The North Pole is at the top of the Earth and the South Pole is at the bottom, right?
Except that the Earth is kind of a ball, and they don't really have tops and bottoms. Of course Earth isn`t exactly spherical and isn`t spinning through space - spinning about an imaginary axis of rotation.
One of the points where that axis goes through the Earth, in the Arctic ocean, where in 2007 Russians planted their flag on the bottom of the sea is called the North Pole. More precisely it is the "geographic" North Pole.
Because just like there are different definitions for what a "year" is (tropical year, sidereal year, anomalistic year); there are different "North Poles".
The Earth's axis of rotation – and hence the position of the North Pole – was commonly believed to be fixed (relative to the surface of the Earth) until, in the 18th century, the mathematician Leonhard Euler predicted that the axis might "wobble" slightly. Around the beginning of the 20th century astronomers noticed a small apparent "variation of latitude," as determined for a fixed point on Earth from the observation of stars. Part of this variation could be attributed to a wandering of the Pole across the Earth's surface, by a range of a few meters. The wandering has several periodic components and an irregular component. The component with a period of about 435 days is identified with the 8 month wandering predicted by Euler and is now called the Chandler wobble after its discoverer. The exact point of intersection of the Earth's axis and the Earth's surface, at any given moment, is called the "instantaneous pole", but because of the "wobble" this cannot be used as a definition of a fixed North Pole (or South Pole) when metre-scale precision is required.
It is desirable to tie the system of Earth coordinates (latitude, longitude, and elevations or orography) to fixed landforms. Of course, given plate tectonics and isostasy, there is no system in which all geographic features are fixed. Yet the International Earth Rotation and Reference Systems Service and the International Astronomical Union have defined a framework called the International Terrestrial Reference System.
An Azimuthal projection showing the Arctic Ocean and the North Pole. The map also shows the 75th parallel north and 60th parallel north.(Credit: CIA)
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It should not be confused with the North Magnetic Pole.
The North Pole is the northernmost point on the Earth, lying diametrically opposite the South Pole. It defines geodetic latitude 90° North, as well as the direction of true north. At the North Pole all directions point south; all lines of longitude converge there, so its longitude can be defined as any degree value.
Gerardus Mercator's map of the North Pole from 1595. Author: Gerardus Mercator (1512–1594)
While the South Pole lies on a continental land mass, the North Pole is located in the middle of the Arctic Ocean amid waters that are almost permanently covered with constantly shifting sea ice. This makes it impractical to construct a permanent station at the North Pole (unlike the South Pole). However, the Soviet Union, and later Russia, have constructed a number of manned drifting stations on a generally annual basis since 1937, some of which have passed over or very close to the Pole. Since 2002, the Russians have also annually established a base, Barneo, close to the Pole. This operates for a few weeks during early spring. Recent studies have predicted that the North Pole may become seasonally ice-free due to Arctic ice shrinkage, with timescales varying from 2016 to the late 21st century or later.
The sea depth at the North Pole has been measured at 4,261 m (13,980 ft) by the Russian Mir submersible in 2007, and at 4,087 m (13,410 ft) by USS Nautilus in 1958. The nearest land is usually said to be Kaffeklubben Island, off the northern coast of Greenland about 700 km (430 mi) away, though some perhaps non-permanent gravel banks lie slightly closer. The nearest permanently inhabited place is Alert in the Qikiqtaaluk Region, Nunavut, Canada, which is located 817 km (508 mi) from the Pole. (Wikipedia)
Featured image: Video screenshot (Credit: Minutephysics)