Article of the month November

Checking the Pattern

Have you just drawn the hour line pattern for a plane sundial, or copied it from an existing sundial?
Would you like to know if it is correct?
Then perform this simple check.

Below is the hour line pattern of an inclining and declining plane sundial.
We just need the pattern. Other data, such as latitude, longitude adjustment etc. need not be known.

Indication in apparent solar time

In the pattern, draw an auxiliary line parallel to an existing hour line. Here is one, drawn in red, parallel to the ‘ten hours’ line.
Determine the intersection with an hour line 90 degrees (6 hours) away. In this case, with the ‘four hours’ line.
With this point as the centre, draw (semi-) circles through the intersections of the auxiliary line with the hour lines on one side.
These (semi-) circles should also go through the intersections of the hour lines on the other side.
If they do not, then the pattern is not correct.

The same check has been carried out in blue, using the ‘five hours’ line.

It does not matter if the hour line pattern has been longitude adjusted. The same check applies, as shown in the third figure.

Longitude adjustment 40 minutes