Article of the month May

Capucin card dial

A card dial such as shown here belongs to the family of the altitude dials.
This example shows a Capucin card dial.
Card dials can be made of a variety of materials, such as cardboard, plastic, metal, wood, et cetera.

There is a slit in the card, alongside of which is a date scale, here marked according to the zodiac.
From the slit, at the proper date mark, hangs a weighted string.

With the string at the proper date, it is pulled taut over the XII hour point. A moveable bead on the string is adjusted to this point. The card dial is now ready for use.

The times of sunrise and sunset may be read immediately, by pulling the string straight down and reading the hour scale.

The card dial is provided with a sight. In the example, it is the small pin in the upper left.

Keep the card in the vertical plane, and aim the sight at the sun. The shadow of the pin will then fall over the sight line on top.

Let the string hang free and read the time at the bead on the hour scale.
You will need to know if it is morning or afternoon.

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It is not difficult to make a card dial like this. An explanation follows.

Draw a coordinate system and a semicircle on the horizontal axis.
Divide the arc in twelve parts of fifteen degrees each, and draw the hour lines to the horizontal diameter.

Plot the latitude from point 12 to P on the vertical axis. P will be the middle of the date line, which is square to the line 12-P.

Divide the date line according to the zodiac by measuring off, on either side of line 12-P, angles of 11.5, 20, and 23.5 degrees.

Draw a vertical line from end point Q of the date line. This sets the earliest and latest times for which the card dial is useable.

Draw circular arcs from the ends of the date line, through the twelve-hour point, to the vertical through Q. Erase the parts of the hour lines outside these arcs. This will give the hour scale of this card dial its characteristic shape.

Finally, here is a card dial by Eise Eisinga.
It appears in Gnomonica or Sundials, a book that Eisinga wrote in 1762, when he was only eighteen years old. In it, he draws and describes about one hundred sundials on variously oriented planes.