The seasonal shift in daylight prompts many to wonder about the precise moment the sun begins to stay out longer after the dark days of autumn. While Earth’s orbit and tilt set the overall pattern of decreasing and increasing daylight, the specific times of sunrise and sunset follow a more complex rhythm than simply clocking the shortest day. Understanding this change requires looking closely at the difference between the total duration of light and the timing of the evening fade, as these events do not all align on the same calendar date.
The Winter Solstice: The Shortest Day
The astronomical turning point for daylight occurs at the Winter Solstice, which falls annually around December 21st or 22nd in the Northern Hemisphere. This moment marks the precise time when one of Earth’s poles is tilted farthest away from the sun, resulting in the minimum total amount of daylight for the year. The Solstice is defined by the absolute shortest period between sunrise and sunset, making it the deepest point of winter in terms of solar illumination.
Immediately following the Solstice, the total duration of daylight begins its increase. This change is subtle at first, often adding just a few seconds of light per day, making it nearly impossible to notice without precise measurements. The Solstice is the true moment when the sun starts staying out longer, as the arc the sun traces across the sky begins its ascent northward. This annual event has been recognized for millennia as the symbolic rebirth of the sun and the start of the lengthening days.
The Earliest Sunset: Why Daylight Seems to Increase Earlier
Many people perceive that sunsets begin getting later several weeks before the Winter Solstice, giving the impression that days are already lengthening. For mid-latitudes in the Northern Hemisphere, the earliest sunset of the year actually happens in early December, often around the 7th to the 10th. Consequently, by the time the Solstice arrives, the sun has already been setting later for nearly two weeks.
This discrepancy arises because the timing of the earliest sunset is not the same as the day with the shortest total daylight. During early December, the sunset is getting later, but the sunrise is simultaneously getting later even faster. This rapid shift in the morning causes the total duration of light to continue shrinking until the Solstice, despite the afternoon feeling slightly extended. The latest sunrise of the year, conversely, does not occur until early January, sometimes around the 5th, well after the shortest day has passed.
The Cause of the Daylight Shift
The reason the earliest sunset and the shortest day do not align is due to a slight misalignment between our 24-hour clock and the actual solar day. A solar day is the time it takes for the sun to return to its highest point in the sky, known as solar noon. This true solar day is not exactly 24 hours long throughout the year, a phenomenon described by the Equation of Time.
This variation occurs because Earth’s orbit is an ellipse, not a perfect circle, and the planet’s axis is tilted. When Earth is closer to the sun in early January, it moves faster in its orbit, causing the true solar day to be slightly longer than 24 hours. Since the time between solar noons is greater than 24 hours in December, the moment the sun reaches its peak shifts later each day relative to our clocks. This daily delay pushes both sunrise and sunset times later, which is strong enough to cause the sunset time to start getting later before the Solstice, even while the overall duration of daylight is still decreasing.