Twilight. Scientifically defined as that time of day between dawn and sunrise or sunset and dusk where sunlight scattering in the upper atmosphere illuminates the lower atmosphere and the Earth’s surface. Romantically expressed, perhaps, as that special time in the morning where slumber gradual recedes and the promise of a new day springs forth or the soft light of the fading sun casts a special glow upon loved ones and familiar surroundings. Twilight is a favorite of artists and romantics. It is a time for reflection upon the events of the day with the hope of yet another day of life to follow tomorrow. Whether viewed scientifically or romantically, the twilight does indeed possess a special quality.
At the South Pole we are able to enjoy twilight for days rather than minutes, but we only enjoy it twice a year. The sun sets in March and comes up in September. We are now experiencing what is called nautical twilight, for twilight is broken down into three widely accepted categories: civil twilight, nautical twilight, and astronomical twilight.
Civil twilight is the brightest of the three. It begins at sunset and ends when the geometric center of the sun is 6 degrees below the horizon. At the South Pole that occurs between 21 March and 4 April. The brightest of stars and planets (such as Venus, the “morning star” or the “evening star”) can be seen during this time. There is still enough light to continue outdoor activities without artificial light and terrestrial objects are still clearly distinguishable. Because of the usual inversion conditions at the Pole (air temperatures are colder near the surface than at higher altitude), we were able to catch a glimpse of a sliver of the sun after it has already set since the image was refracted through the denser, colder air. Good thing, or we would have missed the departing sun entirely. We had solid 8 days of clouds, winds, and blowing snow during sunset!
Nautical twilight is when the sun is between 6 and 12 degrees below the horizon, which occurs here between 5 and 21 April. It ends when the horizon at sea can no longer be used for navigation. During nautical twilight, Sailors are able to make reliable star sightings and navigation calculations using a visible horizon for reference. General outlines of objects on the ground are still distinguishable. Militarily, nautical dawn and nautical dusk have been used when planning operations as far back as the mid-18th century, since combatant would often choose those times to launch attacks. We use the beginning of nautical twilight at the South Pole to finish last minute preparations for the dark winter ahead.
The final flag lines are put in place to allow safe outdoor travel in conditions of diminished visibility. All windows in the elevated station are “boarded up” with either foam blocks or cardboard to prevent artificial light from interfering with the sensitive cameras on the roof that study the aurora. This gives the station the ambiance of the world’s largest submarine, although I can still step out on the porch if I want to experience the Antarctic conditions unprotected…at least for 10 to 20 seconds at a time.
Finally, astronomical twilight occurs here between 21 April and 12 May when the sun is between 12 and 18 degrees below the horizon. The end of astronomical twilight is when the sky is dark enough for all astronomical observations and the faintest stars visible to the naked eye can be seen.
Temperatures have been falling with the fading sunlight. On 7 April we reach minus 100.1 degrees Fahrenheit (minus 135 with windchill). Everything exposed to the elements was cloaked in a covering of ice. Every exhaled breath produced a strange, unearthly sound as the water vapor in expired air instantly froze, producing billowing clouds of ice crystals. Bare hands were useful to manipulate camera controls for only about 10 seconds before the pain became unbearable and dexterity rapidly faded. One does not touch bare metal for fear of leaving behind flesh on the railings or door handles.
There are other things occurring at the South Pole when temperatures drop into the triple digits below zero, but that will have to wait until next time…