Why are there no grazing impacts on the Moon? Round craters seem to indicate a 90 degree impact from a meteor...  But meteors should come from all directions, shouldn't there be some elliptical craters and some trenches dug ending in essentially half craters.
 A: A projectile hitting a water surface under any angle causes a circular wave. The reason is that the surface is a uniform medium so the propagation speed does not depends on direction. The same is the case for a solid surface.
A: There are actually a lot of grazing impacts on the Moon. Messier is a good example. 45 degrees is the most common impact angle, with decreasing likelihood as 90 degrees (vertical) and 0 degrees (horizontal) are approached. The probability of impact angle follows a sin curve.
Part of the issue is the energies involved. In planetary scale impacts, the energy imparted to the surface is so high and creates such a large excavation event that no trace of the obliquity (fancy term for "low angle") remains in the crater itself. It just looks round. It isn't until the impact angle gets low, really low, like 5 degrees or less, that planetary impacts leave an elongated crater (like Messier).
The easier way to tell if a crater impacted obliquely is the ejecta pattern. Oblique craters create this characteristic pattern called a butterfly pattern in which there is a forbidden zone in the uprange direction (back in the direction from which the impactor came) and a zone of reduced ejecta (sometimes forming a forbidden zone) in the downrange direction. Changes in ejecta pattern occur as high as 45 degrees with an uprange forbidden zone. The full butterfly pattern doesn't really start until about 15 degrees or less.
And, to make matters horribly complex, those angles all change based on target properties (how strong is the material, is it solid rock or sand? Or maybe it's ice! Is there layering in the material; if so, what are the properties of the different layers? Does it have water in it that will vaporize and add energy to the impact event? Is there an atmosphere? What's the surface gravity?) and impactor properties (is it solid, an aggregate of tiny particles clumped together? Is it rock or ice? Is it traveling at a few kilometers per second or 50 km/s?).
A: National Geographic had an article in the early to mid 1960's about this topic (1964 I think).  They shot grains of sand from an accelerator into a bed of sand.  Anything but an extremely oblique angle would simply make a round crater.  Unless it is updated with more accurate tests to the direct contrary, there is your answer.
A: I also suspect there’s a gravitational aspect to this.  The craters we see were big enough to make a sizable pock mark, but small enough to not catastrophically destroy the moon.  That means the moon would have a significant gravitational influence on a small incoming meteor, creating a tendency toward more vertical impacts.  A completely unsupported hypothesis on my part that may not hold up well to extremely high projectile velocities.
