In Proposition Eight, Newton tried to demonstrate the correctness of his explanation by calculating the mass, surface gravity and density of the known planets. To determine the mass, he needed to know the angle between a line from the center of the Earth to the Sun, and a line from a point on Earth to the Sun. Routine Class Assignment

Modern science has put that number at about 8.8 seconds. (A second is one-3,600th of a degree.) Newton believed the figure to be 10.5 seconds, but he mysteriously used 11 seconds in the equation. That is the error Mr. Garisto discovered when he repeated those calculations as part of a routine class assignment.

''When I found the discrepancy, my initial reaction was 'Wow!' '' he said. But he simply turned in the assignment, assuming students were expected to find the discrepancy. That was mid-February. But the significance of the find eluded even Prof. Noel Swerdlow, who gave Mr. Garisto an A-plus for his paper. He had made the assignment because he had never been able to get the numbers in Proposition Eight to agree with each other.

In April, Professor Swerdlow and Mr. Garisto attended a lecture on the ''Principia'' by Prof. Subrahmanyan Chandrasekhar, a univeristy astrophysicist who is a Nobel laureate. Immediately afterward, Mr. Garisto and his teacher realized what the student had uncovered.

Mr. Garisto wrote a more extensive paper, submitted it to the scientific honor society Sigma Xi and won the university chapter's Prize for Excellence in Science, no small feat in a physics department that has 18 Nobel Prize winners among past or current students and faculty.