Often said of people who meet for a brief but intense moment and then part, never to see each other again. These people are like two ships that greet each other with flashing lights and then sail off into the night. From a poem by Henry Wadsworth Longfellow.
It means two entities that meet once or erratically. | From Dictionary.com - Individuals who are rarely in the same place at the same time. For example, 'Jan works the early shift and Paula the late shift -- they're two ships that pass in the night'. This metaphoric expression comes from Henry Wadsworth Longfellow's poem "The Theologian's Tale" (published in Tales of a Wayside Inn, 1873).
Tales of a Wayside Inn. Part iii. The Theologian’s Tale: Elizabeth. iv.
(a poem by Henry Wadsworth Longfellow)
Ships that pass in the night and speak each other in passing;
Only a signal shown and a distant voice in the darkness;
So on the ocean of life we pass and speak one another,
Only a look and a voice; then darkness again and a silence.