Hi Dave,
I also want to thank Fer for posting the image of the
Foster-Lambert dial.
Dave Bell wrote:
>
> What defines a Foster-Lambert dial, and how are they designed?
> I don't recall seeing any references to this type on your page!
>
I understand, to have a vertical gnomon, it would be designed for a new
latitude by thinking of moving along the north-south meridian until the
gnomon is vertical.
That would be at latitude 90 degrees (the north pole). (see below
G=(90+L)/2) But the dial plate would still be tilted at at angle G with
respect to the gnomon. To have a horizontal gnomon, it would be thought
of as being at zero degrees latitude (equator) with a tilted dial plate
of G found from G=(90+L)/2 where L is the latitude of dial.
Is this correct thinking? Any comments are appreciated.
Warren Thom
Mac Oglesby had written that Mayall described the dial. This is part of
the text:
page 192 SUNDIALS by Mayall and Mayall 2nd edition
The Foster/Lambert Dial
This dial does not use a perpendicular gnomon. However,
it must lie in the plane of the meridian YZ and make an
angle G (fig. b) with the meridian equal to I/2 the sum of
900 and the latitude of the place. The gnomon or style PH
must be constructed so that its angle will remain the same
as it is moved north and south along the meridian.
To use this dial, place it on a level surface with YZ in the
plane of the meridian and the hour point 12 to the north.
When the foot of the gnomon or style P is placed on the
corresponding date, the position of the shadow on the circle
will indicate the time. If you wish to compute the various
elements, the formulae are:
Let
E = distance OE, the eccentric
C = radius of circle (distance 0-I2)
L = latitude of the place
D = declination of sun
R = distance from O north or south of WX on YZ,
for the daily setting
G = angle gnomon makes with the meridian line YZ
Then
G=(900 + L)/2
R = E tan D
C = E tan G, or C = E cot (900 - G)
If the radius C is known, then:
E = C tan (900 - G) or E = C cot G
