MoonWatch
Procellarum's southern area is low on spectacle but high in terms of geological interest. An irregular bay on the ocean's edge contains the craters Billy and Hansteen, two features useful in finding your way around this region. Billy is a dark floored crater 46 km in diameter, and is named in honour of the French Jesuit astronomer Jacques de Billy (1602-79) who was one of the first to reject the role of astrology in science, along with superstitious notions about the malevolent influence of comets.
Billy is an old impact crater whose rim rises to 1,300 metres above its dark flat floor. The floor is one of the darkest spots on the Moon's face, and can easily be seen any time when it is illuminated, even at full Moon. Billy contrasts with Hansteen, a light coloured 45 km crater with a hummocky floor. The relationship between Billy and Hansteen is somewhat similar to that between dark floored Herodotus and brilliant Aristarchus (35 km and 40 km respectively) some 1,130 km due north, also in Oceanus Procellarum.
According to the American lunar expert Ralph Baldwin in "Measure of the Moon" (1963 - pre-Apollo), Billy, Hansteen and Herodotus are all age class 5, indicating very ancient craters that have been flooded with lava (or, he supposed, volcanic ash). Billy and Herodotus are impact craters pre-dating the later lava flows of Oceanus Procellarum, and therefore formed more than 3.2 billion years ago, their floors being subsequently filled with lava. Hansteen was formed after vulcanism had ceased in Procellarum and is younger than 3 billion years old. However, Aristarchus is far younger than Hansteen, having been formed by an asteroidal impact a mere 250 million years ago. Whilst Aristarchus is one of the brightest areas on the entire Moon, Billy is one of the darkest, and both features are easy to locate, even through a pair of steadily held small binoculars.To the north of Billy is the mountain massif of Mons Hansteen, a triangular outline 30 km broad, shaped like the Inner Hebridean Isle of Mull (only slightly smaller), with peaks rising to similar heights.
At the time of my observation the sunlight is illuminating Billy from the east, having risen just 17 degrees high. It is mid-lunar morning at Billy, and detail on the floor comprises subtle tonal variations that hint towards slight undulations in the level of the floor. These may be lava flows or traces of the original floor structure before it was buried by lava. Billy can be seen any time this month between the evening of 8 February and the morning of 22 February. At a suitable low illumination a small telescope will show the triangular shape of Mons Hansteen and indicate the roughness of Hansteen crater's floor. Larger instruments will reveal structure on the floors of Hansteen and Billy, the small hills to the north of Hansteen and the narrow rille that runs next to the outer western rim of Hansteen.
