Basically, how can we measure the Earth’s rotational axis accurately enough to be able to erect a physical [pole](https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Fantarctic-logistics.com%2Fwp-content%2Fuploads%2F2016%2F08%2Fdestination-south-pole-1.jpg&f=1&nofb=1) that goes through it? And how did expeditions in the early 20th century know when they successfully reached the poles?
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Before radionavigation such as GPS people mainly used dead reconing and celestial navigation. Dead reconing is when you measure the direction and the distance you travel and plot this on a map to find out where you are. If you look at some images of the polar explorers you might see that they often leave a wheel trailing behind them which have a counter so they can count how far they have traveled. When they are stationary they can use celestial navigation which is done by measuring the angles to different celestial bodies such as the sun, moon or stars to the horizon at different times. This is done by the use of a sixtant or octant and a good clock. The first mission to the South Pole spent hours on the pole making celestial observations to make sure they were in the right place. A third technique of navigation which could be used on land was triangulation. This is when you measure the direction to known landmarks so you can plot your position on a map. This was the most accurate way of navigation but only works when you are in an area of mapped landmarks. Also to note is that even with highly accurate GPS technology all these older methods is still being used either as backup to GPS as they are more reliable or as primary systems because they are more accurate.
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