[Paul R.]

Why would the “habital zone” not be essentially spherical shape?

(The definition of “habital zone” seems pretty squirrely to me, anyway.)

[LibWhacker]

A planet moves in an elliptical path around a star. Put any planet in orbit around a star, and its orbital path around the star will be an ellipse. Furthermore, it must move. It can't just sit there not moving in that spherical region you're talking about, or else it would fall into the star. But there is a sphere, yes. Good point. But the entire sphere is not relevant for any planet that wants to be in "the habitable zone." That's why they don't talk about the whole sphere. If a planet tries to sit unmoving somewhere in that sphere, it'd soon be drawn into the star and life wouldn't even get a start on it.

[LibWhacker]

Why would the “habital zone” not be essentially spherical shape?

Ooops, my mistake! The habital zone is not essentially spherical in shape. It is a torus, i.e.,it is shaped like a tire (see that greenish region in the first pic at the top of the page). If your planet orbits too close to the star and never enters that torus, it'll be too hot for liquid water to exist on the surface and life as we know it couldn't exist there. But if your planet's elliptical orbit is entirely within that torus, and has water on it, there is at least some chance it is inhabited. No one knows just how good that chance is; there isn't enough data, etc., I'm sure you know the argument.