The cyclical change in daylight hours is a fundamental aspect of life on Earth. When people ask when the “days get longer,” they are referring to the period between sunrise and sunset, known as the length of daylight. This duration changes throughout the year as our planet moves through its orbit, creating a predictable pattern of increasing and decreasing light.
Defining the Shortest Day: The Winter Solstice
The days begin to get longer precisely at the moment of the Winter Solstice. In the Northern Hemisphere, this typically occurs on December 21st or 22nd. This astronomical event marks the shortest day and the longest night of the year, defined as the instant when the Sun reaches its lowest point in the sky.
From that moment forward, the amount of daylight begins to increase, even if the gain is only a few seconds at first. This change is an instantaneous reversal of the months-long process of shortening daylight hours. The rate of daylight gain is minimal near the solstice but accelerates as the Earth moves toward the spring equinox.
The Role of Earth’s Axial Tilt
The underlying mechanism for this yearly cycle is the Earth’s constant axial tilt of approximately 23.5 degrees. This tilt causes different hemispheres to receive varying amounts of direct solar radiation throughout the year. This axial tilt, not the Earth’s distance from the Sun, is the primary cause of the seasons and the fluctuation in day length.
During the Northern Hemisphere’s winter, the North Pole is tilted farthest away from the Sun, causing sunlight to strike the region at a more oblique angle for a shorter duration. Once the planet passes the solstice point, the Northern Hemisphere begins to tilt back toward the Sun. This gradual shift means the Sun’s path across the sky becomes slightly higher and longer each day, progressively extending the hours of daylight.
Why the Earliest Sunset Happens First
A common point of confusion is that the earliest sunset often occurs in early December, several weeks before the shortest day. This phenomenon is due to a slight mismatch between standard clock time and true solar time, known as the Equation of Time. While our clocks measure a mean solar day of exactly 24 hours, the actual time it takes the Sun to return to its highest point varies slightly.
This variation is caused by the Earth’s elliptical orbit and its axial tilt, which make the solar day slightly longer than 24 hours around the solstice. Consequently, the time of solar noon—the Sun’s highest point—shifts later each day. This later solar noon pushes both sunrise and sunset times later; the sunset time starts shifting later before the solstice, while the latest sunrise occurs after the solstice, typically in early January.
The Opposite Cycle: Southern Hemisphere Timing
The cycle of day length variation is reversed for the Southern Hemisphere. When the Northern Hemisphere experiences the Winter Solstice and the shortest day in December, the Southern Hemisphere simultaneously experiences its Summer Solstice and the longest day. Their shortest day occurs during their winter, which is the June Solstice, typically falling on June 20th or 21st. Following that June date, the days in the Southern Hemisphere begin increasing their daylight hours.