The South Pole, located in Antarctica, experiences extreme seasonal variation. For half of the year, the sun never fully sets, providing continuous daylight. The other half brings a complete reversal, plunging the pole into continuous night, known as the polar night. This cycle of light and darkness is a direct consequence of Earth’s orientation in space as it travels around the sun.
Defining the Polar Night Phenomenon
The “24 hours of darkness” at the South Pole is formally called the polar night, a period when the sun remains below the horizon. At the exact pole, this period lasts for approximately six months. It is not an instantaneous switch to total blackness, but a gradual transition lasting weeks. The definition of darkness depends on the sun’s position below the horizon, which dictates the type of twilight experienced.
The polar night is categorized into different stages of twilight, even after the sun has set.
- Civil twilight occurs when the sun is less than six degrees below the horizon, providing enough scattered light for most outdoor activities.
- Nautical twilight takes place when the sun is between six and twelve degrees below the horizon, allowing the horizon to still be visible for navigation.
- Astronomical twilight, the darkest phase, ends when the sun is eighteen degrees below the horizon.
At this point, no refracted sunlight is visible, and true night begins.
The Role of Earth’s Axial Tilt
The mechanism behind the polar night is driven by Earth’s constant 23.5-degree axial tilt. The planet spins on an axis that is not perpendicular to its orbital path around the sun. As Earth revolves, this tilt causes one pole to lean toward the sun while the other leans away, alternating throughout the year.
When the Southern Hemisphere is tilted away from the sun, the South Pole remains on the planet’s dark side, even during daily rotation. The angle of the sun’s rays is too low to reach the pole, keeping it in perpetual shadow. This six-month period creates the long winter and the continuous darkness. The tilt’s fixed orientation in space ensures this predictable, alternating cycle of extremes at the poles.
Specific Timing of South Pole Darkness
The South Pole’s six-month period when the sun is continuously below the horizon begins around the autumnal equinox in the Southern Hemisphere (near March 20 or 21). On this date, the sun sets for the first and only time that year, initiating the polar night. The sun remains below the horizon until the vernal equinox, approximately six months later (around September 22 or 23).
While the period of polar night lasts for six months, the duration of absolute astronomical darkness is shorter. True night, where the sun is more than eighteen degrees below the horizon, typically lasts for about eleven weeks at the South Pole. This period of deepest darkness occurs around the winter solstice in June, lasting from approximately mid-May to early August. The weeks before and after this deepest phase are characterized by civil and nautical twilight, which still provides some light.
The Opposite Season: 24 Hours of Daylight
The counterpart to the polar night is the polar day, or “midnight sun,” a period of continuous daylight. This season begins when the sun rises around the vernal equinox (near September 22 or 23). The South Pole is tilted toward the sun for the next six months.
During this time, the sun is continuously above the horizon, circling the sky once every 24 hours without setting. This period of perpetual daylight lasts until the sun sets again around the autumnal equinox (March 20 or 21). The constant illumination is a result of the same 23.5-degree axial tilt, which directs the Southern Hemisphere toward the sun.