RISING MOON
Out of round
STANDING OUT IN THE DEEP SOUTHWEST of a waxing gibbous Moon lies the striking shoeprint form of Schiller. Only half of it is visible on the 19th, a few days before Full, sporting rim shadows reaching toothily into the darkness.
Lit up on Feb. 20th, Schiller’s mostly flat floor and overly elliptical shape catch the eye. Of course, all circular craters close to the Moon’s limb appear oval through foreshortening, but Schiller seems squashed twice as much as any other in the area. The following night, the shadows in Schiller are much shorter.
Earlier lunar observers were left scratching their heads trying to explain its form. Research into high-velocity impacts a few decades ago showed that a single projectile at a grazing angle could produce unusual craters, including ones like Schiller. Well after the initial impact, lava welled up through fractures in the crust to make a smooth floor. Messier A in the Moon’s southeast is the poster child for lowangle impacts.
To the northeast, you can’t miss the complex elliptical crater Hainzel. Look closely and you can figure out that this feature was created over time by separate impacts. On the northwest side is a roughly circular form with a classic central peak. The southeast region seems to have come later because the floor of the overlapping area is a better match for texture and albedo. The southern component was first not only because the other two overlay it, but also because its rim is softer, a result of wear and tear from long-term bombardment.
When you’re done, take a close look at the staggeringly bright Aristarchus in the northern third of the Moon. When Earth’s atmosphere is very turbulent or the Moon is very low to the horizon, refraction effects can sometimes give the appearance of outgassing and colored vapors.