No. 1 - SEA2.jpg
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Side view, facing east. |
The gnomon of a properly oriented equatorial sundial points toward the Celestial North Pole (CNP): the gnomon of the
Webster Park sundial is capped with an Earth globe; the North Pole of the
globe points toward the CNP. The gnomon should be parallel to the Earth's
axis, inclined at an angle equal to the latitude of the observer. The dial
plate should be parallel to the plane of the Earth's Equator
(perpendicular to the gnomon), inclined at an angle equal to the
complement of the observer's latitude. This angle is also known as the colatitude. The Seattle, WA sundial gnomon angle of
inclination = ~48 degrees; dial plate angle of inclination = ~42 degrees.
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No. 2 - SEA6.jpg
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Upper dial face.
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Upper dial face as viewed from the east-northeast (facing west-southwest).
The dial plate is 30 inches in diameter; the Earth globe is eight (8)
inches in diameter. Young woman shown for scale.
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No. 3 - SEA4.jpg
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Closer view. |
The gnomon is the part of a sundial that casts the shadow used to tell
time. For equatorial sundials, the gnomon is a pole (or rod) of varying
thickness. The gnomon shadow falls on the upper dial face from the MAR Equinox to SEP Equinox
(shown left); the gnomon shadow falls on the lower dial face from
the SEP Equinox to MAR Equinox (see photo No. 10).
On the day of the equinoxes, the declination of the Sun equals zero
degrees (0°) and the Sun's apparent path across the sky follows the
Celestial Equator. Since the dial plate of an equatorial sundial
represents the plane of the Earth's Equator, the Sun is directly over the
edge of the dial plate on the March and September Equinoxes, therefore the gnomon (or
style) of a properly
oriented equatorial sundial will not cast a shadow on the dial plate.
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No. 4 - SEA3.jpg
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Close-up view.
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Upper dial face as viewed from the north (facing south). The terrazzo
surface of the dial face shows a crisp, hard-edged shadow! The sundial is
designed so that time is told by estimating the center of the gnomon
shadow. Photo taken on 01 JUN 1997; time shown = ~9:20 a.m. Local Apparent Time (L.A.T.) or ~10:17 a.m. PDT.
The apparent discrepancy between solar time and standard time is caused
by: 1) the Equation of Time (EoT); 2) the difference in longitude between the location of the
sundial and the Standard Time Meridian (STM) for the Pacific Time Zone
(PTZ STM = 120°W); and 3) Daylight Saving Time (DST). Note that the Webster
Park sundial features a built-in correction for longitudinal offset from
the Standard Time Meridian, therefore ~9:20 a.m. L.A.T. minus ~3 min
EoT + 1 hr DST = ~10:17 a.m. PDT.
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No. 5 - SEA5.jpg
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Lower dial face.
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Photo taken on 01 JUN 1997, nearing the date of the June Solstice, the
first day of astronomical summer (meteorological summer is the three-month
period from JUN-AUG). Note that the gnomon shadow does not fall on the
lower dial face. Also note that the shadow of the dial plate falls along
the segment of the gnomon that extends below the dial plate.
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No. 6 - SEA16.jpg
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Facing west-northwest.
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View from the east-southeast (as indicated by Cardinal Points markers along base of sundial).
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No. 7 - SEA15.jpg
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Facing east-northeast.
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View from the west-southwest (as indicated by Cardinal Points markers
along base of sundial).
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No. 8 - SEA8.jpg
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Close-up, Earth globe (showing part of Eastern Hemisphere).
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The Earth globe is made of brass, approximately 1/4" thick. Relative
to the Sun, the Earth globe is oriented just like Planet Earth. As
a result, light and shadow on the globe appear the same as day
and night on Earth (for the current time). The globe features the
following lines of latitude (from north-to-south): Arctic Circle; Tropic
of Cancer; Equator; Tropic of Capricorn; Antarctic Circle. Around the
globe, there are 24 meridians or hour lines; every hour is exactly 15 degrees wide. On the Earth globe (as well as on
Planet Earth), it is local solar noon when the Sun crosses Seattle's line
of longitude. Relative to the sundial, the Earth globe is
mounted on the gnomon so that Seattle's meridian is oriented 180° from
the 12 noon PST hour lines shown on the dial faces
(remember that shadows fall in the opposite direction as the Sun).
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No. 9 - SEA9.jpg
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Time marked on dial plate; date marked along gnomon.
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The dial faces are marked to indicate Pacific Standard Time as well as Daylight Saving Time. Date markers appear as rings
inscribed along the gnomon. Sundial motto (partially visible): 'We do not
inherit this land from our parents, we borrow it from our children.'
Native American Saying
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No. 10 - SEA11.jpg
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December Solstice.
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An afternoon gnomon shadow near the date of the December Solstice, the
first day of astronomical winter in the Northern Hemisphere
(meteorological winter is the three-month period from DEC-FEB).
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No. 11 - SEA12.jpg
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Note shadow along gnomon.
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When the Sun's declination is negative (i.e., when the subsolar point is south of the Earth's Equator), the shadow of the dial plate falls along
the upper segment of the gnomon (shown left). When the Sun's declination
is positive (the subsolar point is north of the
Equator), the shadow of the dial plate falls along the lower segment of
the gnomon (see photo No. 1 & No. 5).
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No. 12 - SEA14.jpg
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Closer view.
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DEC 20, near the date of the December Solstice. Sun glint from the surface
of the Earth globe shows that the subsolar point is located south of the
Equator, along the Tropic of Capricorn.
Capricornus (DEC 22-JAN 19) is one of the signs of the Zodiac, the band of constellations through which
the Sun, Moon, and planets move in the course of a year. Remember that
signs of the Zodiac are 'Sun signs,' meaning that they appear in the day
sky on your birthdate (the Sun is 'in' your sign).
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No. 13 - SEA13.jpg
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Close-up, DEC 20 date marker.
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DEC 20. In addition to the dates of the equinoxes and solstices, the dates of important
anniversaries (e.g., family birthdays) are often marked on sundials. DEC
20 is the birthdate of Mr. Nafziger's niece, which also marks the
approximate date of the December Solstice. Other date markers include
the birthdates of Chuck's children.
Gnomon Date Markers
| Northern Segment |
Southern Segment |
| DEC 20 |
SEP 20/MAR 20 |
| DEC 1/JAN 10 |
SEP 5/APR 5 |
| NOV 20/JAN 21 |
AUG 20/APR 20 |
| NOV 6/FEB 4 |
AUG 4/MAY 8 |
| OCT 22/FEB 18 |
JUL 21/MAY 21 |
| OCT 6/MAR 6 |
JUL 11/JUN 1 |
| SEP 20/MAR 20 |
JUN 20 |
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No. 14 - SEA1.jpg
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From conceptual model...
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Conceptual model of equatorial sundial displayed at public planning event
for Webster Park, circa 1996.
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No. 15 - SEA10.jpg
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...to dedication ceremony.
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Sundialist Chuck Nafziger giving speech at Webster Park dedication
ceremony, 01 JUN 1997.
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No. 16 - SEA7.jpg
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Commemorative plaque.
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Smaller plaque, located on the western side of the sundial base (see photo
No. 11), showing sundial title,
location, and credits:
EQUATORIAL SUNDIAL
LATITUDE: 47° 40.7' NORTH
LONGITUDE: 122° 23.7' WEST
DESIGNED AND CONSTRUCTED BY CHUCK NAFZIGER
A larger two-panel plaque, located on the eastern side of the sundial base
(see photo No. 3), features detailed
information regarding the sundial. Left panel section titles
include: To Tell the Time; Sundial Description; Sun Time and Clock Time;
Date and Daylight Features of this Dial. Right panel section titles
include: Dial Time-Clock Time Conversion.
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