Abstract. The long-period perturbations in the ornit of the Lageos satellite due to the earth's albedo have been found using a new analytical formalism. The earth is assumed to be a sphere whose surface diffusely reflects sunlight according to Lambert's law. Specular reflection is not considered. The formalism is based on spherical harmonics; it produces equations which hold regardless of whether the terminator is seen by the satellite or not. Specializing; to the case of a realistic zonal albedo shows that Lageos' orbital semimajor axis changes periodically by only a few millimeters and the eccentricity by one part in 10 5 . The longitude of the node increases secularly by about 3 x 10 -4 arc sec yr -1 . The effect considered here can explain neither the secular decay of 1.1 mm d ly -1 in the semimajor axis nor the observed along-track variations ;n acceleration of order 2 x 10 -1 ` ms--2.

An eclipse of the sun by the moon as seen by the LAGEOS satellite can affect the orbital semimajor axis at the centimeter level. The weakened radiation pressure acting on LAGEOS perturbs the orbit differently from that due to full sunlight. This difference amounted to less than 2 mm in the semimajor axis for 23 of the 30 eclipses LAGEOS experienced between launch in 1976 and the end of 1983. However, it was 17.6 mm for the eclipse on March 28, 1979, and 11.2 mm for the one on December 15, 1982. Differences such as these generate large enough along‐track errors to make it worthwhile to include eclipses in complex orbit determination programs such as GEODYN which integrate the orbit. Eclipses cannot explain the presently unmodeled variations in along‐track acceleration which have a magnitude of about 3×10−12 m s−2.