Possibly Larrieu's Dam in the Crater Polybius
When the moon is around 6-days old, the rising light of the sun first hits the crater
Polybius K, where it catches an internal steep wall that shines as a thin bright line in
the telescope. This was first recorded by A.C. Larrieu, a French astronomer, on
May 27, 1955. He reported it as “... It appears to me, in the telescope, like a bridge
or an Hydraulic barrier!, the bridge was brilliant, very thin and rectilinear.”
There was some discussion during the mid-1950s that it might be an artificial structure,
but such discussions have long passed due to our increased knowledge about the
moon. Still it is fun to try and find this imaginary “Dam” or “Bridge” or “Ramp” in the
telescope. No two drawings of Larrieu’s Dam look the same. This is because very
slight differences in the sun’s angle easily creates large changes in the morning lighting
on the rim and ridges of this small crater.
Update -- Lunar simulations indicate that what I labeled as Larrieu's Dam my not be.
The simulations show that the Dam would not show up until several hours after my
drawing was made. A target for further observations for sure.