Sunset, when the sun’s upper edge dips below the horizon, marks the end of daylight. Sunset times shift throughout the year, with days shortening in autumn and lengthening again. This article explores the scientific reasons behind these changes, clarifying when the sun begins to set later after the darkest part of the year.
The Earth’s Tilt and Orbit
The Earth’s axial tilt, approximately 23.5 degrees relative to its orbital plane around the sun, primarily drives the seasonal variations in daylight. This tilt means that as Earth travels along its elliptical path, different hemispheres receive varying amounts of direct sunlight. When the Northern Hemisphere tilts towards the sun, it experiences summer with longer days, and when it tilts away, it experiences winter with shorter days.
The Earth’s orbit is not a perfect circle but an ellipse, which influences Earth’s speed around the sun. This elliptical orbit combined with the constant axial tilt causes the sun’s apparent position in the sky to change over the year, impacting sunrise and sunset times.
Understanding the Winter Solstice
The winter solstice marks when one of Earth’s poles reaches its maximum tilt away from the sun. In the Northern Hemisphere, this typically occurs around December 21st or 22nd, marking the shortest day of the year. This astronomical turning point signifies the moment when the sun reaches its southernmost point in the sky. After the winter solstice, daylight gradually increases each day. However, this does not mean sunset instantly begins to occur later.
Why Earliest Sunset Differs from Shortest Day
The earliest sunset of the year does not coincide with the winter solstice, which is the shortest day. This difference arises due to a phenomenon known as the “equation of time,” which accounts for the slight daily variations in the length of a solar day. A solar day is the time it takes for the sun to return to the same position in the sky, specifically for solar noon to occur.
Solar noon, the moment the sun reaches its highest point in the sky, shifts slightly throughout the year because of Earth’s elliptical orbit and its axial tilt. Around the winter solstice, the time of solar noon occurs progressively later each day. This daily delay in solar noon means that even as the total daylight period slowly starts to increase after the solstice, the sun is rising later and setting later.
However, just before the solstice, the effect of solar noon shifting later is more pronounced than the daily increase in daylight. This causes the earliest sunset to occur several weeks before the winter solstice, typically in early December in the Northern Hemisphere. Conversely, the latest sunrise occurs a few weeks after the winter solstice for similar reasons. Therefore, the sun generally starts setting noticeably later only after this earliest sunset date, not precisely on the shortest day of the year.
Factors Affecting Your Local Sunset Time
While the astronomical principles dictate general trends, several practical factors influence the exact sunset time experienced in a specific location.
Latitude plays a significant role, as locations closer to the poles experience more dramatic seasonal shifts in day length, with much shorter winter days and longer summer days.
Longitude within a time zone also affects local sunset times. As Earth rotates eastward, locations further west within the same time zone experience sunrises and sunsets later than those to their east.
Daylight Saving Time (DST) is another factor that alters observed sunset times. When DST is in effect, clocks are typically moved forward by one hour, making sunsets appear to occur an hour later by the clock. This adjustment is a human convention, not an astronomical change, but it significantly impacts the perceived timing of sunset for daily life.