In an age of radar and GPS, lighthouses are often thought of as tourist attractions and part of Maine’s coastal scenery. But well into the 20th century, they were critical aids to navigation and saving lives. And today, they still play a role for mariners.

Halfway Rock was a known hazard to captains approaching Casco Bay in the early 19th century. But it wasn’t until several ships were wrecked and sailors killed that Congress finally authorized enough money to build a beacon on the two-acre ledge. It went into service in 1871.

The 76-foot tower, made from granite blocks quarried on nearby islands, doubled as the keepers’ dwelling on the first two floors. In 1884, a wooden boathouse was constructed in the lee of the tower, with two bedrooms in the loft. A wood ramp ran down the rocks into the sea.

Three keepers were assigned to the lighthouse, two on duty at all times and one on shore leave. Because of its small footprint and remote location – a barren rock 10 miles from Portland and more than four miles from Harpswell – Halfway Rock was known as a “stag” lighthouse. No spouses or families allowed.

The last keeper left in 1975, when the Coast Guard automated the beacon and foghorn with solar power. That’s how it functions today. The light flashes red every five seconds and can be seen 19 miles away. The fog signal sounds every 30 seconds.

Halfway Rock Lighthouse remains a visual reference point for boaters in Casco Bay, according to Curtis Rindlaub of Portland, co-author of “A Cruising Guide to the Maine Coast” and a smaller guide to Casco Bay. In his Casco Bay guide, Rindlaub warns fishermen to avoid the area unless the weather is settled.

“This would be a remote and exposed place to have an accident,” he writes.

Rindlaub said he still finds Halfway Rock to be a reassuring presence, a benchmark for heading toward Portland Harbor or sailing east to Small Point and Down East.

“I like to see it, because I know I’m halfway across the bay,” he said.