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Sundial Terms

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Traditional: The horizontal type of sundial, which has hour lines that radiate out from a central, vertical triangle.

Analemmatic: Hours are marked by vertical points on an ellipsis; the human shadow serves as the gnomon; during different months of the year, one stands at a different place, indicated in the construction of the sundial itself.

Equatorial: This sundial is constructed of a disc perpendicular to a north star-oriented gnomon. These are very easy to make.

Gnomon: From the Greek for ‘indicator’, this is the shadow-casting object that marks the hours on a sundial.

Polaris: The North Star; used in positioning some kinds of sundials.

Latitude/Longitude: Your exact position on earth; used in calculations for best accuracy in constructing the sundial.

Orientation: The direction the sundial must face to be accurate, depending on one’s location. In traditional sundials, the orientation is fixed, parallel to the earth’s axis.

Declination: Used to calculate the angle of the gnomon, this is the angle between the rays of the sun and the plane of the earth’s equator. This will depend on your latitude.

Mean time: Calculated by the ‘average’ amount of hours, minutes and seconds in a day, this has been described as ‘time as we wish it was, rather than time as it actually is’. Mean or standard time is also known as ‘clock time’. Clock time assumes twenty-four hours between one noon and another. In reality, this is not the case.

Solar time: This is influenced by the proximity of earth to the sun at different times of year, as well as by the earth’s axis. It is predictable rather than exact, and can vary depending on the time of year. There can be a twenty-second discrepancy between solar time and clock time. This is why some people may deem a sundial ‘inaccurate’, when in fact it is measuring one kind of time and they another.

For further study:

www.steveirvine.com
www.plus.maths.org/issue11/features/sundials
www.sundials.org
www.sundials.co.uk
www.shadowspro.com

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