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Hand for the determination of true solar time - shows true solar time on the face of the clock. True solar time differs from mean time that has been adopted by humanity for measurement by mechanical (and later electronic) timepieces. Over the year, the difference between true solar time and mean solar time ranges between -14 minutes 22 seconds and +16 minutes 24 seconds, while the change in the length of consecutive calendar days can reach 50 seconds. There are two main factors that contribute to this phenomenon: the elliptical, rather than circular, orbit of the Earth around the Sun, and the tilt of the Earth's axis of rotation. While moving around its elliptical orbit the Earth, little by little, changes its position relative to the Sun. The distance between them increases and decreases which influences the length of the true day (midnight to midnight). In the second half of the eighteenth century sundials were still extensively used and they could by their nature only show true solar time. These fairly simple devices for determining time could be found everywhere in city streets, in parks and squares, even on more elaborate milestones. The spread of pendulum clocks that displayed mean time was an irritation to many less sophisticated people and so when he produced his mechanical masterpiece Johann Georg Strasser included a mechanism for determining true solar time. |
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