Hemispherical sundial

Greek Sundial

The first part of this article discusses Sundials. The second part of the article discusses Water Clocks.
For this second part see Water Clocks

In the time of the ancient Greeks and Romans, the earth was considered the centre of the universe, which was itself a sphere containing all the stars. This celestial sphere rotated from east to west, carrying not only the stars but also the sun and the planets. Therefore, the sun revolved around the earth. This is what caused day and night. The earth did not rotate. For the purpose of understanding sundials, it is perfectly acceptable and convenient to adopt this geocentric view. The sun did not travel around the earth in a circle at right angles to the earth's axis (which was also the axis of the celestial sphere) as the stars did. Rather, the sun traced a circle along the celestial sphere, centred on the earth, known as the ecliptic.

The ecliptic plane meets the equatorial plane at approximately 23.5°. This is known as the obliquity of the ecliptic. The circle of the ecliptic more or less intersects the twelve constellations of the zodiac, and the time of year (corresponding to modern months) was reckoned by what sign of the zodiac the sun was traversing. (Regardless of the exact location of the zodiac constellations, the ecliptic was divided into 12 equal arcs of 30° each, leaving most of the constellations off-centred and often not entirely in their designated 30° region.) The sun's motion along the ecliptic circle takes a (solar) year. The dual motion of the sun (on the celestial sphere and along the ecliptic) means that the sun follows a different path in the sky each day. From the perspective of the northern hemisphere, during the summer, the sun is higher in the sky and remains visible for a longer period of time. Since the ancients always divided the daylight into twelve equal hours, these summertime hours were longer. In the winter months, the sun is lower in the sky and visible for a shorter period of time. Consequently, the winter hours were also shorter.

Time in the ancient world was first measured by naturally occurring events, such as sunrise, sunset, and meal times [1]:-

In the early ages of Rome and even down to the middle of the fifth century after the foundation of the city no other divisions of the day were known than sunrise, sunset and midday, which were marked by the arrival of the Sun between the Rostra and a place called Graecostasis.

The single greatest literary source that exists for the sundials of Greece and Rome is Vitruvius's Ten Books on Architecture written about 25 B.C. In Book 9, Vitruvius gives a list of a variety of dials and their inventors [2]:-

Berosus the Chaldaean is said to have invented the semicircular one carved out of a squared block and undercut to follow the earth's tilt. The hemisphere, or scaphê, is attributed to of Samos, and he also invented the disk on a plane. The Spider was invented by the astronomer; some say by . The Plinth or Coffer, of which an example is set in the [region of the City known as the] Circus of Flaminius, was invented by Scopinas of Syracuse; Parmenion invented the "Sundial for Examination"; and Andrias the sundial "For Every Climate, " Patrocles the Axe, Dionysodorus the Cone, the Quiver. The men named here invented other kinds, and many others have left us still other kinds, like the Spider-Cone, the Hollowed Plinth, and the Antiboreus ("Opposite the North")

Vitruvius's analemma is the system of lines and curves that denote the changing hours and months on the face of a sundial. His previous chapter is devoted to determining the analemma based upon the observance of the shadow of a gnomon at noon on the equinox. (The gnomon was the upright stick that cast its shadow on the dial face. Depending on the design of the dial, either the side of the shadow's length or the position of the tip of the shadow was used to determine the time.) Unfortunately, Vitruvius ends his discussion of sundials with the list given above and writes of water clocks for the rest of Book 9.

Before the Greeks developed the sundial into the forms Vitruvius lists, the more ancient civilizations of Egypt and Mesopotamia had shadow measuring devices as early as 1500 B.C. Though this is the date of the earliest surviving sundials [3]:-

A funerary text from 1290 B.C., referring to astronomical events in the 19th century B.C., gives instructions on how to construct a "shadow stick."[4]

This shadow clock consisted of a base with an upright stick at one end. Because of the angular shift in the shadow over the course of the day, it has been speculated that the upright had a crossbar added to it to widen the shadow so that it would always fall on the clock. Neither the funerary text nor surviving examples have the crossbar, though one specimen has holes on either side of its upright which may suggest such an addition.[4]

In practice, the shadow clock needed to be rotated once a day at noon in order to be able to mark the time in both the morning and afternoon [5]:-

With the head to the east 4 hours are marked off by decreasing shadow lengths after which the instrument is reversed with head to the west to mark 4 afternoon hours.

Two hours are said to have occurred before the sun struck the clock in the morning, and another two hours passed after the sun left the clock but before night began. The assumption is that the morning twilight before sunrise was counted as one hour, and that another hour passed between sunrise and when the upright cast an observable shadow on the clock. (The shadow at sunrise would be infinite in length, and so useless for marking the hour.) Two hours similarly passed in the evening.[5] The markings on the clock indicating the four hours were very inaccurate, and were possibly not based on observation but rather some fallacy of celestial geometry.[4]

Sundials resembling the kind of which Vitruvius speaks were in use in Egypt from at least 1200 B.C. These were vertical hanging sundials, semicircular in shape with a horizontal gnomon at the top and centre. "The shadow would sweep around such a dial more rapidly in the early morning and late afternoon than around midday, but the Egyptians simply divided the dial into 12 15° sectors or 'hours'. This is perhaps the crudest order of gnomon use and provides little of either theoretical or empirical interest for the Greeks."[4] Further Egyptian development in timekeeping seems to have waned until the Assyrian invasion in the 7th century B.C.[4]

A near complete sundial was found at Kantara, Egypt dating back to approximately 320 B.C., well over a thousand years after the shadow clocks were in operation [6]:-

Source: www-groups.dcs.st-and.ac.uk
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