<p>Speaking as the author of Robbins (2019 [I always get the date messed up since 2018 was when it was available, but technically the volume it's in is a 2019 year), you are seeing artifacts of how the database was constructed.</p>
<p>All craters in the database were traced manually, by hand.  Tiny, imperceptible biases in the exact hand motions are not perceptible for a few craters, but they are noticeable when you have hundreds of thousands of features.  I tend to start tracing rims near the 7:00 position and go clockwise, and I'm left-handed, and so there are slight ~1 pixel biases to be elongated along a roughly - but not quite! - N/S direction.  For this reason, you can't trust ellipticities smaller than a certain threshold, and you can't trust the ellipticities of craters smaller than a certain size (more on that below).</p>
<p><a href="https://i.sstatic.net/3boG3.jpg" rel="noreferrer"><img src="https://i.sstatic.net/3boG3.jpg" alt="Figure 6 from Robbins (2019)" /></a> In the paper, I did a fairly robust test to determine if there were any trends in ellipticity across the surface as a function of size for ellipticities and crater sizes I trust (more on that next).  Figure 6 from the paper (reproduced above) shows the results of those tests, and the gist is that the directionality was independent from randomness.  So, no N/S trend.</p>
<p><a href="https://i.sstatic.net/fRhq7.jpg" rel="noreferrer"><img src="https://i.sstatic.net/fRhq7.jpg" alt="Figure S2 from Robbins (2019)" /></a>
Another test I did was relegated to the supplemental material and is the figure S2 copied above.  You can ignore the colored contours for the moment and focus on the points.  They show what you noticed, that there is a concentration of craters with a tilt angle close to 90°, but not quite.  There's the offset due to exactly my biases when tracing rims.  They are most noticeable at the smallest diameters -- 1 pixel offset doesn't affect a 100 km crater nearly as much as a 1 km crater.  Now, looking at the contours, they start to go flat as you get to diameters of about &gt;~8 km.  Therefore, I don't trust most smaller ellipticities for craters &lt;~8 km.  Now, if an ellipticity is particularly large, like 1.5 (the major axis is 50% larger than the minor axis), and the crater is ≈1 km average diameter, then that's probably trustworthy.  But not an ellipticity of 1.1 or even 1.2 at that size.</p>
<p>Hope that answers your question, let me know if something wasn't clear and I can edit my answer.  Given that it's my database, I'm probably the best person to answer questions about it!</p>

CC BY-SA 4.0
2024-01-29T11:18:10.543Z
That's amazing. Thank you. We were mostly just playing with the data and had that leap out at us. Thanks for all the hard work putting that together!
David McKee
CC BY-SA 4.0
2024-01-26T16:26:48.060Z
2024-01-26T17:04:28.660Z
2024-01-26T17:04:28.660Z
Stuart Robbins
<p>Loading the dataset from <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JE005592" rel="noreferrer">Robbins 2018</a>, <em>A New Global Database of Lunar Impact Craters &gt;1–2 km: 1. Crater Locations and Sizes, Comparisons With Published Databases, and Global Analysis</em> which I believe to be <a href="https://astrogeology.usgs.gov/search/map/Moon/Research/Craters/lunar_crater_database_robbins_2018" rel="noreferrer">this dataset</a> implies that lunar craters have about 4 times more chance of being north-south aligned (<code>DIAM_ELLI_ANGLE_IMG</code> around 90) than being east-west aligned (<code>DIAM_ELLI_ANGLE_IMG</code> near 0 or 180); this appears to hold for a subset of highly elliptical craters too.</p>
<p>Glancing at equatorial equators, this seems plausible.</p>
<p>What leads to this preferential orientation? Or have I mangled my analysis?</p>

CC BY-SA 4.0
2024-01-23T15:21:23.423Z
A non-circular crater is by definition elliptical. The cited paper states in its abstract "More elliptical craters are found than past work, orientation of D ≥ 10 km craters are random, and many spatial density trends are discussed". Did you read the discussion and maybe some of the relevant references (or papers referencing this)? You certainly did. It would be nice, if you could summarize for us your findings and insights to see where you stand, and to get us to  a point to not pointlessly re-iterate what is known and to find a good place to start answering. It's an interesting question!
planetmaker
CC BY-SA 4.0
2024-01-23T16:00:55.860Z
Also relevant: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JE006728 https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023EA002863 See also https://ui.adsabs.harvard.edu/abs/2018LPI....49.2443R/abstract from 2018 where he talks about bugs in earlier versions for ellipse fitting... so check whether the claim regarding orientation holds at all.
planetmaker
<the-moon><asteroids><impact><crater>
Why do lunar craters seem preferentially north-south aligned?
David McKee
# Why do lunar craters seem preferentially north-south aligned?

(Asked by: username_0 on 2024-01-23)

Loading the dataset from [Robbins 2018](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JE005592), *A New Global Database of Lunar Impact Craters >1–2 km: 1. Crater Locations and Sizes, Comparisons With Published Databases, and Global Analysis* which I believe to be [this dataset](https://astrogeology.usgs.gov/search/map/Moon/Research/Craters/lunar_crater_database_robbins_2018) implies that lunar craters have about 4 times more chance of being north-south aligned (`DIAM_ELLI_ANGLE_IMG` around 90) than being east-west aligned (`DIAM_ELLI_ANGLE_IMG` near 0 or 180); this appears to hold for a subset of highly elliptical craters too.

Glancing at equatorial equators, this seems plausible.

What leads to this preferential orientation? Or have I mangled my analysis?

**Tags:** the-moon, asteroids, impact, crater

> **Comments:**
> **username_1**: A non-circular crater is by definition elliptical. The cited paper states in its abstract "More elliptical craters are found than past work, orientation of D ≥ 10 km craters are random, and many spatial density trends are discussed". Did you read the discussion and maybe some of the relevant references (or papers referencing this)? You certainly did. It would be nice, if you could summarize for us your findings and insights to see where you stand, and to get us to  a point to not pointlessly re-iterate what is known and to find a good place to start answering. It's an interesting question!  
> **username_1**: Also relevant: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JE006728 https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023EA002863 See also https://ui.adsabs.harvard.edu/abs/2018LPI....49.2443R/abstract from 2018 where he talks about bugs in earlier versions for ellipse fitting... so check whether the claim regarding orientation holds at all.  

---

## 1 answers

### Answer by username_2 on 2024-01-26. Score: 15 | Accepted answer

Speaking as the author of Robbins (2019 [I always get the date messed up since 2018 was when it was available, but technically the volume it's in is a 2019 year), you are seeing artifacts of how the database was constructed.

All craters in the database were traced manually, by hand. Tiny, imperceptible biases in the exact hand motions are not perceptible for a few craters, but they are noticeable when you have hundreds of thousands of features. I tend to start tracing rims near the 7:00 position and go clockwise, and I'm left-handed, and so there are slight ~1 pixel biases to be elongated along a roughly - but not quite! - N/S direction. For this reason, you can't trust ellipticities smaller than a certain threshold, and you can't trust the ellipticities of craters smaller than a certain size (more on that below).

[![Figure 6 from Robbins (2019)](https://i.sstatic.net/3boG3.jpg)](https://i.sstatic.net/3boG3.jpg) In the paper, I did a fairly robust test to determine if there were any trends in ellipticity across the surface as a function of size for ellipticities and crater sizes I trust (more on that next). Figure 6 from the paper (reproduced above) shows the results of those tests, and the gist is that the directionality was independent from randomness. So, no N/S trend.

[![Figure S2 from Robbins (2019)](https://i.sstatic.net/fRhq7.jpg)](https://i.sstatic.net/fRhq7.jpg)
Another test I did was relegated to the supplemental material and is the figure S2 copied above. You can ignore the colored contours for the moment and focus on the points. They show what you noticed, that there is a concentration of craters with a tilt angle close to 90°, but not quite. There's the offset due to exactly my biases when tracing rims. They are most noticeable at the smallest diameters -- 1 pixel offset doesn't affect a 100 km crater nearly as much as a 1 km crater. Now, looking at the contours, they start to go flat as you get to diameters of about >~8 km. Therefore, I don't trust most smaller ellipticities for craters <~8 km. Now, if an ellipticity is particularly large, like 1.5 (the major axis is 50% larger than the minor axis), and the crater is ≈1 km average diameter, then that's probably trustworthy. But not an ellipticity of 1.1 or even 1.2 at that size.

Hope that answers your question, let me know if something wasn't clear and I can edit my answer. Given that it's my database, I'm probably the best person to answer questions about it!

> **Comments:**
> **username_0**: That's amazing. Thank you. We were mostly just playing with the data and had that leap out at us. Thanks for all the hard work putting that together!  

---

