Too sharp edge of the Earth's shadow during a lunar eclipse

[10110111]

See the screenshot from Stellarium:

And here's how the same view looks on a photo (taken here):

As you can see, real-life photo a has much smoother gradient, demonstrating the gradual hiding of the Sun behind the Earth for a lunar observer that moves into the eclipsed part of the Moon.

[gzotti]

The problem here is the incredible brightness contrast between bright (opposition surge!) full moon, zone of deep penumbra and total eclipsed zone. This can hardly be photographed correctly. You could have exposed 2x longer to have overexposed the right side and would see a distinct shadow edge on the left. When I last tweaked the Earth shadow I concentrated on the central shadow. To approximate the naked-eye view I increased brightness of the final white part (thus highlighting the dark red shadowed part), just as most photographers increase exposure time near totality. This edge is surprisingly sharp (as it is that magical moment when an observer on the Moon would finally experience totality), and observing it covering craters used to be a scientific endeavour. Where you can play is the gradient in the penumbra, or you can probably rebalance this "overexposure". This was 2015-09-28.

[gzotti]

Here is another.

[10110111]

Your time series of the eclipse photos in the previous comment is really nice. And the current render in Stellarium looks very artificial and reminds me of the games from the 90s.

Why can't we have a similarly nice render in Stellarium, acting like so:

• When most of the Moon is lit, we render the dark part simply dark, as a human would see in reality;
• As the lit part decreases, we gradually increase exposure, letting the lit part of the lunar surface get overexposed in order to expose the red shaded part;
• At the totality we have only the shaded part, so there're no blown out colors?

This is similar to what we do with solar corona at solar eclipses, but more gradual. This way we get both the visibility of red and the realism of all phases of the lunar eclipse.

BTW, do we have a user-accessible exposure knob in Stellarium?

[Atque]

Maybe the shadow edge could appear fuzzy (and the shadow completely dark) until a certain percentage is covered (30-40 or so), and then let the edge sharpen and exposure increase? Or would that require another texture? Perhaps two textures (one fuzzy and one sharp) could merge together as the eclipse progresses? I'm just brainstorming.

[gzotti]

1) The moon should generally become as bright as possible without burning out even when atmosphere is on. 2) During eclipses, the current shadow texture can be used, maybe tweaked with a stronger falloff in the partial phase. The shadow edge must NOT become fuzzier. Conventional photographs do not do justice to the contrast range. 3) Close to totality the current "exposure boost" can be modfied. However, don't overdo it! 4) The outer edge of the penumbra is invisible. We show a symbolic edge. 5) The eclipse is visible with the unaided eye when eclipse magnitude in the penumbra exceeds 70%. Photographically, it is detectable earlier simply by required exposure time adjustments. This could influence Lunar magnitude, but the brightness on screen should not change, to have reserve for the actual eclipse.

One challenge of (1) is getting the Oren-Nayar constants right. I think the current solution is "textbook". Maybe something can be tweaked. Check photos of crescent phases to match terminator brightness. I remember processing a crescent photo on TechPan2415 in my (chemical) darkroom. The film captured a great range of brightness, most likely better than many digital sensors. There was some manual dodging necessary to have both terminator visible and bright edge not overexposed. Today probably an HDR photo could capture more, but many look also artificial.

[gzotti]

Just to give an idea about exposure times, deep totality with a certain configuration required more than 10s exposure time, about 4 at totality end (bright outer umbra with slightly overexposed edge), quickly reducing to 1/4s to avoid white burnout as the moon egressed to 75%, 1/13 for 50%, and 1/100 for 25% and less. With this exposure, the shadow edge is not clearly visible but the Moon, still fully shaded by penumbra, is not overexposed. I can send more pics, but can continue on this only in 2 weeks, sorry.

[gzotti]

I just found my files from my last texture tweak (2018). Will try to further modify the penumbra side of the shadow edge. Compare 1.0 to 0.18.1.