The Vineyard's outermost lighthouse is celebrating a birthday. The Cape Pogue Lighthouse on Chappaquiddick is 200 years old, and for most of those years it has stood as a constant and reliable sentinel for ships making passage across the sometimes treacherous waters of Nantucket Sound.

To celebrate its importance during the past two centuries and to show off some of the restoration work that has been done on this beautiful landmark, the lighthouse will be open to the public this Saturday, with free tours available from 2 to 5 p.m. The staff and friends of The Trustees of Reservations (TTOR) and U.S. Coast Guard are hosting the celebration.

Of the Island's five working lighthouses - four, if you don't count the lighthouse on the grounds of the Martha's Vineyard Historical Society in Edgartown - the Cape Pogue Lighthouse is the hardest to reach.

Located on the northeast corner of Chappaquiddick, it is accessible only by a four-mile drive on an off-road vehicle. There is only one other option, though just as difficult, by boat.

To make celebrating easier, The Trustees of Reservations are providing free vehicular rides to the lighthouse. The shuttle service will start at the Chappaquiddick ferry point. Suzan Bellincampi, TTOR environmental education naturalist, suggests that anyone planning to take the tour should allow about 90 minutes, which includes the tour as well as the travel time to and from the lighthouse. Expect, also, for the trip to be one of the most memorable on the Vineyard, for the lighthouse oversees some of the Island's most pristine landscape - the Cape Pogue Wildlife Refuge.

The refuge is among the most heralded places in the world, with more than 516 acres of open beach and meadow as well as a small forest known as The Cedars. As popular as all the Island lighthouses are with seasonal visitors, this one is a distant jewel. It stands just in from a cliff and rises 60 feet above sea level. This structure has been moved many times to avoid going over the edge. It was first moved in 1838, then again in 1844, 1907, 1922, 1960 and 1987. In 200 years this lighthouse and its two predecessors have been moved more than 800 feet.

While the nautical value of the lighthouse may be more aesthetic today than it was years ago, throughout its existence it has been an essential navigational aid to the traffic crossing Nantucket Sound. Before the construction of the Cape Cod Canal, every vessel sailing between New York city and Boston depended on the lighthouse as they traversed the waters between Cape Cod and the Vineyard - waters made difficult by such nautical hazards as shoals, the East and West Chops, and the Gay Head cliffs.

The Cape Pogue Lighthouse is the only one on the Vineyard constructed almost entirely of wood. Its exterior walls are covered with painted cedar shingles, its top beacon room is made of steel. Four steel guy wires connected to the ground keep the structure upright. The tower was built 108 years ago.

The building has had its share of lightning strikes, a good deal of evidence to this effect found by restorers a few years ago. It has also survived many hurricanes and northeasters, the worst storms the Atlantic Ocean might conspire to send this way. Easily seen from the shoreline from East Chop to Cow Bay, the light marks the northernmost part of Chappaquiddick. It is also readily visible from the Bend in the Road beach (Joseph A. Sylvia State Beach).

Arthur Railton, editor of the Dukes County Intelligencer, has written extensively on the lighthouse. In an article that appeared in the Autumn 1994 edition of The Keeper's Log, Mr. Railton wrote: "Sometime in December 1901, Cape Pogue Light had its 100th birthday, but there was no celebration. Keeper George E. Dolby didn't mention the anniversary in his journal; he probably was not aware of it. In those 100 years, many improvements had been made inside the lighthouse tower. The first lighthouse, in 1801, had a multi-wick, smoky spider lamp that burned sperm oil inefficiently. . . .

"One hundred years later, a concentric-wick kerosene lamp burned bright and clean within a jewel-like lens of precisely ground optical glass that rotated to send brilliant red and white flashes to the horizon."

Today, the lighthouse flashes white every six seconds. The light is electrical, powered by solar cells that charge a battery by day. The light can be seen from a distance of up to nine miles.

The lighthouse interior, which includes an informational display about the building, has been handsomely renovated by the Trustees. The winding stairwell and banister make a complete 360-degree turn for anyone ascending the stair's 32 steps.

Although the Coast Guard still owns it, The Trustees of Reservations took charge of the lighthouse in 1994.

Dan Geary, a summer resident of Chappaquiddick and a summer employee for the Trustees, said he had more than a few reasons for feeling fortunate about working on the old maritime heritage structure.

"My father in law, Don Langley, used to work on the lighthouse 50 to 55 years ago. He was in the Coast Guard," Mr. Geary said. "He chipped paint."

Mr. Geary and others like him have taken charge of the lighthouse. This week they could be found painting the 35-foot-high tower to ready it for the Memorial Day weekend.

This is a big event for the Trustees. The Cape Pogue Light is their mascot. They use a silhouette of the lighthouse in their off-road vehicle beach stickers.

Tomorrow, van shuttles will run continuously from 2 to 4:30 p.m., between the ferry and the entrance to the Cape Pogue Wildlife Refuge, with over-sand trucks providing transportation for the journey to and from the lighthouse.

After tomorrow, lighthouse tours will be on a fee basis. Throughout the summer, nonmembers of the Trustees will pay $15 (adults); and children 15 years of age or under will pay $10. The fee includes transportation to and from the lighthouse.