Even very careful observers, such as Tycho Brahe, were able to obtain only fairly accurate results.

But by applying Gascoigne's invention to the telescope almost absolute accuracy became at once possible.

The principle of Gascoigne's micrometer was that of two pointers lying parallel, and in this position pointing to zero.

These were arranged so that the turning of a single screw separated or approximated them at will, and the angle thus formed could be determined with absolute accuracy.
Huygens's micrometer was a slip of metal of variable breadth inserted at the focus of the telescope.

By observing at what point this exactly covered an object under examination, and knowing the focal length of the telescope and the width of the metal, he could then deduce the apparent angular breadth of the object.