Dial analemmatic

The example presented here is located at Saint-Etienne (the Loire) France. with 45° 27 ' and 4°5e.Il main roads of 2 meters have and were designed so that a person of 1 m 20 top, can see her shade projected there to read the hour at any time of the year. The position of the sun varies in height for the same hour, during the year. heights are identical (example at the equinoxes or on January 20th and on November 22nd). It is necessary to be placed along a line where different dates are indicated. It is obvious that this type of dial is more made to give the principle of the reading of the hour to the sun, to allow the reading with exactitude of it with a margin of 1 minute or 5 minutes! Indeed, the person is used as index. It is thus necessary to take account of the size of the shade of its head, and its correct placement along a line which shows the dates of placement.

Here an image of the dial seen since the south. The pink granite studs mark the 5 hours hours of the morning (on the left) at 7 p.m. (on the right)

Fundamental plans and axes

With the fundamental plans, horizontal and equatorial, and the polar axis, here what that gives. The ellipse on the ground is the projection of the equatorial circle illustrated in blue here.

Detail of the line of the dates: For the calculation of the important data, here 3 intermediate and symmetrical dates. (it there at the same distance between the central indent, March 21st, 23 seven which is related to the equinoxes and the solstices. (30,4 cm here) dated May 21st is to 25,7 cm. It is possible to make the drawing in proportion for the same latitude with 45,5°) For calculation, it is necessary to use the data:

D = variation of the sun,

L= latitude of the place of observation and

R = radius of the large circle.

The formula the length S between the indent of equinox, centers dial and the indent of the desired date is S = R X Cos L X Tan D

Construction

For a placement of the hours without calculation, here a graphic method:

1) To trace the large circle G (2 m on the joint example). One takes the angle of the latitude and one obtains a second circle P, the small one.

2) To divide into angles of 15° (360°/24) milked in blue on the drawing To trace lines parallel with the East-West axis passing by the red circle and a division of 15°, then with the axis North-South and passing by the black circle and the same division of 15°.

3) The intersections related to the same angle give the points hours… One could proceed with subdivisions to have the 1/2 hours, but it is not realistic, readable, with this type of dial.

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