Lunar Day Five
On the lunar surface tonight, let us begin with a look at Mare Serenitatus - the “Serene Sea” (Figs. 6.1 and 6.2). On its northeast shore, binoculars will have no trouble spotting the shallow ring of crater Posidonius. Almost flat from eons of lava flows, this crater shows numerous variations in texture along its floor in small telescopes. This huge, old, mountain-walled plain is considered a class V crater and could be as much as 3 billion years old. Spanning 84 × 98 km, you can plainly see Posidonius is shallow - dropping only 2,590 m below the surface. Tonight it will resemble a bright, elliptical pancake on the surface to smaller optics with its ring structure remaining conspicuous to binoculars throughout all lunar phases. However, a telescope is needed to appreciate the many fine features found on Posidonius’ floor. Power up to observe the stepped, stadium-like wall structure and numerous resolvable mountain peaks joining its small, central interior crater. It has its own interior rimae that is especially prominent to the east and a smashing view of trio Posidonius O, I, and B on the north crater rim. Adding crater Chacornac to the southeast makes things even more interesting! Did you spot the small punctuation of Daniell to the north?