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Project
specific results are generated by allowing input
for the following:
-
site location
(by geographic co-ordinates)
-
date(s)
-
time(s) of
day
-
correction
factor known as the "equation of time"
-
adjustments
between "watch" time (Standard,
Daylight) and local solar time.
We
developed the computer model from the principles
of astronomy and geodesy, taking into account a
number of key elements that are so often missing
in published sun charts.
Published sun charts tend to simplify the complex
geometry involved in calculating the sun's
position relative to an observer on the
ground. Such charts do not account for an
observer's actual location based on site specific
latitude and longitude. They usually
present sun azimuth and altitude figures based on
an average latitude for a general area, and on a
longitude equivalent to the standard meridian of
the time zone within which the chart
applies. As a result, a correction of up to +/- 1/2-hour must be applied.
The average position of the sun is based on an
average 24-hour day. Since the velocity of
the earth varies as it moves through its
elliptical orbit around the sun, the sun's actual
position (i.e. the position that determines
shadow location) varies from the average by up to
sixteen minutes of time. This variation is
equivalent to as much as 4 degrees longitude -
almost 450 kilometres - and, therefore, the
correction factor known as the "equation of
time" should be taken into
account. Our model achieves this.
Ignoring the above factors alone can result in an
'error' of up to about three quarters of an hour in sun
position and shadow calculations.
Sun charts usually provide solar positions in hourly intervals only, requiring interpolation
for times other than those listed. It
should be noted that azimuth and altitude figures
cannot be interpolated linearly, making
interpolation difficult and prone to error.
Sun charts also typically provide data for
selected dates only, making it difficult to
accurately determine the sun's position and to
assess shadow impacts for dates not listed.
Our solar model takes into account all of the
above so that the sun's position relative to an
observer can be precisely calculated for any
location, for any date, and for any
local time - historical, present and future. The model generates not only the
extent and position of shadows at any given point in time, but
it also calculates the duration of shadow
impact - a key indicator when assessing degree
of impact.
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