### ObsLon

The geodetic longitude of the observer

#### Description:

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.

Type:
String.

#### Applicability

##### 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)