Ancient earthworks in Louisiana may be the world's largest astronomical calculator, far larger than Britain's fabled Stonehenge, according to recent measurements and astronomical calculations by two scientists.

The 3,800-year-old earthworks are arranged in six great octagons, one inside the other, with passages radiating out of all eight corners. The whole monument, made of dirt ridges, measures about three quarters of a mile across. Each side of the outermost octagon is about 1,300 feet long.

Stonehenge is the best known of the world's ancient astronomical calendars, but the ring of huge stones that forms it is only a few hundred feet across.

The great monument at what is called Poverty Point, in northeastern Louisiana, was built by people who carried basketfuls of dirt and heaped them up to form the ridges.

The scientists say half a million cubic yards of dirt must have been moved, making it the most massive of all monuments in the ancient world. It is lower but far more massive than the great pyramids of Egypt, according to archeologist William Haag, who recently retired from Louisiana State University and has studied the Poverty Point structures for 25 years.

The unusual ridged mounds have been known to archeologists since the turn of the century, but the array is so large that the concentric octagons could not be seen until 1954 when the first aerial photos were taken.

"Then the whole earthwork just appeared to us," Haag said. "We were aware of the ridges, but the magnitude of them never entered our heads, or that they were manmade, until we saw those pictures."

Not until the mid-1970s did Haag began to think that this great array not only was manmade but might serve as an astronomical calendar as the Stonehenge monument does. In the last two years he and astronomer Kenneth Brecher of Boston University made the measurements and calculations to check the idea.

Haag said that, for an observer standing in the middle of the earthworks and looking down the aisles that radiate from the center, each aisle probably pointed to a special position of the sun, moon or some bright star.

Three lanes remain uneroded. At the summer solstice, the longest day of the year, the sun sets directly in the center of one of the aisles. At the winter solstice, the shortest day of the year, the sun sets in the other still-intact western aisle. A third aisle points at the North Star, Haag said.

Haag believes the monument was used by people of the area--about 5,000 at the time occupied a village at the site--to mark the key times of the year for hunting and gathering wild food.

Haag finds it interesting that all the greatest ancient monuments --Stonehenge in England, the great pyramids of Egypt, the Poverty Point earthworks--were begun at about the same time, around 3,800 years ago. Such coincidences in science are not unusual, Haag says, because knowledge and discovery often come to a head at the same time in different parts of the world. Isaac Newton and Gottfried Leibnitz in England and Germany respectively, for example, independently invented calculus at the same time in the 17th century.

David Thomas of the American Museum of Natural History said that some scholars, though impressed by the size and pattern of Poverty Point, are doubtful that this and other ancient monuments were used as astronomical calendars.

The positioning of the monuments might well be accidental in some cases, he said. He pointed out as an example that, seen through the gateway arch to St. Louis, some monuments and the street pattern of the entire downtown area all are aligned on an east-west axis. If someone were to dig the city up a few millenia from now, the city might appear to be laid out as some astronomical setup.

Haag responds that the probability of more than one arm of the monument coinciding exactly with key points in the solar year is very small. No major features of St. Louis or any other place make a pattern corresponding to astronomical points in that way, he said.