Although the moon undergoes an eclipse tomorrow evening, casual observers here will have difficulty in distinguishing any real marked change in the appearance of the celestial body even if the sky is clear, according to Robert B. Watson, of the Harvard Observatory.

Instead of being completely blotted out by the earth's shadow, the moon will merely become darker, remaining visible throughout the celipse, he added.

Known as a "penumbral" eclipse, the process will begin at 9:28 o'clock and will end at 1:25 o'clock Monday morning, reaching its maximum between 11 and 12 o'clock midnight. At its height, the eclipse will be marked by a darker shadow on one side than the other.

In contrast to a complete lunar or "umbral" eclipse, when the moon passes through the direct shadow of the earth, this affair is called "penumbral" because the moon merely travels through the outer, partial shadow, just missing the earth's direct shadow by a few minutes.

Watson, an instructor in Physics and Communication Engineering, explained that this type of an eclipse takes place on the average of about once every two years, this one being visible throughout the United States.

Dr. Watson recommended observers to try to realize the relative small size of the moon's diameter of 2000 miles to the 866.000 mile diameter of the sun. He also explained that the reason eclipses of the moon do not take place more often is the fact that the orbit of the moon varies five degrees from that of the earth.