ARCturus

In the early times of history, the practical way of defining time was to observe and interpret natural phenomena. The signs of seasonal variations were seen in nature, but that did not help in everyday life. Motions of the Sun and phases of the Moon were monitored and registered in many ways. In the ancient times, the signs were marked on trees, cliffs and even the ground. When the reading literacy improved, scientists used symbols, letters and numbers to making notes on natural surfaces and subsequently on paper.

The scientists detected several regularities in the motions of the Sun. The altitude of the Sun above the horizon varies in the succession of the seasons.  Likewise, the seasonal variations also change the horizontal point of the sunrise in the east and the sunset in the west. Due to the rotation of the Earth, it seems as if the Sun rose in the east and set in the west. In daytime, the Sun being above the horizon, an opaque obstacle creates a shadow that slowly shifts from the west to the east. This obstacle can be a tree, rod or even a short stick fixed perpendicularly on the yard. The even movement of the shadow was the basis for developing a sundial.

There are myriad models, starting from a simple stick version and ending up to beautiful artistic sundials. The picture shows a unique model on the roof of a building in Palma (picture IH, Finland).

An equatorial sundial is constructed of a plate parallel to the level of the equator with a semi-circle of the clock face. Fix gnomon, i.e. a stick, which creates a shadow, on the centre of the semi-circle in the direction of the Earth axis and perpendicular to the plate. Fix the plate and the stick on a horizontal base so that the angle between the stick and the base indicates the latitude of the city. The noon mark on the face is turned towards the north because the Sun is in the south at noon. The time measured is local. If the location of the sundial is not at zero meridian, the time measured should be updated once in a while (see ARCI Sundial).

The equatorial sundial can only be used when the Sun is above the equatorial level. It means, for instance, that the sundial is useless in midwinter in northern Finland. One solution to this problem is to project the scale of hours on a shadow plate which is lying on the ground. This procedure changes the equatorial sundial to a flat sundial.

On the Internet, there are plenty of models and pictures of sundials. Use an advanced search in order to find the best model for your purposes. On the list below, there are some website addresses with useful information on a wide range of topics concerning sundials.

 

 

www.sundials.co.uk

An English pages with a manifold collection of sundials

www.sundials.org/

North American Sundial Society NASS, versatile and equivalent to the English pages

www.sundial.net/

Armillaries and Garden Ornaments, a site introducing decorative sundials.

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