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.\" RCSid "$Id"
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.TH GENSKY 1 4/24/98 RADIANCE
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.SH NAME
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gensky - generate a RADIANCE description of the sky
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.SH SYNOPSIS
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.B "gensky month day time"
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[
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.B options
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]
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.br
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.B "gensky -ang altitude azimuth"
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[
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.B options
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]
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.br
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.B "gensky -defaults"
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.SH DESCRIPTION
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.I Gensky
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produces a RADIANCE scene description for the CIE standard
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sky distribution at the given month, day and time.
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By default, the time is interpreted as local standard
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time on a 24-hour clock.
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The time value may be given either as decimal hours, or using a
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colon to separate hours and minutes.
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If the time is immediately followed (no white space)
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by a North American or European time zone designation,
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then this determines the standard meridian, which may
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be specified alternatively with the
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.I \-m
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option.
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The following time zones are understood, with their corresponding
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hour differences from Greenwich Mean Time:
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.sp .5
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.nf
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Standard time:
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YST PST MST CST EST GMT
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9 8 7 6 5 0
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CET EET AST GST IST JST NZST
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-1 -2 -3 -4 -5.5 -9 -12
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Daylight savings time:
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YDT PDT MDT CDT EDT BST
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8 7 6 5 4 -1
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CEST EEST ADT GDT IDT JDT NZDT
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-2 -3 -4 -5 -6.5 -10 -13
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.fi
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.PP
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If the time is preceded by a plus sign ('+'), then it is interpreted
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as local solar time instead.
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It is very important to specify the correct latitude and longitude
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(unless local solar time is given) using the
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.I \-a
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and
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.I \-o
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options to get the correct solar angles.
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.PP
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The second form gives the solar angles explicitly.
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The altitude is measured in degrees above the horizon, and the
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azimuth is measured in degrees west of South.
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.PP
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The third form prints the default option values.
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.PP
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The output sky distribution is given as a brightness function,
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.I skyfunc.
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Its value is in watts/steradian/meter2.
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The x axis points east,
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the y axis points north, and the z axis corresponds to the zenith.
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The actual material and surface(s) used for the sky is left
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up to the user.
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For a hemispherical blue sky, the description might be:
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.sp
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.nf
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!gensky 4 1 14
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skyfunc glow skyglow
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0
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0
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4 .9 .9 1 0
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skyglow source sky
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0
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0
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4 0 0 1 180
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.fi
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.sp
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Often,
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.I skyfunc
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will actually be used to characterize the light coming in from
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a window.
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.PP
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In addition to the specification of
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a sky distribution function,
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.I gensky
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suggests an ambient value in a comment at the beginning of the
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description to use with the
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.I \-av
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option of the RADIANCE rendering programs.
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(See rview(1) and rpict(1).)
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This value is the cosine-weighted radiance of the sky in
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watts/steradian/meter2.
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.PP
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.I Gensky
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supports the following options.
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.TP 10n
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.BR \-s
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Sunny sky without sun.
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The sky distribution will correspond to a standard CIE clear day.
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.TP
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.BR \+s
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Sunny sky with sun.
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In addition to the sky distribution function, a source
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description of the sun is generated.
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.TP
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.BR \-c
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Cloudy sky.
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The sky distribution will correspond to a standard CIE overcast day.
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.TP
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.BR \-i
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Intermediate sky without sun.
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The sky will correspond to a standard CIE intermediate day.
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.TP
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.BR \+i
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Intermediate sky with sun.
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In addition to the sky distribution, a (somewhat subdued) sun
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is generated.
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.TP
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.BR \-u
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Uniform cloudy sky.
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The sky distribution will be completely uniform.
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.TP
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.BI -g \ rfl
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Average ground reflectance is
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.I rfl.
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This value is used to compute
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.I skyfunc
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when Dz is negative.
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Ground plane brightness is the same for
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.I \-s
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as for
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.I \+s.
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(Likewise for
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.I \-i
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and
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.I \+i,
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but see the
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.I \-r
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option below.)
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.TP
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.BI -b \ brt
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The zenith brightness is
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.I brt.
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Zenith radiance (in watts/steradian/meter2) is normally computed
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from the sun angle and sky turbidity (for sunny sky).
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It can be given directly instead, using this option.
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.TP
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.BI -B \ irrad
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Same as
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.I \-b,
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except zenith brightness is computed from the horizontal
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diffuse irradiance (in watts/meter2).
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.TP
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.BI -r \ rad
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The solar radiance is
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.I rad.
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Solar radiance (in watts/steradian/meter2) is normally computed from
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the solar altitude.
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This option may be used to override the default calculation.
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If a value of zero is given, no sun description is produced, and the
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contribution of direct solar to ground brightness is neglected.
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.TP
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.BI -R \ irrad
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Same as
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.I \-r,
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except solar radiance is computed from the horizontal direct
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irradiance (in watts/meter2).
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.TP
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.BI -t \ trb
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The turbidity factor is
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.I trb.
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Greater turbidity factors
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correspond to greater atmospheric scattering.
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A turbidity factor of 1.0 indicates an ideal clear atmosphere (i.e.
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a completely dark sky).
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Values less than 1.0 are physically impossible.
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.PP
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The following options do not apply when the solar
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altitude and azimuth are given explicitly.
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.TP
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.BI -a \ lat
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The site latitude is
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.I lat
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degrees north.
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(Use negative angle for south latitude.)
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This is used in the calculation of sun angle.
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.TP
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.BI -o \ lon
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The site longitude is
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.I lon
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degrees west.
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(Use negative angle for east longitude.)
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This is used in the calculation of solar time and sun angle.
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Be sure to give the corresponding standard meridian also!
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If solar time is given directly, then this option has no effect.
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.TP
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.BI -m \ mer
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The site standard meridian is
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.I mer
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degrees west of Greenwich.
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(Use negative angle for east.)
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This is used in the calculation of solar time.
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Be sure to give the correct longitude also!
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If solar time is given directly, then this option has no effect.
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.SH EXAMPLE
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To produce a sunny sky for July 4th at 2:30pm Eastern daylight time at a
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site latitude of 42 degrees, 89 degrees west longitude:
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.IP "" .2i
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gensky 7 4 14:30EDT +s -a 42 -o 89
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.PP
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To produce a sunny sky distribution for a specific sun position but
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without the sun description:
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.IP "" .2i
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gensky -ang 23 -40 -s
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.SH FILES
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/usr/local/lib/ray/skybright.cal
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.SH AUTHOR
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Greg Ward
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.SH "SEE ALSO"
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rpict(1), rview(1), xform(1)
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