For centuries a sailor far from land could answer one of the two questions that mattered, and only one. The question he could answer was how far north he was.
The instrument was a sextant, and the measurement took a minute. He looked at the star that sits almost exactly above the north pole and measured the angle between it and the horizon. That angle is his latitude. Not roughly — that is what latitude means.
Stand at the equator and the star sits on the horizon: an angle of nothing, and a latitude of nothing. Stand at the pole itself and the star is straight overhead: a right angle, and the highest latitude there is. In Tashkent it stands about forty-one degrees above the northern horizon, which is exactly how far north Tashkent lies.
The second question — how far east or west — needed an accurate clock, and for a long time no clock could keep time on a moving ship. Ships were lost for want of it.
What makes the first measurement work is that a triangle's angles do not care about its size. The triangle joining the sailor, the centre of the Earth and the direction of the star is enormous, and he measures one of its angles from the deck of a small ship with a piece of brass.