Australian Sky & Telescope — July 2017

60 AUSTRALIAN SKY & TELESCOPE July 2017

LUNAR SHOWCASE

9.Pico and PitonThe titanic impact that created the

Imbriumbasin3.9billionyearsagoleftbehindanenormous

multi-ringed depression some 1,380 kilometres across. It also

fracturedthecrusttogreatdepth,andintimemoltenrock

escaped along these pathways and flooded the basin’s floor,

creatingMareImbrium.Todaylittleremainsofthebasin’s

innerring,butafewbitsofitjutupthroughthelavaplain

southofPlato,andtwooftheseareMonsPicoandMons

Piton (monsisLatinformountain).Theycastlong,dramatic

shadowsacrossthelavaplainwhenevertheSunisjustabove

the local horizon. But actually their heights are modest —

only about 2.5 km each.

10.ArchimedesLocated near the eastern shore of Mare

Imbrium, this ‘ghost crater’ has lost all of its rays, central

peaks and many other details. That’s because lava erupted from

below and inundated the floor with a layer at least 1.5 km

deep. Still, the 80-km-wide rim displays interesting terracing

that becomes more obvious when sunlight strikes it obliquely.

11. Montes Apenninus Distinct mountainous arcs to the
    east and south of Mare Imbrium mark what remains of
    the Imbrium basin’s outer rim. Of these, the Apennine
    Mountains (as they’re commonly known) are most
    prominent and rise up to 4.5 km above the surrounding lava
    plain. To the southwest of the Apennines, the peaks taper to
    an arc of summits north of Copernicus known as Montes
    Carpatus. Their extension to the northeast, abutting Mare
    Serenitatis, is called Montes Caucasus.
    12.PtolemaeusThisbig,obviouscrater(150kmacross)
    sitsalmostdead-centreinthelunardiskandshowsupbest
    aroundthetimesoffirstandlastquarter.Lookcloselyatits
    floor,andyou’llseeassortedshallowdepressionsthatare
    probably buried craters.
    13.ArzachelAlthough not particularly large (95 kilometres
    across),thiscraterhasalottooffertelescopicobservers.It’s
    relativelyfresh,witharugged,terracedrim.A1.5-km-high
    centralpeakrisesfromalava-coveredfloorthat’scrisscrossed
    by faulting.
    14.Rupes RectaMore commonly called theStraight Wall,
    this ancient fault slices across the eastern edge of Mare
    Nubiumfornearly110km.It’smostconspicuousnearthe
    timesoflocalsunrise(9daysafternewMoon)orsunset(22
    days).It’snotcompletelystraight—there’sashortbendat
    eachend.Thisfaultscarp,about300metreshigh,probably
    formed when the great mass of mare lava to the west caused
    theunderlyingrocktocrackandsink.
    15.Tycho Eventhoughit'snotespeciallylarge,85-km-wide
    Tychosportsthemostimpressivesetofspoke-likeraysof
    anycraterontheMoon.Thisbrightsplashradiatesforupto
    1,600kilometresinalldirections,andit’sespeciallyobvious
    aroundthetimeoffullMoon.Theraysaresoobvious

S MYSTERIOUS WHORL The unusual marking Reiner Gamma itself is only about 35 km wide, but narrow surface swirls extend far to the upper

left and lower right of this frame. At right is the 30-km-wide crater Reiner. Lunar Orbiter 4 took this photo in 1967.

S URIOUS AND COMPLEX C This high-magnification view shows

the complex terracing in the walls of Aristarchus crater. Above it lies the

meandering rille known as Vallis Schröteri (Schröter’s Valley).

WES HIGGINS

NASA / LPI