The angle AOB *(at the centre)* is twice the angle ACB *(at the circumference)*, unless you are unlucky and have rounding errors as this page is using integer values!

Try the following:

- Move C. Neither of the measured angles should change.
- Move A or B. The angles should change - but AOB will remain double ACB.

Obviously this assumes you keep C in the same segment ie on the same side of AB; otherwise you would be looking at different angles.