Strange Moon limb

[Pig]

Can anyone confirm what I am seeing on the limb of the Moon at approximately the 7/8 O'clock position. It appears to have a broken circumference line... i.e. Not so circular, it is like there is a step/bite. Not something I have noticed before, it could be that the seeing is just particularly good tonight and the detail is better.

[Charic]

Wish I could help! Moons not up yet? Tree line!  but happily sitting here with three Beers  waiting for the clouds to part 

[Chinapig]

Clouds full of rain down here, but will keep peeping out and dash out with a scope if it clears.

Sounds curious - would be keen to have a look.

[Pig]

LoL ... sounds good ?

[Pig]

1 minute ago, Chinapig said:

Clouds full of rain down here, but will keep peeping out and dash out with a scope if it clears.

Sounds curious - would be keen to have a look.

It is most unusual to see it like this but it is quite obvious.

[Charic]

......just got a glimpse through my 15x70's so orientation is 'normal' and it appears as if something  has taken a 'bite' in the  2 O'clock position,  (7/8  through a scope!)

No it's not the beer?

[Chinapig]

14 minutes ago, Charic said:

......just got a glimpse through my 15x70's so orientation is 'normal' and it appears as if something  has taken a 'bite' in the  2 O'clock position,  (7/8  through a scope!)

Curiouser and curiouser...

Down here the score is:  Me: Nil, Weather: 1

Spot moon nice & shiny through gaps in the clouds.  Put scope & eyepieces ready by back door, nip down the shed to get the mount out.  And it's now RAINING!

Ah well, staying awake for a while with 15x70s handy...

[jabeoo1]

Okay this is weird, I have just been out and had a look, I am very confused right now.  There may well be an easy explanation, but right now I am looking at a view I did not expect.  

This is odd ! 

[Charic]

I believe this phenomenon is caused by Mare Crisium (Sea of Crisis), sat right on the Moon terminator, and due to not seeing the back or far side of the feature, makes it appear, or looks as if somethings missing?
 I'm only viewing  with  15x70's.
Far from a sharp image, yet  the phone captured this?  the 'bite' is just visible!.......if that's what were seeing, otherwise the beer is starting to affect performance!

[jabeoo1]

18 minutes ago, Charic said:

I believe its Mare Crisium, and not seeing the back or far side of the feature, makes it appear, or looks as if it's missing?

I think your right Charic.  That feature lit up as a 95.5% waning gibbous, makes it look *(nothing) like a Pacman.  Crazy  

[Pig]

Charic, this is not what I was referring to, the area I am referring to in your picture is around the 10'Oclock position and is just about visible if you zoom in, it was much clearer through the scope. I think it is a crater but it must have one heck of a depth to it.

[Charic]

I initially tried to observe in the 7 ~ 8 O'clock  area that you first mentioned ( waiting for the lighter gaps in the cloud )  but it was my assumption that a scope v. binoculars ( orientation of the image)  and the resulting image I produced, that we were actually  looking at the same thing, however.......no!
Even on the poor image I supplied, I'm finding it hard to see anything in the 10 o'clock, no matter........ At least we both managed to see something! last night, despite the differences!

[Pig]

I think it helps as I saw it live and I can see it in your image ? I agree I had a good night and early morning session ?

[spaceboy]

If I remember correctly I read in one of my sky at night magazines that there are features on the limb (not terminator) only visible at certain times. We often only consider the terminator to vary in appearance during phase's  but the limb also changes ever so slightly.  My guess Pig is that you were one of the lucky ones who inadvertantly stumbled across this more rare feature. They are though I believe cataloged in the lunar 100 so you could look it up. I cannot remember if this happens at a particular phase or a time of year etc. I know the terminator X / Y are only visible at certain times although these dates and times are cataloged so I assume it's the same for limb features.

[furrysocks2]

Is it this you are looking at?

I used Dial-A-Moon (https://svs.gsfc.nasa.gov/4537) and downloaded the high res file, then overlayed the other one with an approximate arrow, scaled and rotated to match. That matches up with a similar bite in the nasa pic.

I think it's about 38 or so degrees north not accounting for libration, which might put you somewhere near Lorentz??

Taken from The Moon and How to Observe It...

 

Shame I'm forecast cloud for the next week. Hope this is what you were looking at, interesting.

[furrysocks2]

Here's a topo map of the region, taken from https://pubs.er.usgs.gov/publication/sim3316

Libration won't get us as far as 240 degrees, so Lorentz is still my best guess.

[furrysocks2]

I've been having a bit of a play again with this, still not sure if I'm seeing the same thing as you but it all seems to fall into place somehow...

I used the software aaplus to calculate the libration in latitude and longitude for last Friday - approx -4 deg and +5.6 deg respectively. I used a program called OCCULT to plot a lunar profile for the approximate area... axis angle measured from north 0, anti-clockwise, I think, so with Lorentz at approx 32 degrees latitude and my early estimate of the dip at 38 degrees, I tried to include that region. I think the profile comes out in reverse as we'd view it, the northern end being at the left.

It shows a reasonable dip or bite to my eye, particularly at 52 degrees, which counting from north at 0, corresponds perfectly to my estimate of 38 degrees latitude (except it's not quite that due to libration)... all this might actually be misleading, but I think that's right. So I took a portion of the NASA image (shown in red), then rotated and stretched it vertically x3 to compare with the expected profile from OCCULT.

It's a good match. 

I've no real understanding of what this plot shows, but it looks good.

So trying to work it all out, I used OCCULT's plot of the region to match against the topo map. The region to the right was an earlier attempt, the one to the left was centered on the 52 degree dip.

 

I don't know exactly what the dashed line and red cross signify in this, but assuming the dashed line is the angle of observation, then the lowest point of the dip should have been the gap between Nernst and Avicenna in Lorentz, with elevation rising to the sides being a combination of the two crater walls and anything else in the foreground/background.

You may have been looking almost straight down this northern Lorentz valley.

 

[Pig]

Thank you, it all looks pretty impressive and seems to explain it all ? I  am still surprised that I could see such detail at such low magnification  ?

[furrysocks2]

You're welcome - never studied a limb profile like that before. I'm surprised how much can be worked out from a date and a few missing pixels!

Was it indeed that little snicket that you were looking at then?

[Pig]

Yes ? Quite remarkable that I must have been looking at it just as it was in a prime position to show its full depth. It's also fortunate that  Charic snapped that image......

[furrysocks2]

Indeed, only a couple of days by the looks of it, but once a month.

Spent way to much time on this, but October 5th...

 

This animation (loops forwards and backwards) represents 6 days of half a day per frame, centered around the deepest point on the 5th.

 

This one shows a wider view, and it also shows another snicket at almost the same time, not dissimilar in size, centered between 59 and 60 degrees, so 7-8 degrees to the south. You could look for it, but I haven't worked out why it's there.

 

[furrysocks2]

Not trying to drag this out, but just saw that @michael.h.f.wilkinson posted a lovely full disc mosaic taken a day later, night of 9th September.

Just out of pure curiosity, I zoomed in and here's another capture of the same (I didn't recompute the expected profile, but it nevertheless matches up well)...

Too much time looking at the limb profile plots in OCCULT and vertically stretched portions of the limb give a false sense of the elevation difference... looking at the snippet of the original image it's not a huge feature. All the same, something for which I intend to try timing an observation for myself... an excuse to drag the scope out.