Literally, an optical phenomenon that occurs only rarely. As such, this would hardly seem to merit discussion. Yet the literature is replete with discussions of frequently occurring optical phenomena that are nevertheless described by authors as being rare. The explanation for this discrepancy is to be found in the difference between a rare event and a rarely observed event. The authors who characterize, say, a green flash as rare are undoubtedly doing so on the basis of their having rarely seen one. Yet green flashes can be seen frequently if one knows when and how to look for them. Clearly, a rarely observed event owes as much to the observer as it does to the event. For example, if one were to characterize the frequency of sunrises on the basis of most observer's frequency of observation, one would have to conclude that sunrises are very much rarer than sunsets. There certainly are rare optical phenomena, but there are many more common optical phenomena that are merely rarely observed due to personal habits.