Small Telescope Moon Observing Challenge
A small telescope such as a 50- or 60-mm refractor or a four- to six-inch reflector will bring into sharp detail all the lunar features previously seen with a pair of binoculars. Details of crater walls and interiors are now revealed and non-crater geological features are evident.
At crescent Moon, the crater pair of Hercules (left) and Atlas (right) allow us to see the difference between an ordinary impact crater like Hercules and a volcanically modified crater like Atlas. Compared to Hercules, the floor of Atlas is rough and cracked as a result of volcanic pressure that pushed the crater's floor upward. Posidonius crater also experienced the same fate. Because their floors are cracked by volcanic uplift, these craters are known as “floor-fractured” craters. On the southeastern limb, the crater Petavius similarly shows cracks across its floor. Still further south lays Janssen, a wide, ruined, almost unrecognized crater. Janssen is among the oldest of the lunar craters and has been ruined by massive waves of debris thrown from subsequent impacts on the Moon.
The crescent Moon terminator also reveals the curved wall-like structure of the Altai Scarp. This mountainous arc is the outer rim of the Nectaris Basin containing Mare Nectaris.
At 1st quarter Moon, sunrise creates tooth shadows from the Apennine Mountains. Circular Mare Imbrium comes into view and reveals the unusually smooth floored crater Archimedes. To the north the dark-floored crater Plato catches the eye and soon becomes a favorite among lunar observers. East of Plato, another crater pair, Aristoteles (top) sand Eudoxus allow a look at the collapsed terraced walls common with large craters.
Near the middle of the moon lie three adjacent craters that become a popular spot with lunar observers. Ptolemaeus at the north, Alphonsus in the middle, and Arzachel at south show how age affects lunar craters. Ptolemaeus formed first nearly four billion years ago and was subsequently smoothed over by debris thrown from later impacts. Arzachel formed only a couple of hundred million years later, but escaped the storm of debris that buried Ptolemaeus and retains a fresh appearance.
South of the triple craters lies one of the favorite non-crater features on the Moon, a structure affectionately known as Straight Wall. This 110 kilometer long linear structure is a geologic feature known as a graben, or the slumping of the landscape between two parallel faults. The surface slopes down 500 meters to the west, thus the structure casts a dark shadow at sunrise and shows as a bright white line when the sun illuminates the face of the slope.
The massive ray structure surrounding Tycho crater is subdued as sunrise greets the crater, but a small telescope now reveals the crater pinpointed by the rays. As the Moon approaches full phase, the Tychonian ray system blossoms to extend half way across the face of the Moon. The intriguing parallel rays known as the “railroad track” extend to the northwest of Tycho. Copernicus crater also reveals its structure and shows why it was once called the “Monarch of the Moon”. Other brightly rayed craters, Kepler and Aristarchus, lie west of Copernicus on the dark background of Oceanus Procellarum. Irregular Procellarum is the only dark region on the Moon that was designated as an ocean, not a sea.
Gassendi crater on the northern shore of Mare Humorum is another excellent example of a volcanically modified floor-fractured crater. Along the southwestern limb, the crater Schiller stands in oddly elongated contrast to its round neighbors. To the west, massive volcanically-flooded Schickard also stands out because of its unusually smooth floor.