Sundial of the month July
Homogeneous analemmatic sundial
with pole style
We examined the homogeneous analemmatic sundial with vertical gnomon in “Sundial of the Month” for June 2008.
As we will show, we can also construct this sundial using a pole style.
The ellipse of an analemmatic sundial is the result of the vertical projection of the equator onto the ground. See Figure 1.
Figuur 1
When we project along the polar axis, or the pole style, we get another ellipse.
The ratio of the major and minor axes is the same for both ellipses.
However, the major axis of the pole style ellipse is directed north-south.
The moving centre disk of the homogeneous analemmatic sundial now moves east-west.
In Figure 2, we see a version of this sundial in plastic.
Figuur 2
The blue disk is turned such that the shadow of the pole style falls over the red dot, marked in the right photograph with a red circle.
One then reads the time on the homogeneous hour scale on the spot marked with the red square.
The inner yellow disk moves along an east-west line when the sundial is operated.
Because the sundial is homogeneous, it may be adjusted for longitude. For that purpose, all time zones (and some “summer time zones”) are marked on the bottom. See Figure 3.
There is also a longitude scale.
When the geographical longitude of the place of interest is turned to the correct time zone, the sundial indicates civil time there.
The sundial does not adjust for equation of time.
Figuur 3