The seconds are ticking down for the castle that once kept the world running like a clock, a victim of changing times.

Herstmonceux Castle, the home of the Royal Greenwich Observatory, went on sale Thursday for an estimated $11.2 million to $18.7 million.The observatory, where Greenwich Mean Time was once calculated as the reference point for the world's time zones, is expected to move into Cambridge University in 1990.

Time, apparently, has passed the operation by.

"The reason for the move from Herstmonceux Castle is that most of the observatory's observational work is now done overseas, atmospheric conditions abroad being far more favorable than anywhere in the British Isles," said Bill Yates, a London real estate agent.

"As a result, a new home base more suited to the requirements of modern astronomy is to be built at Cambridge."

Astronomers were up in arms two years ago when the government said it would sell the castle in southern England, 21 miles northeast of Brighton in the county of East Sussex, and move the observatory.

Greenwich Royal Observatory clocks, first at Greenwich and then at Herstmonceux, once calculated Greenwich Mean Time for the world. But in the past decade, Britain has only been a minor contributor to a Paris-based international time service that uses 250 clocks around the world.

The observatory said in 1985 that for financial reasons it would let its six atomic

clocks stop after 101 years of calculating Greenwich Mean Time. The clocks contain the atomic element cesium, which must be replaced every few years at a cost of nearly $200,000.

The observatory's main functions now are confined to supporting its new telescopes in the Canary Islands and in Hawaii, where clean air permits observation, but Britain's astronomers still opposed the move to Cambridge.

They said it would scatter the observatory's archives and rare books and end three centuries of independent excellence.

The observatory was founded in 1675 by King Charles II. It moved to its present location a 20th century copy of the original Herstmonceux castle in 1946 because pollution interfered with stargazing at its Greenwich location in southeast London.

The buildings at Greenwich, along with the brass strip marking longitude zero, the Prime Meridian from which the world measures its time zones, remain open as a museum.

Yates predicted the sale of the castle, which stands on 368 lush acres, will generate inquiries from around the world.