Vertical sundial

The vertical sundial is a sundial with a vertical dial . It is usually located on a building wall, where there is often free space and should be embellished. It is therefore the most common sundial.

Vertical sundial with pole rod for true local time on a wall slightly deviating from the west (the scale contains one more hour line for the afternoon than for the morning)

description

In contrast to the horizontal sundial, this is not an all-day sundial . For the longest time (on the northern half of the earth) a wall facing south is illuminated by the sun, namely 12 hours on the days of the equinox . On the longer summer days it is even less than 12 hours, and the early morning and late evening hours can be shown on a north wall sundial . With a corresponding limitation of the display time, any wall can be fitted with a vertical sundial. An east or west sundial shows between sunrise and noon ( true local time ) or between noon and sunset. Twin sundials, which combine east and west sundials, can occasionally be found in large courtyards of castles or monasteries.

calculation

If the wall points due south, the dial is symmetrical and the following equation applies:

${\displaystyle \alpha =\arctan(\cos \phi \cdot \tan \tau )}$

α is the angle between the hour line and the meridian (vertical) which, in addition to a function of the hour angle τ, is also one of the location constants Φ ( geographical latitude ). The angle α = 0 ° applies to the 12 o'clock line ( true local time ).

Most of the time the wall used does not point exactly south. It's twisted a little east or west. For the now asymmetrical scale the equation applies:

${\displaystyle \alpha =\arctan(\cos \phi \div (\sin \phi \cdot \sin d+\cot \tau \cdot \cos d))}$

d is the angle of rotation of the wall (positive if it deviates from the west). The 12 o'clock line remains vertical (τ = 0 ° → α = 0 °).

literature

• Arnold Zenkert: The fascination of the sundial. Publisher Harri Deutsch , Thun and Frankfurt am Main 2002.

Individual evidence

1. ↑ Out of a total of 3341 sundials in Austria, 3161 are vertical sundials (status: January 1, 2006; Karl Schwarzinger: Catalog of fixed sundials in Austria. Austrian Astronomical Association , Vienna 2006).
2. ^ Arnold Zenkert: Fascination Sundial. Verlag Harri Deutsch, Thun and Frankfurt am Main 2002, page 54.
3. ^ Arnold Zenkert: Fascination Sundial. Verlag Harri Deutsch, Thun and Frankfurt am Main 2002, page 62.