Azimuthal sundial
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An azimuthal sundial measures the azimuth of the sun. With the additional use of the sun's declination angle, which must be known (or otherwise measured), the time of day can be read off the dial of this sundial .
Azimuthal sundials are stationary, their shadow thrower is a vertical rod. The dial is mostly horizontal.
The most common azimuthal sundials are the spider sundial and the analemmatic sundial .
Measuring principle
With a sundial, the hour angle τ of the sun, which is a direct measure of the time of day, is also measured directly. In addition and also directly, the second equatorial coordinate (fixed system) is often also used with the same sundial : the declination angle δ as an approximate annual date (two-purpose sundial).
The elevation angle h and the azimuth a are the two sun coordinates in the horizontal coordinate system . Sundials that measure these coordinates and thus allow conclusions to be drawn about the time of day are the elevation sundials or the azimuthal sundials .
One set of conversion equations between the equatorial and horizontal coordinate systems is
- tan a = y / x
- x = sin φ cos δ cos τ - cos φ sin δ
- y = cos δ · sin τ
That turns into
- tan a = sin τ / (sin φ cos τ - cos φ tan δ ),
which shows that the hour angle τ is a function of the azimuth a and the declination angle δ . The declination δ is therefore used as an input variable in order to determine the hour angle τ - that is, the time of day - via the azimuth a . The parameter δ appears on the dials of the azimuthal sundials as a family of date lines ( sundial spider ) or is used to position the shadow object ( analemmatic sundial ).
Azimuthal sundials are also dependent on the geographical latitude φ. They are set up for a certain width.
Individual proof
- ↑ Sundials Handbook , Working Group Sundials in the German Society for Chronometry, 2006, page 20