A bit of light in our dim sea

In these dark winter months, one must work a little harder to find the light we need to thrive. It’s a metaphor, but also perhaps a bit practical. We sailors often find winter somewhat of a trial — the wind isn’t great, if there’s any at all, and it’s cold. We in the Bay Area can at least take comfort in the fact that our waterways don’t frozen over, but frankly that’s (pardon me) cold comfort

So how ’bout this: Consider the leading lights that have helped keep us mariners safe over the decades. I’m thinking of our region’s lighthouses, and Point Bonita Light in particular. This is the light at the rocky southwest extremity of Marin County that guides ships safely into the Golden Gate.

So many interesting factoids and historical anomalies are balled up in this one site. You could think of it as a sailor’s winter reverie. It’s a warm, welcoming glow above a harsh, wind- and fog-blown sea. Yada, yada.

Enough of the flowery prosery. Here are some facts… 

Point Bonita Light was the erected in 1855, the third lighthouse constructed on the West Coast of North America. The first was Alcatraz light, one year earlier, followed by a spate of lights up and down the coast. Unfortunately, they came too late for the hundreds of vessels that ran aground or sank in the approaches to the Golden Gate during the California Gold Rush that began in 1849.

Sadly Point Bonita Light was also a failure. 

Because of the curvature of the earth, a lighthouse needs to be high up to be visible to ships far at sea. For that reason engineers placed Point Bonita Light atop a 56-foot tower constructed on a bluff 306 feet above sea level.

Where it was completely useless. 

Sure, on clear days, the light might be visible more than 20 miles out at sea. But clear days weren’t the problem. The problem was the famous fog that so often envelopes the Golden Gate. At that height, Point Bonita’s beam was too often hidden in the mists above the Gate’s murderous rocks. 

So in 1877, the light was moved to a outcropping just 124 feet above the surface of the ocean. It wasn’t an easy task. Anonymous workers had to chip a 118-foot tunnel through rock to get to the site where the light would be perched. And even that wasn’t enough. By 1940, the trail leading from the tunnel to the lighthouse had eroded and collapsed, necessitating construction of a bridge.

The tunnel and bridge today remain the sole access route to Point Bonita Light, though the bridge is in need of repair and the National Park Service isn’t letting visitors cross it. 

At the time Point Bonita Light was constructed, the typical fashion of creating light in an aid to navigation was via whale oil, which burned brightly and didn’t create much smoke. The light was nevertheless dim by modern standards. That meant exceptional means were required to make the glimmer visible to mariners far at sea. 

It fell to a French engineer named Augustin-Jean Fresnel to design a solution. His lenses, which used concentric glass rings to concentrate light, were found in lighthouses the world over by the mid-1800s. His concepts are still in use today in things light traffic lights. 

The second-order Fresnel lens in the Point Bonita Light was brought around Cape Horn by ship from France. (Fresnel lens orders, or sizes, ranged from first, or largest, to eighth.)

Fresnel’s original lens is still in the lighthouse today, though whale oil is no longer the light source. Over the decades, whale oil gave way to lard oil, then to kerosene, then to oil vapor. For almost 100 years, light came from incandescent bulbs. In 2022, the bulb was replaced by an LED. Through all these changes, the Fresnel lens remained. 

Point Bonita’s position is particularly significant to professional mariners. It forms the northern end of what maritime authorities call the demarcation line for San Francisco Bay. It officially separates the inland waters of the San Francisco Bay and delta from open ocean. The southern end of the line is another lighthouse, Mile Rocks Light, just offshore of San Francisco’s Lands End trail. 

This line is where inland right of way rules give way to the rules used on the open sea. It’s also where mariners licensed to operate only on inland waters are required to turn around.

Point Bonita, like all navigational lights, has a characteristic flash that helps mariners tell it from other aids to navigation. Point Bonita Light’s characteristic is described as an occulting beam. It shines white for three seconds followed by a one-second eclipse. It is thus easily distinguished from Mile Rocks Light, which flashes white briefly every five seconds. 

The trail and tunnel leading to Point Bonita Light are open on Sundays and Mondays from 12:30 to 3:30 p.m. and I thought it’d be fun to schedule visit — let’s call it a land cruise-out for Spinnaker members. I envision discussing some of these facts — a lots of other fascinating details about navigating the Golden Gate over a picnic lunch. 

Point Bonita is also one of the few places in our region where someone interested in celestial navigation can get a clear view from land of an ocean horizon directly to the south. A south-facing ocean horizon is necessary (at our latitude) to perform a noon sight that a navigator could theoretically use to fix his or her position using just one celestial body. So I was thinking I’d bring a couple of sextants and see if we could get some sun sights to add to all the other fun we’ll have.

If you’re interested in this outing, let the folks in the Spinnaker office know and we’ll put this on the schedule.