But Tycho Brahe's sextant proved the forerunner of the modern instrument.

The general structure is the same; but the vast improvement of the modern sextant is due, firstly, to the use of the reflecting mirror, and, secondly, to the use of the telescope for accurate sighting.

These improvements were due to many scientific men--to William Gascoigne, who first used the telescope, about 1640; to Robert Hooke, who, in 1660, proposed to apply it to the quadrant; to Sir Isaac Newton, who designed a reflecting quadrant;[8] and to John Hadley, who introduced it.

The modern sextant is merely a modification of Newton's or Badley's quadrant, and its present construction seems to be perfect.
It therefore became possible accurately to determine the position of a ship at sea as regarded its latitude.

But it was quite different as regarded the longitude that is, the distance of any place from a given meridian, eastward or westward.