In predicting the range at which a light can be seen, one should first determine the geographic range to compare this range with the luminous range, if known. If the geographic range is less than the luminous range, the geographic range must be taken as the limiting range. If the luminous range is less than the geographic range, the luminous range must be taken as the limiting range.
These predictions are simple when using the U.S. Coast Guard Light List because only nominal ranges are tabulated. If one is approaching a light, and wishes to predict the time at which it should be sighted, he first predicts the range. It is then good practice to draw an arc indicating the visual range. The point at which the course line crosses the arc of visual range is the predicted position of the vessel at the time of sighting the light. The predicted time of arrival at this point is the predicted time of sighting the light. The direction of the light from this point is the predicted bearing at which the light should be sighted. Conversion of the true bearing to a relative bearing is usually helpful in sighting the light. The accuracy of the predictions depends upon the accuracy of the predicted a range, and the accuracy of the predicted time and place of crossing the visual range arc.
If the course line crosses the visual range arc at a small angle, a small lateral error in track may result in a large error of prediction, both of bearing and time. This is apparent if the vessel is farther from the light than predicted, in which case the light might be passed without being sighted. If a light is not sighted at the predicted time, the error may be on the side of safety. However, such an interpretation should not be given unless confirmed by other information, for there is always the possibility of reduced meteorological visibility, or of the light being extinguished.
When a light is first sighted, one might determine whether it is on the horizon by immediately reducing the height of eye by several feet, as by squatting or changing position to a lower height. If the light disappears, and reappears when the original height is resumed, it is on the horizon. This process is called bobbing a light. If a vessel has considerable vertical motion due to the condition of the sea, a light sighted on the horizon may alternately appear and disappear. This may lead the unwary to assign faulty characteristics and to error in its identification. The true characteristics should be observed after the distance has decreased, or by increasing the height of eye of the observer.
