|Proper Name||Mare Nubium Quadrangle|
|Quadrangle on||The Moon|
|Quadrangle scale||1:2,500,000 (30 quadrangles)|
|Lunar coordinates||Latitude: 30° S to 0°|
Longitude 45° W to 0°
|Important features||Alphonsus, Arzachel, Bullialdus, Fra Mauro, Gassendi, Letronne, Mare Cognitum, Mare Humorum, Mare Nubium, Oceanus Procellarum, Palus Epidemarum, Ptolemaeus, Purbach, Werner|
One of thirty standard quadrangles that together divide the Moon's surface into regular sections of approximately equal area. Quadrangle LQ-19 runs southward from the lunar equator to a latitude of 30° South, and westward from the Moon's prime meridian to longitude 45° West. These coordinates place the quadrangle southwestward from the centre of the Moon's visible face.
The surface of this part of the Moon mainly comprises the dark, relatively flat basalt plains known as maria or 'seas', broken by numerous rilles running across the entire area. The quadrangle contains three small lunar seas, of which the main example is Mare Nubium, the Sea of Clouds, which fills much the quadrangle's eastern section (and thus the entire quadrangle is sometimes informally known as the Mare Nubium Quadrangle). To the northwest of Mare Nubium is the somewhat smaller Mare Cognitum, the 'Known Sea', while the southwestern corner of the quadrangle is occupied by the circular sea known as Mare Humorum, the Sea of Moisture.
These seas are ringed and separated from one another by ridges of higher land dotted with large craters and walled plains, but the most strongly cratered regions of the quadrangle lie near its western and eastern edges. In the west, northward of Mare Humorum, are two wide but highly eroded ringed plains, Letronne and Gassendi. Along the eastward edge of the quadrangle, the larger craters are much more strongly defined. The largest of these is Ptolemaeus, more than 150 km in diameter, from which a chain of other large craters runs southward along the eastern edge of Mare Nubium. Notable among these are Alphonsus, Arzachel and Purbach. (Albategnius is another significant crater in this same general group, but that crater falls beyond the eastern border of LQ-19 and belongs to the neighbouring quadrangle, LQ-20.)