Shadow thrower
In addition to the dial, the shadow object is an essential part of a sundial . The shadow it creates serves as a pointer, the position of which can be read off a dial scale as the time of day and, to a limited extent, the date of the year.
Point and line pointer
A small sphere or a thin rod is used to create a point or line shadow. The end of a stick ( gnomon ) serves as a point-shaped shadow thrower , or the end of a linear shadow serves as a point-shaped pointer. Instead of a stick, the straight edge of a flat body is also used, or the edge of a flat shadow is used as a linear pointer.
Change of shape: light pointer
A change in shape creates a light pointer, which includes a “light projector” in the form of a perforated or slit screen. Such a pointer is also used in sundials - albeit less often than a shadow thrower.
Central projection
The point-like shadow or light spot is the image of the sun created by central projection. It shifts on the dial, which serves as a picture plane, both during the day and during the year. The position of the sun measured with the hour angle and the angle of declination in the sky can be read off on a sundial using appropriate scales. The hour angle is our measure of the time of day, and the corresponding scale is marked with hours. The declination angle alternates between maximum value (summer solstice) and minimum value (winter solstice). The corresponding sundial scaling contains some daily tracks distributed over the year (often only one per month for the first day of the following zodiac period). From the position of the punctiform shadow, both the time of day (with relatively good resolution) and the season (with moderate resolution) can be read. The point-shaped shadow projector or the pinhole diaphragm are referred to as a nodus (sundial) .
Pole rod
The display of the declination angle can be suppressed by a shadow stick positioned parallel to the earth's axis - the pole stick . Most sundials use this to make them easier to read. The shadow does not contain any information in its longitudinal direction, but this expansion makes it easier to see than a point-like shadow. The point-like display must always be retained if the time of day is to be displayed seasonally, for example: temporal instead of equinox , mean local time (MONT) instead of true local time (WOZ).
Shadow stick, not parallel to the roof
Shadow bars that are not positioned parallel to the earth's axis do not show the hour angle Τ independently of the variable declination δ of the sun. Your shadow can only be interpreted as the time of day, taking into account the declination obtained elsewhere (usually in the form of the year date).
The example in the analemmatic sundial or the sundial spider vertical rod used immediately shows only the azimuth angle a in the sun. After changing the following relationship, the hour angle Τ is displayed indirectly:
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(See conversions between equatorial (τ, δ) and horizontal coordinates (a, h) at rest ).
φ = latitude δ = angle of declination
In the case of an analemmatic sundial, the declination parameter is contained in a separate date scale on which the shadow stick is currently to be set up. With the sundial spider, date circles are superimposed on the hour scale . The time can be read off at the intersection of the shadow line and the current date circle.
With elevation sundials , the elevation angle h of the sun is displayed immediately . In the case of the cylinder sundial , for example , it is measured with a horizontal shadow stick that is held in the direction of the sun. Before that, the shadow bar is placed over the current vertical date line . After changing the following relationship, the hour angle Τ is displayed indirectly:
For the few other sundials with a shadow bar that is not arranged parallel to the roof, further special features must be observed: