Xplanet is similar to Xearth, where an image of the earth is rendered
into an X window.  Xplanet can display several different map
projections, including Azimuthal, Mercator, Mollweide, orthographic,
or rectangular, as well as a window with a globe the user can rotate
interactively.  The other planets and some satellites may also be
displayed.  The latest version can always be found at
http://xplanet.sourceforge.net.

Most of the algorithms are taken from "Astronomical Formulae for
Calculators" by Jean Meeus, published by Willman-Bell.  The rotational
parameters for the other planets and satellites are taken from Davies
et al. (1996), Celestial Mechanics 63, 127--148.  Orbital formulae for
Pluto are taken from Paul Schlyter's page at
http://www.m2c3.com/alpocs/tdl1999/schlyter110199/ppcomp.html 
Formula for conversions to galactic coordinates from sect 29 of
Practical Astronomy With Your Calculator by Duffett-Smith (3rd ed,
1988).

Most of the map projection algorithms are from "Map Projections - A
Working Manual" by John P. Snyder, published by the U.S. Department of
the Interior.

Xplanet is free software, distributed according to the terms of the
GNU General Public License.  See the COPYING file for details.

Xplanet has been compiled and run on Linux, Solaris, HP-UX, FreeBSD,
IRIX, and Darwin platforms.  It has been compiled on Microsoft Windows
using the Cygwin environment.

See the INSTALL file for installation instructions.

Options need only be specified with enough characters to be
unambiguous.  Valid options to Xplanet are:

-animate
Pop up a window using OpenGL or Mesa where the user can rotate the
globe interactively.  The -markers and -label options will be ignored
in this mode.  Valid keys in this mode are:
Home/End:	      Move closer/farther
Arrow keys:	      Rotate body
+/-:		      Increase/decrease rotation speed
r:		      Reverse rotation
h:		      Toggle help screen
q:		      Quit

-background background_file
Use background_file as the background image, with the planet to be
superimposed upon it.  If no projection is explicitly specified,
orthographic is assumed, but this option may also be used with the
azimuthal, mollweide, and peters projections.

-blend
Use bilinear interpolation to compute the color of each pixel instead
of nearest-neighbor interpolation.  It slows down the computation, but
it can look a lot better, particularly if you're using a low
resolution map.

-body body
Render an image of the specified planet or satellite.  Valid values
for body are mercury, venus, earth, moon, mars, jupiter, io, europa,
ganymede, callisto, saturn, titan, uranus, neptune, pluto, and random.

-center x,y
Place the center of the globe at pixel coordinates (x,y).  You can use
this with the -radius option to put a small image anywhere on the
screen.  If no projection is explicitly specified, orthographic is
assumed, but this option may also be used with the azimuthal and
Mollweide projections.

-cloud_gamma gamma
Apply a gamma correction to the cloud image before overlaying.  Each
pixel's brightness is adjusted according to:
new_value = 255 * [(old_value/255)^(1/gamma)]

-cloud_image cloud_file 
Use cloud_file as the image to be overlaid.  Overlaying clouds slows
Xplanet down considerably (but it looks really nice).  New global
composite cloud maps are generated every six hours on
http://xplanet.sourceforge.net.  If you use this option a lot, you
might consider making your own day and night image maps with the
clouds already overlaid to save some time.  You can create a shell
script like the following:

	 wget http://xplanet.sourceforge.net/cloud_800.jpg
	 xplanet -image day.jpg -cloud_image cloud_800.jpg \
	 -shade 100 -output day-clouds.jpg -geometry widthxheight
	 xplanet -image night.jpg -cloud_image cloud_800.jpg \
	 -cloud_shade 30 -output night-clouds.jpg -geometry widthxheight

Where width and height are the dimensions of the image to create.  Now
you can use day-clouds.jpg and night-clouds.jpg as your day and night
images until it's time to get a new cloud map.

-cloud_shade percent 
Only shade the cloud map; do not shade the night map.  This is useful
for creating a new night map with the cloud image overlaid in the
event you're using different map files for day and night.  For an
example, see the -cloud_image option above.

-cloud_ssec cloud_file 
Just like -cloud_image but where cloud_file is an image downloaded
from the Space Science and Engineering Center (SSEC) at the University
of Wisconsin.  The latest image (updated every three hours) can be
obtained from http://www.ssec.wisc.edu/data/comp/latest_moll.gif. This
image is a 640x350 pixel Mollweide composite image with ugly pink
coastlines.  Xplanet will reproject and resize the image as well as
remove and fill in the coastlines.  As with -cloud_image, this can be
time consuming each time Xplanet runs, so consider making a new
composite yourself.  

-cloud_threshold threshold
Cloud pixel values below threshold will be ignored.  The value for
threshold should be between 0 and 255.  A value of 90 is the default.

-color colorname
Set the color for the label/markers to colorname.  The default is
"red".  Any color in the rgb.txt file may be used (usually
/usr/X11R6/lib/X11/rgb.txt).  Colors may also be specified by RGB hex
values; for example -color 0xff and -color blue mean the same thing,
as do -color 0xff0000 and -color red.

-date string
Use the date specified instead of the current local time.  The format
of the string should be "24 Jun 1999 21:02:17" ("%d %b %Y %H:%M:%S" as
read by strptime(3)).  The time is assumed to be local time.  If
strptime is not available on your system the -date option will be
ignored.

-dayside
Render the image as seen from directly above the subsolar point.  If no
projection is explicitly specified, orthographic is assumed, but this
option may also be used with any projection except rectangular. 

-demfile demfile
Use demfile as the digital elevation map.  This file should be an 8
bit image, with 0 being the lowest elevation (corresponding to radius
1) and 255 being the highest elevation (corresponding to radius = 1 +
demscale, defined below).  The -blend option will be ignored if
-demfile is used.  This option implies -projection orthographic. 

-demscale demscale
Assign the highest elevation in the digital elevation map named with
the -demfile option to be at a distance of 1 + demscale from the
planet center.  The default is 0.05.  This option will be ignored if
-demfile is not used. 

-earthside
Render the image as seen from the earth. This option only works with
other planets, specified with -body.  If no projection is explicitly
specified, orthographic is assumed, but this option may also be used
with any projection except rectangular.

-font fontname 
Set the font for the label/markers to fontname.  The default font is
"variable" for X11 or "helr____.ttf" when using the FreeType library.
Either X11 or TrueType fonts may be specified if support for each is
compiled in.  The command "xlsfonts" will list all of the fonts that
are available on X11.  If a TrueType font is specified, this option
implies -truetype.

-fontdir directory
Specify the directory to find the TrueType font to use.  This option
may be given more than once. 

-fontsize size
Specify the pointsize.  The default is 12.  This option only works
with TrueType fonts. 

-fullscreen
Set the width and height of the window or output file to the size of
the root window.  On X11, this will only work if there is a DISPLAY
variable set.  This option implies -window but also can be used with
-output.

-fuzz fuzz
Let the day and night hemispheres blend into one another for pixels
within fuzz degrees of the terminator.  The default value is 6.

-geometry string
Specify the window geometry using the standard X window geometry
syntax, [<width>{xX}<height>][{+-}<xoffset>{+-}<yoffset>]
(e.g. 256x256-10+10 puts a window 256x256 pixels in size 10 pixels
away from the right side and 10 pixels below the top of the root
window).  This option implies -window, but can also be used with
-animate or -output.

-gmtlabel
Same as the -label option, but show GMT instead of local time.

-greatarcfile filename
Use the coordinates in filename to plot arcs of great circles.  Each
line should have the following syntax:

lat1 lon1 lat2 lon2

where all values are in degrees.  In addition, the keywords "color"
and "spacing" are supported as in the example below:

33.9 -118.4 52.3 4.8 color=SpringGreen spacing=0.5 # LAX-AMS

Valid values for "color" are the same as for the -color option.  The
value for spacing defines the distance between dots on the great arc.
The default is 0.1 degree.  Delimiters (whitespace, tabs, foward
slashes, or commas) are not permitted in any of these keyword/value
pairs.  Anything after the # character is ignored.

-grid
Draw a longitude/latitude grid.  The spacing of major grid lines and
dots between major grid lines can be controlled with the -grid1 and
-grid2 options (see below).

-grid1 grid1
Specify the spacing of grid lines. Grid lines are drawn with a
90/grid1 degree spacing. The default value for grid1 is 6,
corresponding to 15 degrees between major grid lines.  This option
implies -grid.

-grid2 grid2
Specify the spacing of dots along grid lines.  Grid dots are drawn
with a 90/(grid1 x grid2) degree spacing.  The default value for grid2
is 15; combined with the default grid1 value of 6, this corresponds to
placing grid dots on a one degree spacing.  This option implies
-grid.

-help                
Display a list of options.

-image image_file
Use image_file as the day map image.  For the earth and moon, it is
assumed that the image goes from [-180,+90] at the top left to
[180,-90] at the bottom right.  For the other planets, the corners are
assumed to be [180, +90] and [180, -90] at top left and bottom right
respectively, where the longitude increases to the west for Mercury
and Mars and the longitude increases to the east for Venus.  This is
confusing but most images you will find adhere to this convention, so
you probably don't need to worry about it anyway.  If this option is
not specified, the day map will default to body.jpg (e.g. earth.jpg).

-label
Display a label in the upper right corner which indicates the current
local time and subsolar point, and the position where the observer is
directly overhead.  For orthographic projections the illuminated
fraction is also displayed.

-labelformat string
Specify the format for the date/time label.  This format string is
passed to strftime(3).  The default is "%a %d %b %Y %H:%M %Z".  Non-US
users may want to use "%c", which shows the date and time in the
locale's appropriate date and time representation.

-labelname
Display the body's name in the label.  This option implies -label.

-labelrot
Display the angle (in degrees) between the body's north pole and the
top of the image in the label.  This option implies -label.

-labelpos string
Specify the location of the label using the standard X window geometry
syntax.  The default position is "-15+15", or 15 pixels to the left
and below the top right corner of the display.  This option implies
-label. 

-latitude latitude
Render the globe as seen from above the specified latitude (in
degrees).  The default value is 0.  If no projection is explicitly
specified, orthographic is assumed, but this option may also be used
with any projection except rectangular.

-localtime time 
This option is equivalent to using the -longitude option with the
meridian at which the local time is the time specified.  The time can
range from 0 to 24.

-longitude longitude 
Place the observer above the specified longitude (in degrees).
Longitude is positive going east, negative going west (for earth and
moon), so for example Los Angeles is at -118 or 242.  The default
value is 0.

-mapdir directory 
When looking for an image, Xplanet will first look in the directory
specified by -mapdir, then the current directory, then the "images"
subdirectory of the current directory, and finally in the default
directory specified at compilation time.  This option may be specified
more than once in order to search multiple directories.

-mapbounds lat1,lon1,lat2,lon2
Assume that each map file read in has its northwest corner at
lat1,lon1 and its southeast corner at lat2,lon2.  This is useful if
you have a high-res map but just want to show a small area.

-markerbounds filename
Write coordinates of the bounding box for each marker to filename.
This might be useful if you're using xplanet to make imagemaps for web
pages.  Each line looks like:

204,312 277,324 Los Angeles

where the coordinates are for the upper left and lower right corners
of the box.  

-markerfile markerfile 
Specify a file containing user defined marker data to display on the
map.  The format of each line is generally latitude, longitude,
string, as in the example below:

33.943 -118.408 "Los Angeles" # USA 

Anything after a # is ignored.  

In addition, Xplanet supports the "align", "color", "font",
"fontsize", "image", "position", "radius", and "transparent" keywords.
If used, keywords must follow the text string.

The "align" keyword is used to place the marker string in relation to
the marker itself.  Valid "align" values are "right", "left", "above",
"below", or "center".  If the "align" keyword is not specified,
Xplanet will attempt to place the marker string so as not to overlap
other markers.

Valid values for "color", "font", and "fontsize" are the same as for
the -color, -font, and -fontsize options, respectively.  If TrueType
fonts are specified with an X11 display, be sure to specify -truetype
on the command line.  At present, you can't mix X11 and TrueType
fonts.

Valid values for "image" are either "none" or the name of an image
file.  If the "image" keyword is not specified, Xplanet will draw a
circular marker.  Xplanet looks for image files in the same places it
looks for map files.

Valid values for "position" are "pixel", "sun", or "moon".  If the
"position" keyword is not specified, Xplanet assumes the two
coordinates given in the marker file are latitude and then longitude.

The "radius" keyword is used to place the marker at the specified
distance from the planet's center, in units of the planetary radius.
A radius value of 1 places the marker at the planet's surface.

The "symbolsize" keyword controls the size in pixels of the circular
marker.  The default is 6.

The "timezone" keyword is used to specify a value for the TZ
environment variable when the marker is drawn.  If this keyword is
specified, the marker string is passed through strftime(3) before
being displayed.  See the earth marker file for more details.

The "transparent" keyword is only meaningful in conjunction with
"image".  The format must be "transparent={R,G,B}" where the RGB
values range from 0 to 255.  Any pixels with this color value will be
considered to be transparent.

Delimiters (whitespace, tabs, forward slashes, or commas) are not
permitted in any of these keyword/value pairs (except for with
"transparent", as shown above).  The text string may be enclosed in
either quotes ("") or braces ({}).

Some sample marker file entries are given below:

33.943 -118.408 "Los Angeles" align=below color=blue font=10x20 # USA 
33.943 -118.408 {Los Angeles} align=below color=blue font=10x20 # USA 
Each of these will draw a circular marker at latitude 33.943,
longitude -118.408, with a text label "Los Angeles" below it, colored
blue and using font 10x20.  

20 10 "This is xplanet" image=none position=pixel 
This draws the string "This is xplanet" at pixel coordinates y=20,
x=10, with no marker.  (0,0) is the upper left corner of the screen.
If y or x is negative, it is taken to be the number of pixels from the
bottom or right side of the screen, respectively.

position=sun image=smile.png transparent={255,255,255}
This draws the image "smile.png" at the subsolar point.  Any pixels
with the RGB values {255,255,255} will be considered transparent.
Using "position=moon" will draw the image at the sublunar point.

-1.12479 251.774 radius=1.09261 {HST} 
This draws a circular marker for the Hubble Space Telescope above
latitude -1.12479, longitude 251.774 degrees, at a distance of 1.09261
earth radii from the center of the earth and labels it "HST".

40.70  -74.00 "New York %H:%M %Z" timezone=America/New_York
This draws a marker with "New York" followed by the time in HH:MM ZZZ
where ZZZ is the string for the time zone (either EST or EDT).

This option implies -markers.  The -markerfile option may be used more
than once if you want to use more than one marker file.

-markers
Enable markers, as in xearth.  

-moonside                   
Render the earth as seen from the moon.  If no projection is
explicitly specified, orthographic is assumed, but this option may
also be used with any projection except rectangular.

-night_image night_file
Use night_file as the night map image.  If this option is not
specified, a default night map will be used for the earth.  If this
file is not found, or for the other planets, the night map will be a
copy of the day map, modified as described in the -shade parameter.

-nightside                   
Render the image as seen from directly above the anti-subsolar point.
If no projection is explicitly specified, orthographic is assumed, but
this option may also be used with any projection except rectangular.

-north north_type
This option rotates the image so that "north" corresponds to the
given coordinate system: "body", "orbit" or "galactic"
(perpendicular to the plane between the sun and the center of our galaxy).
The default value is "body" unless the \-terminator option is
specified where the default becomes "orbit".  The -rotate
option will further adjust the resulting image.  If no projection is
explicitly specified, orthographic is assumed, but this option may also
be used with any projection except rectangular.

-notransparency
Do not update the background pixmap for transparent Eterms and aterms.

-output filename
Output to a file instead of rendering to a window.  The file format is
taken from the extension. Currently .gif, .jpg, .ppm, .png, and .tiff
images can be created.  The image size defaults to 512 by 512 pixels
but this may be changed by the -geometry flag.

-print_coords
Print the observer lat, lon in degrees and then exit.  This might be
useful for satellite tracking, or for getting the position of the sun
or moon.

-projection projection_type
The projection type may be one of ancient, azimuthal, hemisphere,
mercator, mollweide, peters, orthographic, or rectangular.

-quality quality
This option is only used when creating JPEG images.  The quality can
range from 0 to 100.  The default value is 80.

-radius radius 
Render the globe with a radius of radius percent of the screen height.
The default value is 50% of the screen height.  If no projection is
explicitly specified, orthographic is assumed, but this option may
also be used with the azimuthal and Mollweide projections.  If used
with the Mollweide projection, the radius value is the value of the
semimajor (horizontal) axis as a percent of the screen width.  When
drawing Saturn in an orthographic projection, the radius value applies
to the radius of the outer ring.

-random
Place the observer at a random location.  If no projection is
explicitly specified, orthographic is assumed, but this option may
also be used with any projection except rectangular.

-range range
Render the globe as seen from a distance of range from the planet's
center, in units of the planetary radius.  The default value is 1000.
Note that if you use very close ranges the field of view of the screen
can be a lot greater than 180 degrees!  If you want an "up close" image
use the -radius option.  This option implies -projection orthographic. 

-root
Render to the root window.  This is the default mode.

-rotate angle 
Rotate the globe by angle degrees counterclockwise so that north (as
defined by the -north argument) isn't at the top.  The default value
is 0.  My friends in the Southern Hemisphere can use -rotate 180 to
make the earth look like it should!  If no projection is explicitly
specified, orthographic is assumed, but this option may also be used
with any projection except rectangular.  For non-orthographic
projections, the globe is rotated and then projected, if that helps
you visualize what to expect.

-satfile satfile
Specify a file containing a list of satellites to display.  A file
containing NORAD two line element (TLE) sets named satfile.tle must
exist along with satfile.  A good source of TLEs is www.celestrak.com.
A sample TLE entry for the International Space Station looks like
this:

ISS (ZARYA)             
1 25544U 98067A   01286.44085648  .00059265  00000-0  81723-3 0  5959
2 25544  51.6394 213.7002 0007838 194.2620 314.2054 15.56596996165535

Each line in satfile must begin with a satellite ID number (e.g. 25544
for the ISS).  Each ID must exist in the associated TLE file. 

Valid additional keywords are "align", "altcirc", "color", "font",
"fontsize", "image", "position", "spacing", "trail", and
"transparent".  The usage for most of these is identical to the usage
for the -greatarcfile and -markerfile options.  In addition, a string
to be plotted with the marker may be enclosed in either double quotes
(""), or braces ({}).  If a string is not supplied, the marker will
take the name of the satellite supplied in the TLE file.

The "altcirc" keyword draws altitude circles on the surface of the
earth.  The format is "altcirc=angle", where a circle is drawn
bounding the area where the satellite is greater than angle degrees
above the horizon.  For example, altcirc=0 draws a circle bounding the
region where the satellite is above the horizon, while altcirc=45
draws a circle bounding the region where the satellite is more than 45
degrees above the horizon.  This may be specified more than once.

The "trail" keyword is used to specify an orbit trail.  The format
must be "trail={ground|orbit,start_time,end_time,interval}", where the
start and end times and interval are each in minutes.  The start and
end times are relative to the time of the calculation.  Specifying
"orbit" is only meaningful for the orthographic projection.

A few sample entries are given below:

25544
This draws a marker with the string "ISS (ZARYA)" for the
International Space Station. 

25544 "The Space Station"
This draws a marker with the string "The Space Station" for the
International Space Station. 

25544 "" image=iss.png transparent={0,0,0} altcirc=0 
This draws iss.png at the current position of the International Space
Station.  No text string is drawn.  A curve containing the area where
the International Space Station is above the horizon is drawn.

25544 "" image=iss.png transparent={0,0,0} altcirc=0 altcirc=45 trail={orbit,-5,10,2}
As the previous example, but also draw the orbit trail from five
minutes before to ten minutes past the current time, calculated every
two minutes.  A second altitude circle bounding the region where the
International Space Station is more than 45 degrees above the horizon
is also drawn.

If an ID of 0 is specified, every satellite in the TLE file not
otherwise included in the satellite file will be plotted.

0 color=green
Draws a green marker and labels it with the satellite name for each
satellite which does not otherwise have an entry in the file.

The -satfile option may be given more than once in order to use
multiple satellite files.

-sattrackid id
Set the observer latitude and longitude to be those of the specified
satellite.  The -satfile option must also be used for -sattrackid to
work.  For example, -sattrackid 25544 will place the observer above
the latitude and longitude of the International Space Station.

-shade shade
If the night image file is not found, set the brightness of the night
map to shade percent of the day map.  If shade is 100, the day and
night maps will be identical.  The default value is 30.

-spacing spacing 
Set the spacing between points on great arc files to spacing degrees.
This value defaults to 0.1 degree.  It cannot be smaller than 0.001
degree.

-specular_file filename
Use filename as a specular reflectance file.  Normally it's just a
greyscale image where the oceans are set to 255 and the land masses
are set to 0.  This is used to display the reflection of the sun off
of the oceans.  This option only has an effect with the orthographic
projection.

-starfreq frequency
Fraction of background pixels that will be colored white.  The default
value is 0.001. If no projection is explicitly specified, orthographic
is assumed, but this option may also be used with the azimuthal,
mollweide, and peters projections.

-sunrel del_lon,del_lat 
Place the observer directly above (subsolar longitude + del_lon,
subsolar latitude + del_lat).  If no projection is explicitly
specified, orthographic is assumed, but this option may also be used
with any projection except rectangular.

-swap
Swap the red and blue planes in the image.  This option only works 
with the -output option and is useful on big-endian machines.  

-terminator terminator
Place the observer above the specified terminator.  Valid values are
morning or evening.  For non-rectangular projections, the image will
be rotated so the terminator is approximately vertical.  This can be
combined with the -rotate option to orient the terminator any way you
want. 

-transpng filename
Same as the -output option, except set the background to be
transparent when writing a PNG file.  This option doesn't work
properly with X11 fonts, so use truetype fonts with an X display.

-truetype
Use TrueType fonts.  This option is only useful with X11, where X11
fonts are used by default.  You will need to use this if you are using
an X11 display and want to use TrueType fonts in the marker file.  At
present, you can't mix X11 and TrueType fonts.

-version
Display version information.

-vroot
Render the image to the virtual root window.  Some window managers use
one big window that sits over the real root window as their background
window.  Xscreensaver uses a virtual root window to cover the screen
as well.

-window
Render the image to its own X window.  The size defaults to 512 by 512
pixels but this may be set by the -geometry flag.

-xscreensaver
Same as the -vroot option above.

If no options are specified, the program defaults to -root
-projection rectangular.

Xplanet searches for files in the following order:

TrueType fonts: 
First look in fontdir (if the -fontdir option is used), then in the
current directory, then in a subdirectory "fonts" of the current
directory, and finally in $PREFIX/fonts.  The default font is
helr____.ttf.  Note that TrueType fonts are only used if
an X11 display is not available, a TrueType font is specified with the
-font option, or the -truetype option is specified.

great arc files:
First look in the current directory, then in a subdirectory "arcs"
of the current directory, and finally in $PREFIX/arcs.

image files:
First look in mapdir (if the -mapdir option is used), then in the
current directory, then in a subdirectory "images" of the current
directory, and finally in $PREFIX/images.  The default name is
body.jpg (e.g. earth.jpg, neptune.jpg).  The extension of the file by
default is jpg but this can also be set at compilation time.

marker files:
First look in the current directory, then in a subdirectory "markers"
of the current directory, and finally in $PREFIX/markers.  The default
marker file name is body (e.g. earth, neptune).

satellite files:
First look in the current directory, then in a subdirectory
"satellites" of the current directory, and finally in
$PREFIX/satellites.

The value of $PREFIX is set at compilation time in auxfiles.h.  On a
Unix system it is usually /usr/local/share/xplanet, and on Windows it
is usually C:\WINDOWS\Desktop.  See the INSTALL file for more details
on the configuration options.

Xplanetbg runs Xplanet every five minutes or other specified interval
(taken from the -wait option, where the time between updates is
specified in seconds).  I did it this way instead of adding -wait as
an option to Xplanet since letting Xplanet run all of the time would
take up a lot of memory.  Otherwise Xplanetbg has the same options as
Xplanet without the -animate option, but with the additional options
below: 

-hibernate seconds
After the screen has been idle for the specified number of seconds,
Xplanetbg will sleep.  

-idlewait seconds
Don't run Xplanet unless the screen has been idle for the specified
number of seconds.

-nice priority
Adjust the priority at which Xplanet runs.  On most systems a priority
of 0 is normal and a value of 19 is the lowest priority. 

-num_times num_times
Number of times Xplanetbg will execute Xplanet.  Without this option
Xplanetbg will run Xplanet indefinitely.  

-orbit orbit_spec
Successive positions of an orbit according to orbit_spec are used as
viewing positions.  orbit_spec has the form <duration>:<inclination>
where duration is the length of one orbit in hours and inclination is
the initial direction from the position specified via the latitude and
longitude options.  Inclinations of 90 or 270 degrees will result in
a movement towards the north or south pole respectively. 

-output filename 
Base name of the output file(s) to create.  If this option is used
with -num_times, the specified number of files will be created, each
with a unique filename.  As an example, if the options "-num_times 100
-output earth.jpg" are given the files earth000.jpg through
earth099.jpg will be created.  If -output is used without -num_times,
the output file will be overwritten each time xplanet executes.
Currently .gif, .jpg, .ppm, .png, and .tiff images can be created.

-post_command command
Run the specified command after each execution of Xplanet.

-prev_command command
Run the specified command before each execution of Xplanet.

-start_cloud start_image
-stop_cloud stop_image 
Use start_image and stop_image as the starting and ending cloud
images.  Each intervening time step will create a cloud image file
that is linearly interpolated between these two images in time.  This
doesn't look very good unless the cloud images are fairly close in
time, like a few hours. This option must be used with -num_times.

-start_index index
When used with -num_times and -output, the first output file will have
the specified index instead of 0.

-timewarp factor
As in xearth, scale the apparent rate at which time progresses by
factor.  The default is 1.

-wait
Time between updates in seconds.

Hari Nair
hari@alumni.caltech.edu