A feature on the moon known as Purbach’s X (aka Purbach Cross, Werner Cross or Lunar X) is visible several times a year.
Purbach’s X is at the conjunction of three craters; La Caille, Blanchinus and Purbach (the latter named for Georg Purbach, a 15th century astronomer sometimes called the father of western mathematical and observational astronomy).
The feature is an x-shaped illumination effect involving the rims and ridges at this conjunction when they are near the terminator. These rims and ridges are higher than the surrounding area near the terminator so they are lighted and appear as an “X”. The feature is only visible for about four hours. Visibility starts
about 18 hours and 21 minutes prior to First Quarter moon.
The moon feature known as Purbach’s X will be visible starting Friday, August 21 at 8:11 PM. Sunset on that date is at 7:51 PM, civil dusk is at 8:17 PM, nautical dusk is at 8:47 PM and astronomical dusk is at 9:17 PM.
Therefore viewing of Purbach’s X should be good after about 8:30 PM and as late as midnight.