Bill Trandum

Lighten up

We on the Key Peninsula are about as lucky daylight-wise as anybody in the Lower 48 states. When the summer solstice occurs, about June 21 each year, we are blessed with very long daylight hours. That is to say that twilight signaling the onset of darkness lasts for a considerably long time.

Technically, there are three twilights. civil twilight lasts from before sunrise, when the geometric center of the sun is six degrees below the eastern horizon until it is again six degrees below the western horizon in the evening. Here on the Key, that translates to daylight, including Civil twilight, lasting at the summer solstice for 15 hours and 57 minutes. It is the period when you can almost read a newspaper even when there’s no moonlight, the horizon is clearly defined, and the brightest stars are just visible under clear atmospheric conditions. The end of civil twilight is when you’re legally required to turn your headlights on if you’re driving a vehicle.

Nautical twilight starts earlier and ends later than civil twilight. As a result, the sky is darker near its ends. It is defined as the time from when the sun is 12 degrees below the horizon before sunrise until it is again 12 degrees below at sunset. In clear weather, at the beginning and end of nautical twilight, a sailor using his or her navigation tools can see the horizon at sea, but would have to use artificial light to read anything including instruments.

Then there’s astronomical twilight, defined to begin in the morning and to end in the evening when the center of the sun is geometrically 18 degrees below the horizon. From the end of astronomical twilight in the evenings to the beginning of astronomical twilight in the morning, the sky (away from urban light pollution) is dark enough for all astronomical observations. Astronomical twilight, according to the U.S. Naval Observatory, commences when the first bit of light from the sun just starts to brighten the horizon.

Key Center sits at about 47 degrees, 20 minutes north latitude. If we were north of 48 degrees, 30 minutes, at the summer solstice with clear skies, astronomical twilight can last all night long because the center of the sun never dips more than 18 degrees below the horizon, so there’s just a smidgen of light on the horizon all night long.

The western entrance to the Strait of Juan De Fuca is just north of 48 feet 30 inches so a sailor doesn’t have far to travel to experience that phenomenon. Inland, it doesn’t work so well because hills and mountain ranges block the light.

Even though in July we’re just past the summer solstice, we still get to enjoy many hours of daylight and our warmest days of the year are just ahead of us. The longest day, June 21 is not the warmest day because the Earth, like the oven in your stove, takes some time to heat up after you turn it on to high.

Bill Trandum is a retired U.S. Navy captain, an avid boater and a now retired sailboat racer. He has sailed in ocean weather conditions ranging from dead calms to typhoons where he became a student of winds, tides and weathers.

Winds, Tides, and Weather
Winds, Tides, and Weather