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specific results are generated by allowing input for the following:
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site
location (by geographic co-ordinates)
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date(s)
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time(s)
of day
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correction
factor known as the "equation of time"
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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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