xearth for Windows

Table of Contents

1 Introduction

xearth for Windows is a port of Kirk Johnson's original xearth program to Microsoft Windows. xearth renders a shaded image of the earth on your desktop, as seen from your favorite vantage point in space. By default, the image is updated every five minutes according to the current position of the sun.

2 Installation

To install xearth, simply place the executable (xearth.exe) in a folder on your computer. If you would like xearth to start automatically when you log on to your computer, create a shortcut in your Startup folder to the xearth.exe program file.

3 Operation

When xearth is running, a small icon representing the globe is shown in the system "tray" area (near the clock display). To activate the xearth settings page, click with the right mouse button on this icon and select Properties (you may also double click with the left mouse button to activate the settings page).

The following options are available on the right click menu:

Refresh

The Refresh option causes xearth to immediately begin an update of the displayed image. Note that the updated image may not appear right away, because xearth calculates the image as a low priority background task.

Close

The Close option will terminate xearth and restore your original background image.

About

The About option displays program version and copyright information.

Properties

The Properties option displays the xearth settings page. See the following section for detailed information about the options on this page.

4 Description of Settings

• 4.1 Property page: Image

  Orthographic

  The Orthographic projection renders the earth as it would be viewed from space, that is, a sphere. Only half the earth is visible at any one time when using this projection.

  Mercator

  The Mercator projection renders the earth as a flat map of the type often found on wall maps. Distortion is apparent near the top and bottom of the map.

  Cylindrical

  The Cylindrical projection renders the earth as a flat map much like the Mercator projection, but with a different look.

  Rotation

  Specify a rotated viewing position such that the north is not "straight up" in the center of the rendered image. Positive values rotate the rendered image counterclockwise; negative values rotate the rendered image clockwise.

  Galactic

  When this is checked, and the projection is Orthographic, the image is rotated so that the plane of the earth's orbit is parallel to the top and bottom of the screen.

  Magnification

  Specify the magnification of the displayed image. When the orthographic projection is in use, the diameter of the rendered Earth image is this value times the shorter of the width and height of the image. For the mercator projection, the width of the rendered image is this value times the width of the image. The default magnification factor is 1.

  Shift

  Specify that the center of the rendered Earth image should be shifted by some amount from the center of the image. The shift specifier consists of two components, both integers; these components are interpreted as the offsets (in pixels) in the X and Y directions. Positive values of X shift the image to the right; positive values of Y shift the image downward.

  Width and Height

  Specify the size of the image to be rendered. The size specifier consists of two components, both positive integers; these components are interpreted as the width and height (in pixels) of the image.

• 4.2 Property page: Viewpoint

  Default

  The default viewing location is from the sun. The point on the earth where the sun is directly overhead will be in the center of the rendered map.

  Fixed

  The fixed option has two arguments, interpreted as numerical values indicating the latitude and longitude (expressed in decimal degrees) of a viewing position that is fixed with respect to the Earth's surface. Positive and negative values of latitude correspond to positions north and south of the equator, respectively. Positive and negative values of longitude correspond to positions east and west of Greenwich, respectively.

  Sun-relative

  The sun-relative option has two arguments, interpreted as numerical values indicating the offsets in latitude and longitude (expressed in decimal degrees) of a viewing position that is fixed with respect to the position of the Sun. Positive and negative values of latitude and longitude are interpreted as for the fixed option.

  Orbit

  The orbit option has two arguments, interpreted as numerical values indicating the period (in hours) and orbital inclination (in decimal degrees) of a simple circular orbit; the viewing position follows this orbit. Astute readers will surely note that these parameters are not sufficient to uniquely specify a single circular orbit. This problem is solved by limiting the space of possible orbits to those positioned over 0 degrees latitude, 0 degrees longitude at time zero (January 1, 1970, 00:00:00 UTC).

  Moon

  The viewing position is from the current position of the moon.

  Random

  The viewing position is selected at random each time an update occurs.

• 4.3 Property page: Sun

  Use actual position

  When this option is checked, xearth calculates the current position of the sun based on the system clock, and updates it with the progression of time.

  Latitude and Longitude

  Specify a fixed point on the Earth's surface where the Sun is always directly overhead. The Sun position specifier consists of two components, both numerical values; these components are interpreted as the latitude and longitude (in decimal degrees) of the point where the Sun is directly overhead.

• 4.4 Property page: Labels

  Display legend

  Enable/disable the legend. The legend indicates the current date and time and current viewing and sun positions. The position of the legend can be controlled by choosing the appropriate position option.

  Display city markers

  Enable/disable markers. Enabling markers will display small red circles and text labels indicating the location of interesting places on the Earth's surface.

• 4.5 Property page: Quakes

  Show recent earthquakes

  Enable/disable the display of recent tectonic events. When enabled, earthquakes are shown as yellow circles whose diameter is proportional to the magnitude of the event.

  Interval between updates

  This value determines how often the earthquake server at USGS is checked. Setting this value to zero disables automatic earthquake updates.

• 4.6 Property page: Dots

  Display stars

  Enable/disable stars. If stars are enabled, the black background of "space" is filled with a random pattern of "stars" (individual white pixels). The fraction of background pixels that are turned into stars can be controlled with the Star frequency option (see below).

  Star frequency

  Set the density of the random star pattern; this value indicates the fraction of background pixels that should be turned into "stars".

  Percent big stars

  Set the percentage of double-width stars; by default, all stars are a single pixel, but this option can be used to create some stars that are composed of two horizontal pixels. This provides a slightly less uniform look to the "night sky".

  Display coordinate grid

  Enable/disable the display of a longitude/latitude grid on the Earth's surface. The spacing of major grid lines and dots between major grid lines can be controlled with the Grid lines per quadrant and Dots per grid line options (see below).

  Grid lines per quadrant

  Specify the spacing of major grid lines if grid display is enabled; major grid lines are drawn with a 90/value degree spacing. The default value for this value is 6, corresponding to 15 degrees between major grid lines.

  Dots per grid line

  Specify the spacing of dots along major grid lines if grid display is enabled. Along the equator and lines of longitude, grid dots are drawn with a 90/(Grid_lines_per_quadrant * Dots_per_grid_line) degree spacing. The spacing of grid dots along parallels (lines of latitude) other than the equator is adjusted to keep the surface distance between grid dots approximately constant. The default value for this value is 15; combined with the default grid lines per quadrant value of 6, this corresponds to placing grid dots on a one degree spacing.

• 4.7 Property page: Shading

  Enable shading

  Enable/disable shading. When shading is enabled, the surface of the Earth is shaded according to the current position of the Sun (and the values provided for the other shading options, below). When shading is disabled, use flat colors (green and blue) to render land and water.

  Daylight intensity

  Specify the brightness that should be used to shade the day side of the Earth when shading is enabled. This value should be an integer between 0 and 100, inclusive, where 0 indicates total darkness and 100 indicates total illumination.

  Night intensity

  Specify the brightness that should be used to shade the night side of the Earth when shading is enabled. This value should be an integer between 0 and 100, inclusive, where 0 indicates total darkness and 100 indicates total illumination.

  Terminator falloff

  Specify the shading discontinuity at the terminator (day/night line). This value should be an integer between 0 and 100, inclusive. A value of x indicates that the shading should immediately jump x percent of the difference between day and night shading values when crossing from the night side to the day side of the terminator. Thus a value of 0 indicates no discontinuity, and a value of 100 yields a maximal discontinuity (such that the entire day side of the earth is shaded with the daylight intensity shading value).

• 4.8 Property page: Time

  Use current time

  Instead of using the current time to determine the "value" of time-dependent positions (e.g., the position the sun), this option tells xearth to use a particular fixed time.

  Timewarp factor

  Scale the apparent rate at which time progresses by this value.

• 4.9 Property page: Display

  Delay between updates

  Wait the specified amount of time between updates. Setting this value to zero disables automatic updates.

  Gamma correction

  Adjust the colors xearth uses by a gamma value. Values less than 1.0 yield darker colors; values greater than 1.0 yield brighter colors. The default gamma value is 1.0, appropriate for use on systems with built-in gamma correction. For systems without built-in gamma correction, appropriate gamma values are often in the 2.3 to 2.6 range.

5 Known Bugs

The following problems are unlikely to be fixed.

• When earthquake display is enabled and a time other than the current time is selected, xearth still displays the earthquake information for the current real time. xearth will not predict earthquakes in the future.
• When viewing the orthographic projection with stars enabled, the stars shown do not reflect the actual stars visible from the chosen vantage point.

6 Acknowledgements

This program would not exist if it were not for Kirk Johnson's original xearth program for the X window system. Many thanks to Kirk for this fine program.

7 Legal stuff

Permission to use, copy, modify and freely distribute xearth for non-commercial and not-for-profit purposes is hereby granted without fee, provided that both the above copyright notice and this permission notice appear in all copies and in supporting documentation.

Unisys Corporation holds worldwide patent rights on the Lempel Zev Welch (LZW) compression technique employed in the CompuServe GIF image file format as well as in other formats. Unisys has made it clear, however, that it does not require licensing or fees to be paid for freely distributed, non-commercial applications (such as xearth) that employ LZW/GIF technology. Those wishing further information about licensing the LZW patent should contact Unisys directly at (lzw_info@unisys.com) or by writing to

Unisys Corporation
Welch Licensing Department
M/S-C1SW19
P.O. Box 500
Blue Bell, PA 19424

The author makes no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.

THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.