How to make an overlapping shadow effect on one side in illustrator

Question

I'm trying to recreate an effect like the iOS activity app icon where a circle overlaps itself and leaves a shadow. However I'm struggle to recreate it.

I've tried cutting a segment of one of the circles and overlaying it on top of the other one. Using a drop shadow doesn't work though as it appears all over the cut out segment rather than just the end. I tried using a gradient on the "behind" layer as well, but I couldn't position this correctly either to achieve this type of effect. I've also tried adding another full circle in between the two layers with a gradient applied to it, but it doesn't look like a shadow being created from the "top" layer.

If it helps my circles are a solid color rather than a gradient as below, but I just can't work out how to create it.

Answer 1

Several ways are possible, here's one:

It's made of pieces. They are moved apart in the right. The number is the layering order. 1=Top, 4=Bottom. The shadow is partially transparent black blurred copy of the half circle. It's deformed a little to make the shadow effect mostly horizontal. That's not needed on a black background.

Not everyone like blur based effects because they are often rasterized in format conversions and lose the scalability. Another option is to add a 2nd fill color to the bottom shape in the Appearance panel. It's a radial gradient from opaque black to transparency. The intermediate stop is needed for the actual shadow. If you adjust the gradient stops carefully it can create quite plausible illusion of depth with only 2 separate pieces and one colored fill. An example:

NOTE: The gradient is adjusted to elliptic to get a long depth making shading but the seam area is not darkened.