Xearth sets the X root window to an image of the Earth, as seen
from your favorite vantage point in space, correctly shaded for
the current position of the Sun. By default, xearth updates the
displayed image every five minutes; the time between updates can
be changed using either X resources or a command line
option. Xearth can also be configured to render into a top-level X
window or directly into PPM and GIF files; see the man page for
details.