FOR CENTURIES, men have been delving into the mysteries of Stonehenge, that great stone sanctuary which had become a ruin even before the arrival of the Romans in Britain.

And there has been no end to the speculation in modern times.

Indeed, it would seem that the interest in this prehistoric "rock garden" (the facetious reference was made by a New York businessman on tour in Britain) has been directly proportional to the tourist traffic.

Last year nearly 400,000 people from all parts of the world braved the perpetual winds of Salisbury Plain to satisfy their curiosity.

What they saw was not appreciably different from the sight that has excited viewers for hundreds of years.

From the distance on its lonely setting above the fork of the highway, Stonehenge sometimes resembles an abstract sculpture. Perhaps for this reason it has been a source of inspiration to painters and photographers.

But under steady and close surveillance the image shifts. The abstraction begins to assume hard lines and the inevitable questions emerge.

WHAT WAS it? Who built it? How old is it?

Only the last question can be answered with any degree of certainty.

Archaeologists have decided that Stonehenge dates from about 1800 B.C. and that it was reshaped a number of times between 1800 and 1400 B.C.

Stonehenge I dates from the New Stone Age or Neolithic Period; Stonehenge II was constructed about 1600 B.C. and Stonehenge III was rebuilt in three stages, probably between 1600 B.C. and 1400 B.C.

At least some of the stones used in the construction are native to Wales and probably were brought by water along the Bristol Avon, then dragged overland to Salisbury plain. Shaped by pounding them with round stones, they were hauled to the site on tree-fork sledges.

MOST STUDENTS of Stonehenge claim it was a temple during at least part of its active use. It has even been suggested that it once was the most important religious shrine in southern Britain — the Westminster Abbey of its day.

Since Stone Age pottery has been found at the site, it is possible the Druids of the late Iron Age — whose sagacity impressed Caesar and whose bloody sacrifices shocked Tacitus — took over what had been an altar or holy place for 1,500 years.

A popular idea that the Druids built Stonehenge has been discredited by scholars. Some experts even doubt that the Druids had any connection with Stonehenge.

There are historians who contend that Stonehenge was a temple for sun worship, and if this theory has any credence at all, it would seem to fit in with the discoveries of Boston University astronomy professor Gerald Hawkins.

HAWKINS published his views in 1965 in a book entitled, "Stonehenge Decoded." He claims Stonehenge was built as a sophisticated astronomical observatory.

His theory hinges on the basic structure of Stonehenge — a 97-foot ring of 25-ton uprights and horizontal slabs (known as the Sarsen Circle) surrounding five huge trilithons or archways.

In his studies, Hawkins became fascinated by one aspect of construction:

Stonehenge, he discovered, was oriented so that its axis passes through a 35-ton marker stone and points directly to the spot on the northeast horizon where the sun rises at the summer solstice — the longest day of the year.

Standing among the giant slabs, he felt the architects had deliberately limited his exterior view — as though compelling him to sight through an instrument.

USING DATA from Stonehenge in a computer, Hawkins discovered that a number of the Stonehenge alignments pointed with uncanny accuracy to the summer and winter solstice positions of the rising and setting sun and moon.

In this manner he figured that early Britons were able to determine that winter had started on the one day of the year that the rising sun was entirely visible on the horizon through two carefully aligned arches.

Hawkins believes that Stonehenge astronomy noted one phenomenon undetected by modern astronomers — eclipses of the moon occur in cycles of 56 years.

He accidentally rediscovered the cycle when running Stonehenge data through the computer and associated it with the 56 Aubrey (named for their discoverer John Aubrey) holes, or shallow pits of chalk, that ring the arches. lie decided the holes formed a primitive eclipse computer, which enabled operators to determine the probable dates of solar and lunar eclipses.

Most historians and archaeologists are willing to concede him this final conclusion, but a number of them remain skeptical of Hawkins' observatory theory.

Other theories undoubtedly will be offered as research continues. Perhaps future generations will be able to unlock all the secrets of Stonehenge. Until then, the mystery will contribute to the lure.

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