A gnomon is the pin or triangular plate on a sundial which causes a shadow to be cast by the sun onto the graduated dial. Originally the term was applied to structures erected perpendicularly to the horizon, from whose shadow the altitudes, declinations, etc, of the sun and stars may be determined. The gnomon was usually a pillar or column or pyramid erected upon level ground. It was much used by the ancient astronomers, and gnomons of great height, with meridian lines attached to them, were still common in France and Italy at the end of the 19th century. Research Gnomon
A sundial or dial is an instrument for showing the hour of the day from the shadow thrown while the sun is shining by a stile or gnomon upon a graduated surface. The sundial has been known from the earliest times amongst Egyptians, Chaldeans, and Hebrews. From those eastern nations it came to the Greeks. It was introduced into Rome during the first Punic war.
Sundials are of various construction, horizontal, inclined, or upright, the principle in every case being to show the sun's distance from the meridian by means of the shadow cast by the stile or gnomon. The stile is made parallel with the earth's axis,and may be considered as coinciding with the axis of the diurnal rotation. Consequently as the sun moves westwards the shadow of the stile moves round in the opposite direction, falling on the meridian lines so marked as to represent the hours of the day. The sundial of course gives solar time, which, except on four days of the year, is slightly different from that of a well-regulated clock. Since at least 1900 sundials have been rather articles of curiosity or ornament than of use. Research Sundial
Eudoxus was a Greek mathematician and astronomer. He was born at Cnidus, Asia Minor around 400 BC and died in 347 BC. In his early life he attended lectures by Plato and later devised the hypothesis of concentric spheres to explain the stationary points and retrogradations in the motions of the planets. According to Pliny and Strabo it was Eudoxus who first fixed the length of the year at 365.25 days, while Vitruvius ascribes to him the invention of the sundial. He was also a philosopher and was much admired by Marcus Cicero. In mathematics Eudoxus' early success was in the removal of many of the limitations imposed by Pythagoras on the theory of proportion. He also established a test for the equality of two ratios. The model of planetary motion was published in a book called On Rates.
Further astronomical observations were included in two other works, The Mirror and Phaenomena. In a series of geographical books with the overall title of A Tour of the Earth, Eudoxus described the political, historical, and religious customs of the countries of the eastern Mediterranean. He devised the first system to account for the motions of celestial bodies, believing them to be carried around the Earth on sets of spheres. Work attributed to Eudoxus includes methods to calculate the area of a circle and to derive the volume of a pyramid or a cone. Eudoxus probably regarded the celestial spheres as a mathematical device for ease of computation rather than as physically real, but the idea was taken up by Aristotle and became entrenched in astronomical thought until the time of Tycho Brahe. Research Eudoxus
An obelisk is a stone pillar having a square or rectangular cross section and sides that taper towards a pyramidal top. They were often used as monuments in ancient Egypt, where they symbolised the supreme god. They were known as Pharaoh's needles by the Arabs, and fingers of the sun by the Egyptians. The first recorded obelisk was described during the reign of Rameses II about 1322 BC, but the Romans also took to them, emperor Augustus erecting one in the CampusMartius on the base of which was a sundial. Cleopatra's Needle on the Thames Embankment in London is an example of an Egyptian obelisk. Research Obelisk
Abbreviations
Research Gnomon
