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Cavalese dial Analemma

On the left is the picture of a complex vertical analemma sundial for Latitude 46° 71' and Longitude 11° 28' seen in the Italian town of Cavalese.

To the right is a basic vertical analemma dial of the type that can be illustrated using the above analemma designer.
The time shown is 3pm BST and the shadow is following the zodiac line for the month of April or August.

Quick start:

1) Enter your latitude in decimal degrees.
2) Enter 0 (zero) for South facing.
3) Enter a gnomon length of 100.
4) Click the Calculate button.

Rotation from South facing:
Gnomon length:


Perhaps the best way to describe an analemma is to imagine a board stood vertically facing south,and a rod (gnomon) is mounted squarely on the face of the board near the top, then if the tip of a shadow cast by the rod is marked at noon (local time) for every day of the year, the resultant series of marks make an approximation to a figure of eight shape called an analemma.

The shape is generated by a combination of the declination of the sun and the Equation of time throughout the year.

An analemma can also be generated for the other hours of the day in addition to the noon analemma (show in red).


Rotation from south facing:
If 0 (zero) is entered this means the dial is not rotated, and is therefore south facing.

In practice most sundials cannot be mounted on a wall that is exactly south facing.
Entering a value for rotation (in degrees) results in the analemma distorting from the symetrical south facing version.
The shadow tip can fail to touch the dial surface for some hour lines and altitudes of the sun,
in which case the affected lines are not drawn.

Gnomon length:
See the picture to the right to see how the gnomon stands perpendicular to the dial face.
For the purpose of calculation the length referred to is in pixels, so if you enter 100 and you know how many pixels per inch/cm your screen is set to, you can get some idea of scale.

The gnomon length is also useful for viewing the graphics at different sizes. For example if the graphics exceed the size of the window a smaller value for gnomon length will draw to a smaller scale.

Green dot:
The mysterious green dot shows the position where the gnomon stands.

Lines of the Zodiac
The lines that flow from left to right are traced by the tip of the gnomon around the 21st day of each month, and represent the lines of the Zodiac.