"Seeing a 98 per cent eclipse is like kissing your sister," said Jay M. Pasachoff, research fellow in Astrophysics, as he prepared to leave for Mexico Saturday as part of an expedition to observe the total solar eclipse on March 7. The Harvard College Observatory, Smithsonian Astrophysical Observatory, and National Geographic Society have organized the project.

Although only two per cent of the sun will be visible in Cambridge on March 7, this sliver will effectively block out the scientifically interesting portion of the sun, the outer corona. Therefore, in order to perform several experiments which require a total eclipse, scientists from around the world will be convening in southern Mexico.

The Harvard contingent, headed by Donald H. Menzel, Paine Professor of Practical Astronomy and former director of the Harvard College Observatory, will set up camp in Miahautlan, an Indian village south of Oaxaca. This will be Menzel's 13th eclipse. "The sun has been my specialty, and there is still no better method of observing the sun's outer corona than by watching an actual eclipse," he said.

During a solar eclipse, the moon comes directly between the earth and the sun, and its shadow, the umbra, sweeps a narrow path across the earth. On March 7, the 100-mile wide path of totality will start in the eastern Pacifre and cross Mexico, Florida, Georgia, the Carolinas, and Virginia.

After skirting the eastern coastline, the umbra will pass over Nantucket Island. The moon's outer shadow, or penumbra, however, will cause a partialeclipse that will be visible from most of North America.

In Mexico, the total eclipse will occur at 11:30 a. m., and will last for three minutes. It will reach Nantucket at 1:30 p. m. The whole process of the moon passing in front of the sun will last for two hours.

Clear Skies

The Harvard expedition, which includes Menzel, Pasachoff, Winfield W. Salisbury, lecturer in Astronomy, and seven students and technicians, is joining several other camps in Mexico because there is a 70 per cent chance of rain where the totality will cross the United States. They are allowing a month to go through customs, pour concrete piers, and prepare their equipment.

"This expedition is particularly important because there will not be another good eclipse in this century in a well-populated area." said Pasachoff.

Three Experiments

The expedition will carry out three major experiments, all involving the outer corona. This is a tenuous gas, thinner than any man-made vacuum, and heated by two million degrees centigrade. Although the inner part of the corona can be seen through special narrow band filters without an eclipse, a total solar eclipse is essential to study the whole structure, including the outer portion, of the corona.

To study this portion, the scientists will employ three devices. The first, a spectrograph, was designed by James G. Baker, associate of the Harvard College Observatory. This instrument. weighing a ton, uses 16-inch mirrors and a four-by-six-inch grating with 15,000 individually ruled lines in each inch. According to Menzel, spectroscopic photographs of the outer corona have never been made with such sophisticated equipment.

The second experiment involves the use of four special television cameras to measure the intensity of polarization of the outer corona in various colors. The third experiment is a repeat of photography carried out for the Harvard astronomers at the 1968 eclipse in Siberia. Pictures of the eclipse will be taken through polarized filters at different angles with a specially designed telescope.

An additional experiment that was proposed but has since proven unfeasible was to follow the eclipse for an hour in an airplane travelling 2000 miles per hour. "Ground-based studies will give us some kinds of information that could not have been gotten from the plane," Pasachoff said.

"Eclipses are awesome sights, and though astronomers have to travel to inaccessible places to study them, they do allow us to see such different aspects of the sun that these expeditions are well worth while," Pasachoff added.

The next total solar eclipses will occur in 1972 over northern Canada, and in 1973 over the Sahara desert.