4 ms·

... what is a sextant? https://en.wikipedia.org/wiki/Sextant https://en.wikipedia.org/wiki/Sextant

• adrian_b 4y ago
  A sextant requires you to wait the local noon (the maximum rise of the Sun over the horizon), when measuring the angular height of the Sun allows you to compute the latitude and the time when the noon happened allows you to compute the longitude. Instead of a sextant, one could use a vertical rod fixed on a horizontal and plane surface (in a good sundial either the gnomon is inclined, or the base is inclined, or both, so it would be less appropriate for this task), and wait for the moment of the shortest shadow, when the length of the shadow gives the Sun's angular height and the time gives the longitude (and the direction of the shadow gives the North-South direction). With a more sophisticated instrument able to measure the 2 angles of the Sun's direction (height and azimuth), together with knowing the time (including the date, not only the time of the day) and together with knowing the local North-South direction, you could compute the latitude and longitude at any time. The local North-South direction could be given by an inertial compass. A cheap magnetic compass could be used, but then for high precision in computing the position, a detailed geomagnetic map of the Earth would be needed and a complex computation would be required to find a place on Earth where the angles of the Sun at the known time and the indication of the magnetic compass would match the observed values.
• mellavora 4y ago
  Doesn't a sextant also require a rather thick book listing the positions of many celestial bodies at certain times/locations?
  • adrian_b 4y ago
    Only if you use it to measure the angular height of stars during the night. When you use it for the Sun at the local noon, as in traditional navigation, you only need a chronometer that keeps the time from some place of known longitude, and nothing else.