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vendredi, 15 décembre 2017
Learn more about sundials
Article Index
Learn more about sundials
the different kinds of sundials
How to install and direct a sundial
Informations given by the sundial

Informations given by the sundial

First of all, a sundial is a way to know the solar time.

To know the official time (the one of your watch) it is necessary to apply corrections related to the longitude of the location of the dial as well as the equation of time:

In France, one knows that the sun rises earlier to Strasbourg that in Brest (about 1 hour of offset). The correction of longitude permits to take into account this offset in relation to the meridian of reference.

The shadow of the gnomon showing Solar Time on the sundial  is of varying lengths during the year, unlike the Mean Time used in clocks. The difference between the mean time and solar time is expressed by the equation of time (this difference is due to the change in speed of the Earth on its elliptical orbit around the Sun and the eccentricity of the orbit). The correction of equation of the time can go until 16 minutes of advance or delay according to the period of the year.

But a sundial can also indicate the changes of signs of Zodiac, the solstices and equinoxes or birthdays dates .

Some sundials even make it possible to know since how long the sun rose and how long it remains before it lies down.