The preliminary to the Steering and Sailing Rules state: ‘Risk of collision can, when circumstances permit, be ascertained by carefully watching the compass bearing of an approaching vessel. If the bearing does not appreciably change, such risk shall be deemed to exist.’ What is to be taken as ‘appreciable’ when considering a change of bearing is not defined, but the watchkeeper at sea in clear weather has his accumulated experience to call upon and no difficulty is normally encountered in deciding on the degree of risk, except in the case of using radar in thick weather, when the watchkeeper has not the benefit of long experience. The purpose of this note is to consider the conditions under which the nearest approach can be estimated and to determine the accuracy of the estimate.