Light and sky in Antarctica

Earth’s axis is tilted about 23.4 degrees relative to the plane of its orbit around the Sun. Between September and March, the southern hemisphere tilts toward the Sun, and the area around the South Pole experiences whole days of continuous daylight as the planet rotates. Between March and September the opposite occurs.

The Antarctic Circle, near 66.5° south latitude, marks the limit: south of it there is at least one day each year when the sun does not set, near the December solstice, and at least one day when it does not rise, near the June solstice.

The atmosphere bends sunlight and makes the solar disk appear above the horizon when it is actually already a little below it. That is why the midnight sun begins a bit north of the circle and lasts a few days longer than pure geometry would indicate.

The farther south you go, the longer the endless day and endless night become. At the Antarctic Circle, each lasts about one day, near the solstices. At the tip of the Peninsula, farther north of the circle, the sun sets every day, but December nights are short and bright.

At McMurdo, 77.8° S, the midnight sun runs roughly from late October to late February, and the polar night from late April to late August: about four months each, with weeks of transition when the sun rises and sets.

At the South Pole there is only one sunrise and one sunset per year. The sun rises near the September equinox, circles the sky at the same altitude during a single day, climbs to about 23 degrees at the December solstice, and sets near the March equinox. After that come about six months without sun.

At the poles the sun rises and sets very slowly because it moves diagonally, almost parallel to the horizon. For that reason twilight, the phase when the sun is just below the horizon and the sky still has light, lasts for weeks.

At the South Pole, after the March sunset, it takes about a month and a half for the sky to become completely dark, when the sun is more than 18 degrees below the horizon. The return of light before the September sunrise is also gradual.

For those who spend the winter at a station, these phases mark the calendar. The most important day is the June solstice, midwinter, celebrated at all stations with a special dinner and exchanges of messages between bases.

During the polar night, the only light comes from the Moon, the stars and the aurora. At inland stations outdoor work is limited to the essentials, with extreme-cold clothing and red lights, which do not disturb eyes adapted to the dark nor telescopes.

Without the alternation of day and night, the body’s internal clock becomes desynchronized. Stations use strong artificial light, fixed meal and sleep schedules, and medical monitoring. At Concordia Station, the European Space Agency studies the effects of isolation and darkness to prepare for long space missions.

Animals adapt too. The emperor penguin is the only bird that rears its chicks in the Antarctic winter: males incubate the eggs in the dark, huddled in groups to conserve heat, as shown in Life in Antarctica.

The aurora australis is the sky glow caused by particles from the solar wind that the Earth’s magnetic field channels toward the magnetic south pole, where they collide with atmospheric gases. Green, the most common color, comes from oxygen about 100 to 200 kilometers up.

The aurora appears in a ring around the magnetic pole, which lies near the coast of Adélie Land. Much of Antarctica sits under or near that ring, and at the stations the aurora often appears on many clear, dark winter nights.

In summer, with the sun up all the time or the sky too bright, the aurora still occurs but is not visible. That is why almost only those who winter at the stations see it. How auroras form and where they appear elsewhere is covered in Aurora boreal y austral, in The Earth and Tourism.

The cold air full of ice crystals creates light effects uncommon elsewhere. Halos are rings around the Sun or Moon formed by light passing through suspended ice crystals. Parhelia, or sundogs, are bright spots on either side of the Sun.

At the end of winter, nacreous clouds may appear 15 to 25 kilometers high, with mother-of-pearl colors. They form only with extreme cold in the stratosphere and participate in the chemical reactions that open the ozone hole, discussed in Antarctica and Earth’s climate.

Near the horizon, temperature differences between air layers bend light and create mirages: icebergs and mountains can seem to float or stretch. At sunrise or sunset, sometimes the green flash appears, a quick green gleam at the edge of the solar disk.

The Antarctic plateau combines what astronomers look for: very dry, cold, thin air, little turbulence, no city lights and months of continuous night. Water vapor, which blocks part of infrared and microwaves, is at its lowest levels on the planet there.

Dome A, the highest point of the ice cap at 4,093 meters, where the Chinese Kunlun station opened in 2009 is located, was identified in 2020 as the best place on Earth for optical astronomy. There, the air turbulence above a layer of about 14 meters makes the image of the stars roughly twice as sharp as at the best observatories in Hawaii and Chile.

Dome C, at 3,233 meters, houses the French‑Italian Concordia station, opened in 2005 and one of the few inland stations that operate year‑round. In winter, a team of about 13 people stays isolated for roughly nine months, with no flights or resupply.

At the Amundsen‑Scott station the South Pole Telescope operates, with a 10‑meter antenna, inaugurated in 2007. It studies the cosmic microwave background, the oldest light in the universe, and is part of the Event Horizon Telescope, the network that imaged black holes.

Beneath the station lies IceCube, a neutrino observatory embedded in the ice. It has 5,160 sensors attached to 86 strings, between 1,450 and 2,450 meters deep, occupying about 1 km³ of very clear ice. Construction was completed in December 2010.

Neutrinos pass through almost everything without leaving a trace. When a rare neutrino collides with an atom in the ice, it produces a faint blue flash that the sensors record. In 2017 IceCube detected a neutrino from an active galaxy, the blazar TXS 0506+056, the first time a high‑energy neutrino was linked to a known source in the sky.