This is the longitude of the telescope, measured in decimal degrees east from Greenwich. Note that the sign convention for longitudes changed in 1984! The older versions of the Astronomical Almanac gave west longitudes. For photometric data reduction, an accuracy of about 0.003 degree (about 20 arcsec) is required. Current volumes of the Astronomical Almanac give positions to 0.1 arc minute, which is adequate for our purposes.
Astrometric observations, including occultation timings that may be measured with photometric equipment, require much better accuracy. You can find precise positions of telescopes in the pre-1984 volumes of the Astronomical Almanac. You should try to provide as accurate a value as possible.