At sea a single sight can be a few minutes of arc out: the boat rolls, a wave hides the horizon, the drum is misread. So navigators take a run of sights, five or more in a few minutes, timing each one.

Each dot is one sight. The line is the best fit through the dots you keep; the ring is their average. Tap a dot to leave it out or put it back.

Sights used: 7 of 7
Average time and Hs: 10h16m00s, 38°32.7'
Average Hs is off by: 0.9′

One dot sits well off the line: a misread drum, the wrong limb, or a wave hiding the horizon. Averaging it in drags the result off by about a mile. Leave it out.

Over a few minutes a body's altitude changes almost in a straight line. Plot the sights against time and the good ones fall close to a line; a blunder stands out. Leave it out, then use the average time with the average Hs (or the one sight that sits on the line). The result is better than any single sight.

Take several sights, not one