Moonwatch
April’s top lunar feature to observe
Blancanus is well defined with terraced rim walls leading down to a flat floor
It’s sometimes easy to forget that the Moon is a three-dimensional object. When we look at it from Earth, apart from slight libration variations, its features are the same one month to the next. Located around 384,400km away from Earth, the Moon is too far for our eyes to see in a way that allows us to perceive depth. However, looking at it through a telescope emphasises its physical shape and nothing does this better than observing round craters close to the Moon’s edge.
This month’s Moonwatch target is the crater Blancanus located just to the south of the large, impressive walled plain of 225km Clavius. Blancanus itself is also quite large at 106km, but being close to the Moon’s southern edge, it appears foreshortened into an ellipse.
The foreshortening varies depending on the Moon’s libration state. Libration is the phenomenon that causes the Moon’s globe to rock and roll slightly seen from Earth, caused by the Moon’s tilted and elliptical orbit. However, despite being a large lunar feature in its own right, Blancanus never stands out like more centrally located craters such as 93km Copernicus.
To be fair, this is in part also due to the local surroundings for both craters. Copernicus is surrounded by lava and sits at the centre of an extensive ejecta ray system. Blancanus doesn’t fare as well, being surrounded by many other craters. Some are larger, some smaller and many are a similar size. Examples include 110km Scheiner to the west and 119km Klaproth to the southeast.
Blancanus itself is well defined with wide terraced rim walls leading down to a flat floor. Although there’s no demonstrative central mountain peak complex, there is a set of low altitude hills offset to the southwest of Blancanus’s centre. There are numerous tiny craterlets on its floor, the largest of which are 7km Blancanus V and 6km Blancanus A.
Three notable craters interrupt Blancanus’s rim: 7km Blancanus H in the south, and the close pair of 9km Blancanus G and 11km Blancanus N to the west. Below the main crater is 46km Blancanus C which, by virtue of appearing even closer to the Moon’s southern edge, is even more influenced by libration.
The three similar-sized craters Scheiner, Blancanus and Klaproth are quite different in appearance.
Klaproth has a flat floor and is defined by a very irregular rim. Scheiner’s rim is also quite battered and in terms of definition, sits somewhere between Blancanus and Klaproth. All three are devoid of central mountains but Scheiner’s floor looks quite rough towards the east. Its western half is dominated by 12km Scheiner A, 12km Scheiner J and a 12km unnamed crater.
Lunar libration causes large variations in the appearance of Blancanus this month and if you have clear skies, try looking for it anywhere between 2–17 April. The crater will be most notable when the terminator is nearby (as indicated by our ‘Best time to see’ dates) but if you can locate it when the region is more fully illuminated, the view will reveal just how different it can appear between favourable and unfavourable libration states.