From the xearth(1) manual page:

"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 with the -wait option
(see below); updates can be disabled completely  by  using
the  -once  option  (see  below).   Xearth can also render
directly into PPM and GIF files instead of drawing in  the
root window; see the -ppm and -gif options (below)."