When Galileo first looked at the Moon through his tiny hand-made 30 mm refractor (hardly better than a toy telescope of today) he was struck by the view of a rugged world packed with deep craters, jagged mountains and gorge-like valleys. In his book "Sidereus Nuncius" (Starry Messenger, 1610) Galileo wrote "....the surface of the Moon is neither smooth nor uniform, nor very accurately spherical, as is assumed by a great many philosophers....it is uneven, rough, replete with cavities and packed with protruding eminences."
Regardless of the type of telescope used - be it a humble 30 mm refractor at low magnification or a 300 mm reflector at high power - this is exactly the impression that today's observers receive on scanning the lunar terminator (the division between the illuminated and unilluminated lunar hemispheres). Along the terminator, lots of shadow is thrown up by the low elevation of the Sun. Just beyond the terminator, high hills and mountains catch the rays of the Sun and shine as isolated beacons glowing in the darkness of the lunar night. Craters on the terminator are filled to their brims with shadow, the rims of their far walls often appearing as bright narrow broken lines. Near the terminator even the shallowest depression can pretend to be a deep crevasse, and the lowliest hill can give the impression of being a towering peak by vitrue of its long pointed shadow. Under a low Sun, a smooth, gently undulating lunar landscape can be transformed into the Apalachian mountain folds of the USA.
Galileo wasn't just a awestruck spectator who sat glued to his eyepiece. He was a fair draftsman who made the first lunar observational sketches. Also, the mathematically-minded astronomer was the first to attempt to measure the height of lunar mountains. Basing his work on careful measurements of the distance of the illuminated mountain peaks past the terminator, he applied simple geometry to derive some realistic figures for the heights of certain mountains. Although the mountains are of comparable height to the Earth's, they are proportionately larger because the Moon is just one quarter the Earth's diameter.
There is no "sea level" from which mountain heights may be measured, so lunar mountains are measured from their bases. For example, while the summit of a mountain on the floor of Copernicus is lower than the top of a small dome on the crater's outer flanks, we measure that mountain's height from Copernicus' floor, so it is actually deemed to be several thousand metres higher than the dome.Lunar mountains cast sharp sharp shadows during their local morning and evening, and the layperson viewing the terminator through the eyepiece or a close-up telescopic photograph can be forgiven for thinking that the mountains are steep and pointed like cathedral spires. But when the very edge of the Moon is surveyed, generally we are looking at rugged country edgewise around the limb (except where a lunar sea like Australe or Orientale has moved onto the limb). Even at high magnification this looks rather tame, with undulating hills and mountains. Still, many space artists of the 19th and 20th century portrayed the Moon's landscape as a monochrome version of the terrestrial Alps, with sharp Matterhorn-like mountains that towered over incredibly deep craters.Spaceprobes have shown the Moon's mountains at close quarter, and astronauts have visited the bases of mountains in several locations. Billions of years of meteoroidal sandblasting has denuded the lunar surface, wearing away the sharp edges and producing the fine lunar soil in the process.My observation shows a typical mountain cluster, an unnamed group of peaks lying south of the crater Gruithuisen on the border of Mare Imbrium and Oceanus Procelarum. These peaks rise to an estimated 900 metres above the plain. When illuminated obliquely, the nearby ridges of the Dorsum Bucher show up well.The Moon's seas display many examples of mountain clusters and isolated peaks.
Have a look at Mare Imbrium as the terminator sweeps across it during the first week of this month. On the evening of 2 June Mount Piton in eastern Iridum is on display. The following evening you'll notice how Piton's shadow has shrunk, and the Teneriffe Mountains and Mount Pico to the west of Piton have emerged into the lunar morning sunshine. On 4 June you'll be able to see the Straight Range, a linear east-west oriented mountain group on Imbrium's northern shore, about 90 km long with peaks rising to 1,800 metres. Finally, on 5 June the Gruithuisen cluster comes into view. Of course, the Moon boasts many more spectacular mountain ranges and peaks that are worthy of closer telescopic scrutiny.
