It’s funny – when sorting photos into their respective folders (or, often enough, deleting them from the drives,) I often find something to comment upon, or something that I meant to feature earlier but forgot about. In this case, it’s a detail that I didn’t notice at the time, and “the time” was the day following the not-total lunar eclipse. I could have featured it then, but I was more intent on selecting the best and most illustrative images and simply missed the subtle details of this one.
Here’s an image part way into the eclipse, purposefully overexposed to try and get the shadowed portion to show up.
As you can see, I just barely brought out the shadowed portion of the moon, while completely blowing out the sunlit portions. But what I didn’t realize is how well the penumbra shows.
If you recall, there’s this thin, ‘outer shadow’ of the Earth during an eclipse, because of geometry. A light source larger than the object that creates a shadow will have thinner outer edges but a darker cone in the center; the outer edges are the penumbra, and the center cone the umbra. For a lunar eclipse, the penumbra isn’t too noticeable because the light isn’t reduced very much, less than the normal contrast between the highlands and mares of the moon itself. But when I overexposed the image above, I brought out the distinctions a bit better, and what I took to be simply the indistinct edges of the umbra turned out to be the more-visible penumbra – you can actually see the curved edge of it before the moon is bleached pure white. Here’s the same image, but after I dropped the mid-tones a little more:
The penumbra seems smaller than I imagined it, though this is hardly a definitive measurement due to the exposure, but you can see that it clearly has a width to it and is not simply a gradient between the shadow and the sunlight. You can also see the color cast from portions of the light coming through the thin edge of Earth’s atmosphere, possibly enhanced by local humidity conditions.
And then, because I’m me, I stopped typing right here and went back into GIMP with two of the images used previously, to do a comparison between them with an animated gif (pronounced, “gez-OON-tite.”) These two show the moon just before entering the penumbra (so, “full,”) and just before entering the umbra. Shown together this way, it’s a little easier to see that the penumbra is larger than the images above seem to indicate, but the edge distinction is a lot vaguer. There are slight variations in the exposures between the two images, so this isn’t a precise comparison, but it does seem that the penumbra extends past Tycho here.
You might also note that the bottom edge of the moon is actually a wee bit darker in the full phase, but I’m putting this down to scattered clouds more than anything else; the sky wasn’t perfectly clear before the eclipse, though I waited for the clearest conditions before snapping the full image.
Anyway, those are your curiosity illustrations of the day. But I’ll use this space to mention that the Geminids meteor shower is due to peak in three days, so check it out if you like. The moon is more conducive to it this time, closing in on the first quarter (“half”) but setting before midnight, when the storms tend to start increasing activity. The nights have been far from balmy here (and far from clear most nights,) so we’ll just have to see if I’m brave enough to, um, brave them for the abysmal luck that I’ve been having with meteor storms.