# Why does the Moon's terminator look “wrong” in this image?

There is a picture of the moon in National Public Radio's on-line article Get Ready For Halloween By Watching The Moon's 'Occultation' Tonight. It looks wrong to me - specifically, the brightness gradation near the terminator - or lack thereof.

The image is credited "A waning gibbous moon occultation will be visible Tuesday night in parts of the United States. JPL/NASA" but I wonder if this is true. Something just looks "wrong" here.

In the original article the JPL/NASA contains a link to this page, which currently contains this image also shown below. It looks more like an actual photograph.

And here is a NASA/JPL image from https://svs.gsfc.nasa.gov/4404 for 2016-10-19 04:00 UT, which if I uderstand correctly is not actually a photograph, but simulated from LRO data. It also illustrates that the terminator is expected to be graded from light to dark and contain contrast from shadowing.

above: terminator from all three images. Realistic terminator on the right show graded intensity and strong shadowing from the highly oblique incident light.

• @JoeBlow the terminator is an abrupt transition from light to dark, the other two images demonstrate geometrically correct graded transition from light to dark as the incidence of sunlight is increasingly oblique. The "earthshine" in the first image is way too bright. These are unphysical. – uhoh Oct 19 '16 at 10:24
• @JoeBlow Before imaging from lunar orbit, a major source of astronomical information on the moons topography and crater formation came from studying images lit from different angles at different phases. Lunar shadows are serious science. I'm hoping for answers or comments from people actually familliar with lunar geography and/or imaging. – uhoh Oct 19 '16 at 11:54
• I think your question is a good one, but might be easier to take in with only the first and fourth image. – zephyr Oct 19 '16 at 13:10
• @zephyr It's a heterogeneous population of questioners and answerers here. Not everyone would have dug in so deep like you. In hindsight, seeing your answer and knowing you'd be here, I could have "tuned" the question for you. – uhoh Oct 19 '16 at 13:28

The reason that moon image looks wrong is because it is wrong. It is not a real image of the moon $-$ at least the terminator is not real.

The original article you cite has a link just below their image indicating the source of their image of the moon. That source is the night sky planner, hosted by JPL. You'll find the same image on that website, albeit slightly darker (it seems the NPR people lightened up the image a bit).

If you do some more digging, you'll see that the night sky planner got its image of that moon from someone else. Within the html code, they have the image defined as:

<img alt="the moon" src="http://api.usno.navy.mil/imagery/moon.png">


Clearly you can see that the image was taken from the United States National Observatory.

After a bit of digging, I found out what exactly is going on here. The purpose of this site is to show you the current phase of the moon. To do this, they take a single image of the full moon and artificially shade out a region to make it appear as the current phase of the moon. You can see their process here and how it was done by some guy named R. Schmidt. They've broken down the moon phases into 181 images which you can download here, if you're interested.

As you can see, the terminator on that image is wrong because it is not a real image of the current phase of the moon, but rather a computer generated "shading" of the full moon to indicate the current moon phase.

• Wow - that's quite an in-depth investigation! Thank you for "going deep." I think the final image is actually a 3D rendered simulation from topological and surface data, and not even a photograph at all, so I felt strange using it as a "correct" example. – uhoh Oct 19 '16 at 13:12
• @uhoh Yes it does seem that the actual moon image is not a single, full image, but some conglomeration of surface images. At least as far as I can read from the page I linked. I suppose they wanted a high res image of the surface as they could get. – zephyr Oct 19 '16 at 13:15
• You can type in any date and hour in 2016 and a new image appears taking into account illumination angle (phase), distance variation and libration. You can watch the shadows move. So I think it's doing all the basic physics and rendering a completely artificial image. – uhoh Oct 19 '16 at 13:18
• I'm talking about the "DIal-a-Moon" page: svs.gsfc.nasa.gov/4404 That is not from 181 fixed images. It's computed and rendered. That's the only way to get size, libration, and shadows all correct, since they repeat with different periods. – uhoh Oct 19 '16 at 13:32
• @uhoh Ah, sorry, I misunderstood and thought you were talking about a different site. – zephyr Oct 19 '16 at 13:33