This attribute
specifies the geodetic (or equivalently, geocentric) longitude of the observer, in
degrees, measured positive eastwards. See also attribute 
ObsLat. The basic 
Frame class
makes no use of this attribute, but specialised subclasses of Frame may use it. For
instance, the 
SpecFrame, 
SkyFrame and 
TimeFrame classes use it. The default value is
zero.
The value is stored internally in radians, but is converted to and from a degrees string for
access. Some example input formats are: " 155:19:23.2" , " 155 19 23.2" , " 155:19.387"
, " 155.32311" , " E155.32311" , " -204.67689" , " W204.67689" . As indicated, the sign of
the longitude can optionally be indicated using characters " E" and " W" in place of the
usual " "
and " -" . When converting the stored value to a string, the format " [s]ddd:mm:ss.ss"
is used, when " [s]" is " E" or " W" and the numerical value is chosen to be less than
180 degrees.  
  Frame  
 All Frames have this attribute.
  SpecFrame  
 Together with the ObsLon, 
Epoch, 
RefRA and 
RefDec attributes, it defines
the Doppler shift introduced by the observers diurnal motion around the earths axis,
which is needed when converting to or from the topocentric standard of rest. The
maximum velocity error which can be caused by an incorrect value is 0.5 km/s. The
default value for the attribute is zero.  
 TimeFrame  
 Together with the ObsLon
attribute, it is used when converting between certain time scales (TDB, TCB, LMST,
LAST)