The specific time of sunset on December 21st is not a universal constant but changes drastically depending on your precise latitude, longitude, and local time zone. December 21st marks the Winter Solstice in the Northern Hemisphere, which is the day with the minimum amount of daylight for the year. This astronomical event causes the sun to follow its lowest and shortest path across the sky. Although the solstice brings the earliest darkness of the season, the exact timing of sunset is highly localized.
Identifying Your Exact Sunset Time
To determine the exact sunset time on December 21st, you must use tools that calculate astronomical events for a specific point on the globe. Reliable resources are provided by governmental agencies or established astronomical institutions, such as the US Naval Observatory (USNO) and the National Oceanic and Atmospheric Administration (NOAA). These calculators require you to input your exact location, either by city and state or by latitude and longitude coordinates. The calculation accounts for your time zone and the Earth’s atmosphere, which slightly bends sunlight, making the sun visible for a few moments longer.
The Winter Solstice and the Shortest Day
The importance of December 21st stems from the Winter Solstice, the astronomical moment when the Earth’s Northern Hemisphere is tilted farthest away from the sun. The Earth’s axis is consistently tilted at approximately 23.5 degrees. The solstice occurs when this tilt results in the minimum exposure to the sun’s direct rays, causing the sun to trace its lowest arc across the sky for the entire year.
This low arc results in the shortest period of daylight and the longest night of the year. After the solstice, the Northern Hemisphere begins its slow tilt back toward the sun. Consequently, the duration of daylight hours begins to increase daily.
The Earliest Sunset Paradox
A common misconception is that the earliest sunset of the year occurs exactly on the Winter Solstice, the day with the least amount of daylight. The earliest sunset actually takes place a few weeks before December 21st, typically occurring in early to mid-December for most mid-latitude locations. This phenomenon is due to a slight misalignment between our civil clock time and the true solar time, a concept known as the Equation of Time.
The Equation of Time describes the difference between the time indicated by a sundial (apparent solar time) and the time kept by a clock (mean solar time). This difference arises because the Earth’s orbit around the sun is an ellipse, not a perfect circle, and the Earth’s axial tilt complicates the sun’s apparent movement. As a result, the time interval between one solar noon and the next is not exactly 24 hours.
Around the winter solstice, the true solar day is slightly longer than 24 hours, meaning the moment of solar noon (when the sun is highest in the sky) occurs later each day. This shifting of solar noon pushes both the sunrise and the sunset times later. Even though the day length is still decreasing slightly in early December, the effect of the later solar noon causes the sunset time to shift later.
The latest sunrise is similarly affected by the Equation of Time, causing it to occur a few weeks after the solstice, typically in early January. Although the actual shortest day remains the solstice, the effect of the sun’s changing position relative to our clocks separates the dates of the earliest sunset, the shortest day, and the latest sunrise. The difference between the date of the earliest sunset and the solstice varies by latitude, with locations closer to the equator experiencing a greater separation.