Cadran solaire est une horloge qui utilise la position du soleil, une tige jette une ombre sur un plan sur les lignes horaires.
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A sundial is an instrument for measuring time based on the detection of the position of the Sun. Has very ancient origins and in its most general sense indicates sundials present on the walls of buildings. Is made up of a stylus, called the gnomon, it is the rod that casts its shadow on the plate (or dial) of the watch allows timekeeping. In dial hour lines are drawn corresponding to the hours. |
A sundial can be tailored to a particular latitude bending the gnomon so that it is parallel to the axis of rotation of the earth, so that it points toward the north pole in the northern hemisphere and towards the south pole in the southern hemisphere.
The study of sundials is called gnomonic.
To build a sundial you need to know:
the coordinates, latitude and longitude
declination of wall
Equation of time
The accuracy of this information that will determine the accuracy of sundial.
Use the easy software to build the sun dial in few minutes, click the link: Construire un cadran solaire
The earth’s coordinates, latitude is the angle between the equatorial plane and the straight line that passes through that point and is normal to the surface and longitude the angle east or west from a reference meridian to another meridian that passes through that point.
Is very important to calculate it accurately (up to 6 decimal places) to get a more accurate sundial, you can use this tool, find your home, zoom in and move the marker over your wall. Haut de la page
Declination of wall
The declination of a wall is the angle in degree between the south and perpendicular axis of the wall (north for the southern hemisphere), is counted positive towards West and negative towards East.
There are more method to calculate the declination, let’s see some:
A compass indicates the direction of the magnetic pole, but there can be several errors in the measurement, bisurbation like mountains. You can use a easy tool Mesurer sur la carte
select distance mode, move the marker over the wall use max zoom and click to get a second marker parallel to the wall, read the value of the direction and add 90 degrees to get the declination
Get the sun position with this tool la position du soleil
set coordinates date and time (accurately around noon), move the mouse over the sun icon and read the azimuth and elevation, whit this data you can calculate the shadow of a nail, the the difference (theory for south and real value) in degrees is the declination Haut de la page
It is equivalent to the difference of time, early or late, between the meridian passage of the sun (time zones which are adjusted our watches, 15° every hour), and that the longitude of the sundial. Haut de la page
Equation of time and analemma
The dials do not convert the common apparent solar time in standard time. It therefore has a variation of 15 minutes in a year, known as the equation of time, caused by the fact that the Earth’s orbit is elliptical and not circular, and is inclined with respect to the equator (see ecliptic)
The visual representation of this equation is the analemma, in astronomy indicates a particular geometric figure-eight curve that describes the position of the sun on different days of the year, at the same time and in the same locality.
Due to the tilt of Earth’s axis ( 23.5° ) and ellipticity Earth’s orbit, the height of the Sun above the horizon is not the same, day after day, and the combined effect is that of the figure described. The vertical coordinate of each point corresponds to the declination of the Sun at that time, while the horizontal coordinate indicates the deviation of the solar position earlier or later than the average time ( as shown by the clocks ) .
A Simple Expression for the Equation of Time:
n = day of the year.
Δt = 9.873 sin( 4π / 365.242 ( n - 81 )) - 7.655 sin( 2π / 365.242 ( n - 1 )) Haut de la page
Build a sundial manually
The most important element of the sundial is the gnomon, which consists of a rod that casts its shadow on a sundial on which the hour lines must be drawn. By the type of sundial (vertical, horizontal, equatorial ring, etc.), the size, from the coordinates and from the declination determines the length and its position. Haut de la page
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