Well, the point is that unless you are using it for an astronomical observatory, for an equatorial mount you only need it move it in steps of ten degrees or so, possibly more finely if you are using it for a solar furnace. This means you only need move it every forty minutes.
What you need is a large gearing ratio such that the stepper motor will not move when not powered. A common (hobby) servo will not stay put (especially when there is wind shear on the assembly) and will tend to use power to keep itself in place. A common stepper will also require power to stay in position but with appropriate gearing may be powered up only to make individual movements. You really need a stepper and high gearing of the sort that you would of course find in a clock (running at half speed).
In general, this requires something purpose-built.