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 eclipse 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 of 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 whcih integrate the orbit. Eclipses cannot explain the presently unmodeled variations in along-track acceleration which have a magnitude of about 3×10-12 ms-2.