Then there are polar dials, on which the hour scale lies parallel to the gnomon. With this form, the hours are inscribed on a ring with the same centre point as the gnomon. There are several other forms a sundial can take. On equatorial dials, the hour scale is a disc that lies at right angles to the gnomon, on the plane of Earth’s equator. This makes it difficult to read around the time of the equinoxes, so an improved version is the armillary sphere. This particular sundial is designed with numerous miniature slots that are placed at various angles. If you own an equatorially mounted telescope, you may recognise this setup as the definition of the polar axis – parallel to the rotational axis of the Earth. A variation of this form, the vertical dial, is often found on the south-facing side of churches and buildings. French group Mojoptix, a member of the digital design-sharing website and community Thingiverse, has invented an innovative digital sundial that works precisely and accurately and does not make use of moving parts or electronics of any kind. In the modern world, there are much easier and more precise ways for telling the time, but the sundial could still have a place in your garden, serving as a garden ornament. When most people think of a sundial, they picture a garden, or horizontal, dial mounted on a pedestal. On a correctly positioned example, the gnomon will point due north, and its angle to the horizontal will equal the latitude of its location. The sundials are the oldest known instruments for telling time. With the invention of magnetic compasses at the end of the 14th century, portable directional sundials finally became a reality. Greek astronomer Ptolemy at work with a sphere.
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