If we stood exactly at the geographical North Pole, Polaris would always be directly overhead. In the northern hemisphere, there is a moderately bright star near the celestial pole, which is the North Star or Polaris. If we were to take a photograph of the northern sky with a long exposure, we would see from the trails of the stars that they all revolve about a common point, which is the northern celestial pole (Figure 2).įigure 2: Trails of stars at the sky after an exposure time of approximately 2 hours (Credit: Ralph Arvesen, Live Oak star trails,, ) The Earth’s equator becomes the celestial equator and the geographical poles are extrapolated to build the celestial poles. If we project the terrestrial coordinate system of latitudes and longitudes in the sky, we get the celestial equatorial coordinate system. From there, longitudes are counted from 0° to +180° (eastward) and -180° (westward).Įxample: Heidelberg in Germany is located at 49.4° North and 8.7° East. The origin of this coordinate is defined as the meridian of Greenwich, where the Royal Observatory of England is located. This is the line that the Sun apparently crosses at local noon. For a given position on Earth, the longitude going through the zenith, which is the point directly above, is called the meridian. The longitudes are great circles connecting the two poles of the Earth. They are counted from 0° at the equator to ☙0° at the poles. The latitudes are circles parallel to the equator. The equator is defined as the great circle halfway between the poles. The North Pole is defined as the point where the theoretical axis of rotation coincides with the surface of the sphere, and the Earth rotates in a counter-clockwise direction when the pole is viewed from above. Two angular coordinates remain, which for the Earth are called the latitude and the longitude. Instead, we can use spherical polar coordinates originating from the centre of the sphere, which has a fixed radius (Figure 1). The surface of a sphere is a curved area, and using directions like up and down is not useful, because the surface of a sphere has neither a beginning nor an ending. Figure 1: Illustration of how the latitudes and longitudes of the Earth are defined (Credits: Peter Mercator, djexplo, CC0).Īny location in an area is defined by two coordinates.