In a test that scientists said was the most precise experiment ever done, the two Viking spacecraft on the planet Mars and the two in orbit around Mars have confirmed Einstein's Theory of relativity.

The test was a measure of the time it took for radio signals sent to the four spacecraft to return to earth when the sun stood between the two planets on Thanksgiving Day and Mars and the earth were 100 million miles apart.

Einstein's Theory of Relativity predicts that light (or radio) waves moving from one planet to another will be bent off their path by a massive body like the sun, slowing them down for a minute fraction of a second.

"The accuracy of our measurements involved an error of about five feet in a 200 million-mile pass," Dr. Irwin I. Shapiro of the Massachusetts Institute of Technology said yesterday at pasadena's Jet Propulsion Laboratory, where Viking is being directed."That is the highest accuracy of a measurement [of length] ever made by man."

Besides helping to confirm once more that Einstein was right, one of the two Viking spacecraft sitting on the surface of Mars detected what almost surely was a Marsquake. The seismometer on the Viking 2 spacecraft on the Plains of Utopia picked up what appeared to be a tremor early in November that shook Mars with a force equal to about 6.5 on the Richter scale.

"All I can say at the moment is that we've detected an unusually large event on the surface of Mars," said Dr. Donald Anderson of California Institute of Technology. "If it was a selsmic event it was the same size as the San Fernando earthquake in California that killed 64 persons in 1971.

The experiment that confirmed Einstein's Theory of Relativity involved the four Viking spacecraft and about 100 scientists on earth, the most ever to participate in a test of Einstein's theory. The test also provided what some scientists believe was a tenfold improvement in accuracy in testing Einstein's Theory.

"We have not refined all our data and we now have at least a doubling in accuracy," said Shapiro, who bounced radar signals off Venus and Mercury nine years ago in the first space age test of the theory."Our final uncertainty should be well under one-tenth of 1 per cent, which is a tenfold improvement."

In the 60 years since Eistein proposed his theory of general relativity, countlesa physicists have tried to disapprove it and have failed. Those scientists who have suggested theories contradicting Einstein have themselves been contradicted in experiments performed by other physicists.

Einstein theorized that differences in time and space make every measurement ambiguous and therefore relative.

In a series of equations so complex they are fully understood by only a few scientists, Einstein expressed the idea that this "relativity" is made even more ambiguous by the fact that time and space are curved and that this curvature is further altered by the forces of gravity and motion.

As intricate as they might sound to a layman, Einstein's observations formed the foundations of much of the work done in physics in the last 50 years. They have helped scientists explain how and why stars collapse, row stars can collapse so much they became nothing but "black holes" and how the mysterious quasars (for quasi stars) can release more energy than 100 stars of the same size.

Before he died in 1955, Einstein suggested several ways to prove his theory. One was the measure the irregularities in the orbit of Mercury; another was to measure the ways gravity will bend and alter the frequency of light waves.

Shapiro and his team of scientists sent signals to all four Viking spacecraft on Thanksgiving Day and then measured the time it took for the signals to reach the spacecraft and be beamed back to earth. The sun was almost directly between the earth and Mars at the time, meaning the signals would surely be bent by the sun's gravity before they came back to earth.

Under Einstein's theory, the predicted bending of the signals would slow down by two ten-thousandths of a second. This is just what happened to the echoed signals, which were measured to an accuracy of within one ten-million of a second.