Article of the month August

Equator projection sundials 1

The previous month in this feature, we looked at the universal ring dial.
see that article

The next figure, a view in the direction square to the meridian plane, shows its essence.
We see:
- the earth axis, or celestial axis, with a date scale
- the equatorial plane with the hour points.

We parallel project this figure from several directions onto the horizontal plane.
Two special cases were discussed here earlier:
- projection in the vertical direction
- projection in the direction of the celestial axis
In the first case we get an analemmatic sundial with a vertical gnomon; in the second case we get a pole style sundial with a gnomon parallel to the earth, or celestial, axis.
Here is the figure from that article:

read that article.

This month, we project the same figure in a direction halfway between the celestial axis and the vertical.
The projection angle is (45 + 1/2 phi) degrees.
Now the hour scale is circular, and the hour points are evenly divided.
This has the advantage that it is easy to adjust for longitude or equation-of-time, by simply turning the hour scale.
The gnomon meets the horizontal at an angle of (45 + 1/2 phi) degrees.

With this projection, we get a sundial called the Foster-Lambert dial.
You can see a fine example, but on a declining vertical plane, in the photograph below.


See also the October 2007 article.