An Abstract in Honor of the 87% Moon

Among the eight conventionally named lunar phases — new, waxing crescent, first quarter, waxing gibbous, full, and their counterparts on the way back down — there exists what we believe to be an unloved middle child: the moon as it appears approximately three nights before fullness, when its illuminated face has reached, by our reckoning, eighty-seven percent. The present authors, all members of a small society that meets on the second floor of the Briarcliff Public Library on alternating Thursdays, propose that this particular configuration of light — neither the showy full disk nor the demure half — deserves separate consideration, separate naming, and (the bolder among us would say) a small annual tribute of its own.

The 87% moon rises in the late afternoon, often while the sky is still lit, and it lingers — pale, faintly chrome — until well after midnight. It does not announce itself. One steps outside to bring in the trash bins or to walk a dog and finds it has been there for hours, holding a steady amount of light. There is, in this phase, a generosity that the full moon (with its theatrical entrance) does not quite manage. The almost-full moon trusts you to look up on your own.

We document, in Figure 1 below, the precise geometry of the eighty-seven percent terminator, and offer the reader a means by which to navigate to adjacent illuminations and back. We invite — gently, in the manner of a librarian closing a window — a reconsideration of the gibbous interval, and we welcome correspondence from anyone who has, on some recent evening, noticed and been kept company by exactly this moon.