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FUNCTIONALITY & ACCURACY
Harris Morrison, the founder of Shepherd's Watch, has
paid close attention to the functionality and historical
accuracy of the collection to make sure they are faithful
to their originals. To ensure historical accuracy, he
consults with experts in the sundial field:
- Dr. Sara J. Schechner is an authority on historical
sundials and an officer of the North American Sundial
Society. She is presently the David P. Wheatland
Curator of the Collection of Historical Scientific
Instruments at Harvard University. Previously, she
ran her own firm, Gnomon Research, a firm that specialized
in science-and-society exhibits and educational
programs. Her research has won many awards, and
she has curated notable exhibitions around the U.S.
Prior to that, she served as Curator of the Adler
Planetarium and Astronomy Museum in Chicago, home
to the largest collection of time-finding instruments
in North America.
- Dr. Ross McCluney, B.A., M.S., Ph.D. is Principal
Research Scientist at the Florida Solar Energy Center.
Currently he collaborates with artist Susan Miller
on the design of artistic sundials for private clients.
Among other sundial realizations, he was scientific
adviser to the developer of a very large sundial
incorporated into a large office building at the
Team Disney Building in Lake Buena Vista, Florida.
HOW SUNDIALS WORK
Time represented by a sundial is referred to as Local
Apparent Time (LAT). It is based strictly on the apparent
motion of the sun so that when the sun is viewed to
be directly over the observer's meridian, the LAT would
be noon. No two locations share the same LAT noon, or
any other hour, unless they are on the same line of
longitude.
For a horizontal sundial to provide accurate time, it
must be designed for the specific latitude where it
will be located.
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| THE
ANGLE THAT THE GNOMON MAKES WITH THE HORIZONTAL FACE
MUST EQUAL THE LATITUDE OF THE SUNDIAL. |
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The inclination angle that
the gnomon makes with the horizontal sundial plate must
equal the latitude of the dial, and the gnomon must
be oriented to the local meridian, that is, it must
point to the north celestial pole. The hour lines that
appear on the dial plate can be constructed using methods
found in several reference books, or they can be calculated
mathematically using plane trigonometry. To determine
the angle that each hour line makes with the 12 o'clock
line, the following formula is used:
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tan D = (tan t)(sin
l) where:
D = the angle the hour line makes with the 12
o'clock line.
t = the time measured from noon in degrees and
minutes of arc. Remember the sun completes 360
degrees in 24 hours, or 15 degrees in 1 hour,
or 1 degree in 4 minutes.
l = the latitude of the dial.
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Each time zone has what
is called a "standard longitude" (also called a "standard
meridian") for that zone. The further you are away from
that longitude with your time zone, the greater the
difference between sundial time and standard time. Since
the Earth rotates through an angle of about 15 degrees
each hour, the standard meridians for the time zones
are at 15 degree intervals around the Earth. Eastern
standard time, for example, has 75 degrees at its standard
meridian, is the fifth time zone eastward from the zero
one at Greenwich, and passes between Montreal and Ottawa
and just East of Philadelphia as it heads south out
into the Atlantic Ocean just west of Cape May, New Jersey.
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| A STATIONARY
SUNDIAL CAN OFFSET THE FIXED TIME DIFFERENCE BETWEEN
THE TIME ZONE'S REFERENCE LONGITUDE AND THE DIAL'S LONGITUDE. |
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The 75 degree meridian also
crosses the Eastern end of Cuba before running down
the west coast of South America. Since clock times are
the same for all places within a time zone, solar noon
cannot occur at the same clock time everywhere in a
time zone. If we are west of the standard meridian of
our time zone, the sun will rise and set a little later
than at the standard meridian and it will reach its
solar noon position a little later. Thus, solar noon
occurs a little later west of the standard meridian
than it does on its meridian.
Since the Earth rotates 15 degrees each hour, if we
are, for example, 7.5 degrees west of a standard meridian
(half an our of Earth rotation), then solar noon will
occur 30 minutes later at our longitude than it does
on the standard meridian. This correction based on longitude
is the same for all days of the year, for a given longitude.
Once you know how much the sun is fast or slow at your
longitude, compared with the standard time kept by your
watch or clock, you can make this same correction to
your sundial for every day of the year.
Unfortunately, this fixed or constant correction works
by itself only on just four days of the year, April
15, June 13, September 1, and December 25. On other
days in the year there is an additional (varying) correction
which must be made. The reason for this is that our
clocks and watches don't tell time by the Earth's rotation
on its axis but by its rotation around the sun each
year. Instead of dividing the time between solar noons
into equal intervals, clocks divide the time in a whole
year into equal intervals of hours, minutes, and seconds.
(Leap year occurs because there are actually 365 and
a quarter days in a celestial year, and every four years
we have to add the four quarters of a day which have
built up into a whole day, by putting another day into
the month of February, adding a 29th day of the month
in leap years).
The problem is that while the Earth is spinning on its
axis, it is always rotating around the sun, and this
has the effect of putting the sun in a slightly different
location in the sky each day, relative to the galactic
star background.
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| A DAY
DEFINED BY THE LENGTH OF TIME IT TAKES THE SUN TO REAPPEAR
OVER THE SAME EARTH MERIDIAN CAN VARY BY UP TO APPROXIMATELY
16.5 MINUTES DEPENDING UPON THE TIME OF YEAR. |
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Furthermore, the Earth's
orbit around the sun is not circular but elliptical.
Also, the Earth's axis of diurnal rotation is tilted
23.4 degrees away from its axis of yearly rotation around
the sun. The combined effect of these is to introduce
an additional difference between sundial time and clock
time which is different for each day of the year. Called
the "Equation of Time" for historical reasons, this
difference is fortunately the same all over the world,
for a given day of the year. A table of values for the
Equation of Time for several dates in a year is shown
below:
Equation of Time near Solstices, Equinoxes, and Selected
Dates Between
| Month / Day |
|
EOT in hours |
EOT in minutes:seconds |
| 1/19 |
|
-0.1776 |
-11:39.4 |
| 2/17 |
|
-0.2345 |
-14:4.2 |
| 3/20 |
Equinox |
-0.1263 |
-8:34.7 |
| 4/19 |
|
0.0134 |
0:48.2 |
| 5/21 |
|
0.0576 |
3:27.3 |
| 6/21 |
Solstice |
0278 |
-2:40.4 |
| 7/22 |
|
-0.1069 |
-6:25.0 |
| 8/23 |
|
-0.0452 |
-3:42.9 |
| 9/22 |
Solstice |
0.1188 |
7:7.7 |
| 10/23 |
|
0.2607 |
15:38.5 |
| 11/21 |
|
0.2371 |
14:13.4 |
| 12/21 |
Equinox |
0.0345 |
2:4.1 |
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