The ancients considered the Earth to be flat and located at the centre of the Universe, with the sky a crystal dome on which the stars were fixed. It was around 2000 BC that Aristotle realised that the Earth is a globe. There is clear evidence to support this view. For instance, there is the differing altitude of the pole star as seen from different locations and the fact that the most southerly of the constellations are only rendered visible by travelling southwards (the converse is true for people living in the southern hemisphere). More commonly known is the phenomenon of the lower parts of a ship which is travelling away from the shore disappearing from view while its sails are still visible above the horizon. The sails are then seen to ‘sink’ below the horizon as the ship further recedes.
KeywordsCelestial Body Universal Time Local Solar Time Sidereal Time Royal Greenwich Observatory
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