The timing of sunsets changes throughout the year, a phenomenon influenced by intricate astronomical factors. While the general pattern of longer summer days and shorter winter days is widely known, understanding these dynamics reveals the precise mechanisms governing our daily light and darkness.
The Yearly Pattern of Later Sunsets
For those in the Northern Hemisphere, sunsets begin to occur later in the day a few weeks after the winter solstice, which typically falls around December 21. This trend of increasingly later sunsets continues into the new year, extending through spring and well into summer. Sunsets reach their latest point in the year after the summer solstice, which occurs around June 21. Similarly, in the Southern Hemisphere, the pattern mirrors this, with later sunsets beginning after their winter solstice in June.
The earliest sunset of the year does not coincide with the shortest day, the winter solstice, but actually happens several weeks before it. For example, in mid-latitudes of the Northern Hemisphere, the earliest sunset can occur in early December, while the latest sunset happens in late June or early July, after the summer solstice. This observed pattern highlights a subtle but important distinction between the shortest day of the year and the specific timing of the earliest or latest sunsets.
The Astronomical Reasons for Changing Sunset Times
The changing sunset times are dictated by Earth’s axial tilt, its elliptical orbit around the sun, and the “Equation of Time”. Earth’s axis is tilted approximately 23.5 degrees relative to its orbital plane. This tilt is responsible for Earth’s seasons and the varying duration of daylight throughout the year, as different hemispheres are tilted toward or away from the sun.
Earth’s orbit around the sun is not a perfect circle but an ellipse. This elliptical path means Earth’s speed varies as it travels, moving faster when it is closer to the sun (perihelion, in early January) and slower when it is farther away (aphelion, in early July). This varying orbital speed affects the actual length of a “solar day,” which is the time it takes for the sun to return to the same position in the sky. Our clocks, however, measure an average 24-hour day.
The “Equation of Time” quantifies the difference between time measured by a sundial (apparent solar time) and the steady, average time kept by our clocks (mean solar time). This discrepancy arises from the combined effects of Earth’s axial tilt and its elliptical orbit. The tilt causes the sun’s apparent east-west movement across the sky to vary, while the elliptical orbit means Earth’s speed around the sun is not constant. These factors cause solar noon, the moment the sun reaches its highest point, to shift slightly from day to day relative to 12:00 on our clocks. The Equation of Time can result in a difference of up to about 16 minutes between sundial time and clock time over the year.
How Location Influences Sunset Times
Geographical location significantly impacts both the timing and the magnitude of seasonal sunset changes. Latitude, or how far north or south a location is from the equator, plays a substantial role. Closer to the poles, the seasonal variation in sunset times and day length becomes much more pronounced, leading to very long summer days and very short winter days. Conversely, locations near the equator experience minimal variation in daylight hours and sunset times throughout the year, with roughly 12 hours of daylight year-round.
Longitude and the design of time zones also influence the clock time of sunset, separate from the astronomical timing. Time zones are human-defined regions, often spanning many degrees of longitude, where a uniform clock time is observed. A city situated on the western edge of a time zone will experience later clock sunsets compared to a city on the eastern edge of the same time zone, even if they are at similar latitudes. This is because the sun’s actual position in the sky is determined by its geographic longitude, not the arbitrary center of a time zone.
Sunset vs. Day Length and Sunrise
The variation in the length of the solar day, caused by Earth’s elliptical orbit and axial tilt, means that solar noon shifts slightly later each day around the solstices. This daily shift in solar noon has a noticeable effect on the clock times of both sunrise and sunset. Consequently, the earliest sunset in the Northern Hemisphere typically occurs in early December, several weeks before the winter solstice. Similarly, the latest sunrise often occurs in early January, after the winter solstice. A parallel phenomenon occurs around the summer solstice, where the earliest sunrise happens before the solstice and the latest sunset occurs after it.