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Designing a Sundial from Scratch
Roger BaileyNASS 2003 Banff
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Designing a Sundial from Scratch
• No reference books• No computers or design software• No Internet access• Use Greek geometry: Ruler and Compass• Use geometric construction• Extend geometry to derive trigonometry • Protractor? Trig tables? Calculator?• Teambuilding exercise?
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Equinoctial Tangent Line
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Equinoctial Tangent Line• Picture an Armillary Sphere
or equatorial sundial looking down the polar axis at Equatorial plane
• Equatorial disc as a circle, time angles 15°
• Angles (90, 60, 45, 30,15) using ruler and compass
• Horizontal Tangent line
d = r Tan t 8 9 10 11 12 1 2 3 4
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Equinoctial Tangent Line• Equinoctial Line defines the
Polar Sundial where the plane is parallel to the gnomon
• Equatorial Line is the intersection line at the base of horizontal and vertical south facing sundials
• The gnomon intersection point and the distances on define the hour angles for horizontal and vertical sundials
8 9 10 11 12 1 2 3 4
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Planar Sundials
Horizontal Plane
Equato
rial D
isc
Equino
ctial
Tange
nt Lin
e
Horizontal Sundial
Vertica
l Plan
e
Vertica
l Sun
dial
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Gnomon Base Lengths
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Gnomon
r
Vertical B
ase
Lat
Gnomon Bases on the planes are the hypotenuse of the right angle triangles with one side = r and containing the Latitude.
On Vertical Plane: Base = r / Cos LatOn Horizontal Plane: Base = r / Sin Lat
Lat
Horizontal Base
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Gnomon Base LengthsUse the tangent line and r to find the gnomon base lengths for the latitude and co-latitude for horizontal and vertical dials.
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Gnomon Base LengthsUse the tangent line and r to find the gnomon base lengths for the latitude and co-latitude for horizontal and vertical dials.
Draw the gnomon base vertically from the tangent point to set the gnomon intersection with the plane
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Hour LinesUse the tangent line and r to find the gnomon base lengths for the latitude and co-latitude for horizontal and vertical dials.
Draw the gnomon base vertically from the tangent point to set the gnomon intersection point C with the plane
Draw Hour Lines from C to the hour points on the tangent line
C
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Vertical Sundial • Draw right angle triangle to
determine gnomon base length. Latitude angle at centre of disc, radius is from centre to tangent point
• Hypotenuse (Gnomon Base) is = r / Cos Lat. Draw to set gnomon intersection point CVwith vertical plane
• Draw Hour Lines from base to tangent line distances
• Tan HA = r Tan t / base or• Tan HA = Tan t x Cos Lat
8 9 10 11 12 1 2 3
Lat
r
CV
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Horizontal Sundial • Draw right angle triangle to
determine gnomon base length. Latitude angle at tangent line; radius is from disc centre to tangent point
• Hypotenuse (Gnomon Base) is = r / Sin Lat. Draw to set gnomon intersection point CH with horizontal plane
• Draw Hour Lines from base to tangent line distances
• Tan HA = r Tan t / base or• Tan HA = Tan t x Sin Lat
8 9 10 11 12 1 2 3 Lat
r
CH
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Problem with Infinity
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•Six o’clock line is infinite but defined
•7 am and 5 pm are off scale
•Turn the corner at 9 am and 3 pm (45°)
•Box the circle through gnomon base point
•Scale from diagonal line(Waugh’s technique)
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• Delete the construction lines
• Add a border and frame
• Trim the hour lines• Add the numbers, text
and decoration
Finish the Dial
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6
7
8
6
5
4
Life’s But a Walking Shadow
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Vertical Declining Sundial
• Treat declining sundial as south facing equivalent
• Solve VD Tetrahedron for Substyle Distance SD and Substyle Height SH– Vertical Meridian Plane– Horizontal Declination
Plane– Vertical SD Plane– Sloped SH Plane
Horizontal Plane
Vertical Declining Plane
GnomonMeridian
SD
Dec Lat
SH
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Meridian Plane: Latitude Triangle
• For all vertical planes– Gnomon points south– Noon shadow is vertical
• For Meridian Plane– Gnomon is true length– Gnomon angle with
horizontal is true Latitude• CM = G x Sin (Lat)• MS = G x Cos (Lat)
Horizontal Plane
Vertical Declining Plane
GnomonMeridian
SD
Dec Lat
SH
C
M S
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Horizontal Plane: Declination Triangle
• MH = MS x Sin(Dec) MH = G x Cos(Lat) x Sin(Lat)
• HS = MS x Cos(Dec) HS = G x Cos(Lat) x Cos(Dec)
Horizontal Plane
Vertical Declining Plane
GnomonMeridian
SD
DecLat
SH
M
H
S
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Vertical Declining Plane
• Tan (SD) = MH / CM
• Tan(SD) = G x Cos(Lat) x Sin(Dec)/(G x Sin(Lat))
• Tan(SD) = Sin(Dec)/Tan(Lat)
Horizontal Plane
Vertical Declining Plane
GnomonMeridian
SD
Dec Lat
SH
C
M
H
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Inclined Declining Plane
• For Substyle Height SH
• Sin(SH) = HS / G
• Sin(SH) = G x Cos(Lat) x Cos(Dec) / G
• Sin(SH) = Cos(Lat) x Cos(Dec)
Horizontal Plane
Vertical Declining Plane
GnomonMeridian
SD
Dec Lat
SH
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0
5
10
15
20
25
30
35
40
45
0 20 40 60 80 100
Wall Declination
Substyle Distance & Height
• Substyle Distance increases with wall declination from zero to Co-Latitude
• Substyle Height decreases from Co-Latitude to zero with wall declination
SubstyleDistance
SubstyleHeight
Tan SD = Sin Dec / Tan Lator = Sin Dec x Tan CoLat
Sin SH = Cos Dec x Cos Lator = Cos Dec x Sin CoLat
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Sundials from Scratch
• Draw circle and Tangent Line• Draw time segments to Tangent Line• Find gnomon base point using the tangent line• Draw Hour Lines from base point to tangent
points• Scale from diagonal for hours near 6 o’clock• For vertical declining dials, first solve VD
tetragon for Substyle Distance and Height
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