Starship Asterisk*

Tom Glenn wrote: ↑Thu Feb 17, 2022 6:53 am

johnnydeep wrote: ↑Wed Feb 16, 2022 7:57 pm
As for bumpy moon edge, the full moon picture I used above shows it, but the full res version shows it better.

In contrast, this APOD shows a perfectly smooth edge all the way around. Here's a part of the bottom edge:

Perhaps it's just an image processing artifact?

We don't know the processing details for this image, but many LROC images are presented as orthographic projections, which are not the same as Earthbound perspectives, although at first glance they appear similar. In an orthographic projection, the point of perspective is at an infinite distance, and the projection lines are parallel, so it presents the view of a lunar hemisphere as it would appear from an extreme (infinite) distance, which distorts features on the edge. In contrast, when we view the Moon from Earth we get a perspective view in which projection lines are not parallel, because the Moon has significant angular size as viewed from Earth.

But the featured image may not be any specific mathematical projection, as it differs from published orthographic projections in several respects, as well as from the Earthbound perspective, notably when looking at the distance from the limb of key features in specific quadrants. There are regions of the lunar nearside that would appear at mean libration that are not visible in this image. We would need to consult those who processed the image to see what was done in this case.

johnnydeep wrote: ↑Wed Feb 16, 2022 7:57 pm
As for bumpy moon edge, the full moon picture I used above shows it, but the full res version shows it better.

In contrast, this APOD shows a perfectly smooth edge all the way around. Here's a part of the bottom edge:

Perhaps it's just an image processing artifact?

MarkBour wrote: ↑Wed Feb 16, 2022 5:52 pm

neufer wrote: ↑Tue Feb 15, 2022 10:40 pm

johnnydeep wrote: ↑Tue Feb 15, 2022 9:39 pm

Yeah. Now why exactly is that? At least one crater still does have very prominent rays, so why don't all of them?
And another stark difference is the absence of edge bumpiness on this "terminator moon". Again, why?

https://en.wikipedia.org/wiki/Opposition_surge wrote:
<<The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) . . .

I'm not sure why neufer brings up the Seeliger effect here. Today's APOD is composited slices of images, each taken when the sun is off to the side of each section. Art, are you saying that rays are most visible with straight oppositional lighting? Or do they show up best somewhere in between side lighting and oppositional lighting?

As to the lack of bumpiness on the edge, I haven't really seen it in other images to compare. johnnydeep, you must have better examples than I have. I've only noticed it in eclipse images that dealt with Baily's beads. I do see a little bumpiness only at the top of this image. Perhaps the smooth edges were part of the process in which the composite was put together, or again, perhaps the visibility of such features are reduced by the angle of the lighting.

MarkBour wrote: ↑Wed Feb 16, 2022 5:52 pm

neufer wrote: ↑Tue Feb 15, 2022 10:40 pm

johnnydeep wrote: ↑Tue Feb 15, 2022 9:39 pm
Yeah. Now why exactly is that? At least one crater still does have very prominent rays, so why don't all of them?
And another stark difference is the absence of edge bumpiness on this "terminator moon". Again, why?

https://en.wikipedia.org/wiki/Opposition_surge wrote:
<<The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) . . .

I'm not sure why neufer brings up the Seeliger effect here. Today's APOD is composited slices of images, each taken when the sun is off to the side of each section. Art, are you saying that rays are most visible with straight oppositional lighting? Or do they show up best somewhere in between side lighting and oppositional lighting?

neufer wrote: ↑Tue Feb 15, 2022 10:40 pm

johnnydeep wrote: ↑Tue Feb 15, 2022 9:39 pm

MarkBour wrote: ↑Tue Feb 15, 2022 7:28 pm
Very nice, it certainly does bring out some of the features. On the other hand, it reduced the visibility of rays. To the point that I'm like "Where's Tycho?"

Yeah. Now why exactly is that? At least one crater still does have very prominent rays, so why don't all of them?
And another stark difference is the absence of edge bumpiness on this "terminator moon". Again, why?

https://en.wikipedia.org/wiki/Opposition_surge wrote:
<<The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) . . .

johnnydeep wrote: ↑Tue Feb 15, 2022 9:39 pm

MarkBour wrote: ↑Tue Feb 15, 2022 7:28 pm
Very nice, it certainly does bring out some of the features.
On the other hand, it reduced the visibility of rays.
To the point that I'm like "Where's Tycho?"

Yeah. Now why exactly is that? At least one crater still does have very prominent rays, so why don't all of them?
And another stark difference is the absence of edge bumpiness on this "terminator moon". Again, why?

https://en.wikipedia.org/wiki/Opposition_surge wrote:
<<The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. The term is most widely used in astronomy, where generally it refers to the sudden noticeable increase in the brightness of a celestial body such as a planet, moon, or comet as its phase angle of observation approaches zero. It is so named because the reflected light from the Moon and Mars appear significantly brighter than predicted by simple Lambertian reflectance when at astronomical opposition. Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter.

Photomosaic of a panorama taken by Surveyor 7 on the outer rim of the crater Tycho.

---

Shadow hiding: When the angle of reflection is close to the angle at which the light's rays hit the surface (that is, when the Sun and the object are close to opposition from the viewpoint of the observer), this intrinsic brightness is usually close to its maximum. At a phase angle of zero degrees, all shadows disappear and the object is fully illuminated. When phase angles approach zero, there is a sudden increase in apparent brightness, and this sudden increase is referred to as the opposition surge. The effect is particularly pronounced on regolith surfaces of airless bodies in the Solar System. The usual major cause of the effect is that a surface's small pores and pits that would otherwise be in shadow at other incidence angles become lit up when the observer is almost in the same line as the source of illumination.

Coherent backscatter: A theory for an additional effect that increases brightness during opposition is that of coherent backscatter. In the case of coherent backscatter, the reflected light is enhanced at narrow angles if the size of the scatterers in the surface of the body is comparable to the wavelength of light and the distance between scattering particles is greater than a wavelength. The increase in brightness is due to the reflected light combining coherently with the emitted light.>>

MarkBour wrote: ↑Tue Feb 15, 2022 7:28 pm Very nice, it certainly does bring out some of the features. On the other hand, it reduced the visibility of rays. To the point that I'm like "Where's Tycho?"

Chris Peterson wrote: ↑Tue Feb 15, 2022 2:44 pm

pettygrew wrote: ↑Tue Feb 15, 2022 2:26 pm
By looking at the number of craters, can it be determined from which directions the Moon was struck?

It has been struck uniformly. There is no preferred direction.

https://en.wikipedia.org/wiki/Oceanus_Procellarum wrote:

Ancient rift valleys – rectangular structure (visible – topography – GRAIL gravity gradients) (October 1, 2014).

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<<Oceanus Procellarum (lit. 'Ocean of Storms' is a vast lunar mare on the western edge of the near side of the Moon. Oceanus Procellarum is the largest of the maria ("seas"), stretching more than 2,500 km across its north–south axis and accounting for 10.5% of the total lunar surface area. There are several hypotheses about the origin of Oceanus Procellarum and a related asymmetry between the near and far sides of the Moon. One of the most likely is that Procellarum was a result of an ancient giant impact on the near side of the Moon. The size of the impact basin has been estimated to be more than 3,000 kilometers, which would make it one of the three largest craters in the Solar System. The impact likely happened very early in the Moon's history: at the time when magma ocean still existed or just ceased to exist. It deposited 5–30 km of crustal material on the far side forming highlands. If this is the case, all impact related structures such as crater rim, central peak etc. have been obliterated by later impacts and volcanism. One piece of evidence in support of this hypothesis is concentration of incompatible elements (KREEP) and low calcium pyroxene around Oceanus Procellarum.

Procellarum may have also been formed by spatially inhomogeneous heating during the Moon's formation. The GRAIL mission, which mapped the gravity gradients of the Moon, found square formations resembling rift valleys surrounding the region beneath the lava plains, suggesting the basin was formed by heating and cooling of the lunar surface by internal processes rather than by an impact, which would have left a round crater.

Other hypotheses include a late accretion of a companion Moon on the far side. The latter postulates that in addition to the present Moon, another smaller (about 1,200 km in diameter) moon was formed from debris of the giant impact. After a few tens of millions of years it collided with the Moon and due to a small collisional velocity simply piled up on one side of the Moon forming what is now known as far side highlands.>>

P. Nielsen wrote: ↑Tue Feb 15, 2022 5:21 pm Can we do this for the back side?

pettygrew wrote: ↑Tue Feb 15, 2022 2:26 pm By looking at the number of craters, can it be determined from which directions the Moon was struck?

APOD Many of the Moon's (craters)