Why is my World shader with a gradient in 2.8 only showing some of the colours in my colour ramp?

I would like to set the ‘world’ shader to a gradient, which should be one colour at the nadir, another colour at the zenith, and other colours in between, using a ColorRamp node.

There is an answer here, but it’s for an older version of Blender and doesn’t seem to behave quite as described in 2.8. I do get a gradient, but no matter how much I fiddle with it, I can’t seem to get it to cover the whole range of the ColorRamp.

There are also various answers covering how to get a gradient across the window, but that’s not what I want. I’m aiming for more of a sky dome sort of effect.

This is my current set-up, which isn’t quite working – it seems to be correct in the upper half of the sky, but the lower half is a uniform yellow-green.