Then HA' will cut the circular arc in J.
The very faint circular arc could be a 23 or 22° halo.