A Landfall Fix is the position of a sailboat calculated by using a Circular LOP (line of position) and a bearing from the object.   One can obtain the Circular LOP by noting when a lighthouse lantern's light is first seen.  The light's bearing is then taken.   By knowing the height of the lighthouse above tide level and the height of the observer's eye level above sea level, an approximate distance for the first observance can be determined.  This distance is used to draw a Circular LOP on the navigational chart.  The bearing from the lighthouse will then give you your position or Landfall Fix.  If the lighthouse height is entered as zero, the calculator will estimate the distance to the horizon.

The strength of the light and clarity of the night air are also factors which may inhibit the lantern's observance.  You may observe the glow or loom of the lighthouse before you see the light of the lighthouse's lantern.  It is the lantern's light which is used in this calculation.  The lantern's light, even if constant,  may flash on and off as the sailboat rises and descends upon the waves.   The point where the lantern's light is first observed, or it is observed flashing, is used to determine the boat's position.