6. To understand how a sundial works

THE IMPORTANT STEPS to understand sundials, are:-

1. To identify the dial surface which can be of any orientation, any shape, and the marking on the dial surface
2. To identify the gnomon (or style) which is to cast a shadow (by sunlight, of course) on the dial surface
3. To identify a STRAIGHT LINE related to the gnomon that is PARALLEL TO THE AXIS OF THE EARTH. Sometimes this line physically exists, e.g. an edge of the gnomon of an old sundial - See Photos P7, P8 - on the Roof of HKU Main Building, and the dark string of the HKUST red sundial at http://sunsite.ust.hk/hkust/campus/sundial.html.

	Photo P7 - The main building of Hong Kong University is designated a historically conserved building in Hong Kong. This building was opened on 11 Mar 1912 by the British governor of Hong Kong Sir Frederick J D Lugard. On the parapet wall of the West side of the roof of this building, there is a horizontal sundial, possibly of 25 years old, according to somebody who is with HKU for 30 years.
	Photo P8 - A horizontal sundial humbly sits on the roof of HKU - Main Building, for all seasons, in sunshine or in shadow. The wedge of the gnomon is to be parallel to the axis of the earth.

In the equatorial sundial existing in The Forbidden City in Beijing (See Photos P1, P2), the gnomon (i.e. the pointer), which is normal to the dial surface, is parallel to the exis of the earth. This style is inclined at 40° to the horizon because Beijing is at 40° North. The dial surface, therefore, is inclined at 50° to the horizon, and is parallel to the equatorial plane of the earth. Refer to the diagrams of Sunlight-2 heliodons for solar geometries.

Photo P9- A horizontal sundial mounted on a sundial tester

Photo P10- Sunlight-2 heliodon, testing a horizontal sundial

In another design of a horizontal sundial for Hong Kong (shown mounted on a sundial tester which is in fact the Sunlight-2 Heliodon), the LINE PARALLEL TO THE EARTH AXIS is a virtual line joining the tip of the metal pointer and the meeting point of the straight hour lines. This virtual line inclines at 22.37° to the horizontal dial surface because Hong Kong is at 22.37°N.

Note that in some sundials, the gnomon may move (hence the line parallel to earth axis may move too), or a bright spot of sunlight falling on the dial surface tells the time/day instead of the shadow cast. Depending on the time(s) to be read viz. apparent solar time, mean sun time, local standard time (http://www.crest.org/staff/ceg/sunangle/), the gnomon may be of irregular shape.

4. Take the ASSUMPTION that there is practically no difference between topocentric observation (i.e. observing the sun at a location on earth surface) and geocentric observation (i.e. observing the sun fictionally at the centre of the earth) in relation to the variables of concern of solar geometric parameters (http://www.crest.org/staff/ceg/sunangle/index.html) for constructing sundials. Thus the sundial is interchangeable freely to be placed at a point on the surface of the earth and fictionally at the centre of the earth, to obtain the same accuracy. For general purposes including sundial construction, this assumption is acceptable because the distance between the sun and the earth far outweighs the diameter of the earth. This assumption is most prominently illustrated by those universal sundials which can operate for a large range of latitudes.



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