The time the sun disappears below the horizon in the summer varies significantly depending on location and the specific date. Astronomically, sunset is defined as the moment the upper edge of the Sun disappears below the horizon line. This event is governed by Earth’s relationship with the Sun, but its timing is influenced by geography and human convention. Understanding when the sun sets requires examining the astronomical causes of season, geographic factors, and the lingering effects of twilight.
The Role of Earth’s Axial Tilt and the Solstice
The primary reason summer sunsets occur late is the 23.5-degree tilt of Earth’s rotational axis relative to its orbital plane. This tilt causes one hemisphere to be angled toward the Sun for half the year, receiving sunlight more directly and for a longer duration each day. This astronomical geometry causes the seasons and the significant variation in daylight hours.
The Summer Solstice, around June 21st in the Northern Hemisphere, marks the point when the axial tilt is at its maximum toward the Sun. On this day, the hemisphere experiences its longest period of daylight. Consequently, the latest sunset times cluster around this date. The absolute latest sunset may occur a few days later due to the Equation of Time, a slight mismatch between solar and clock time.
After the Summer Solstice, the Earth continues its orbit, and the hemisphere’s tilt toward the sun gradually decreases. This change causes the length of the day to shorten slightly each day, meaning the sunset time begins to creep earlier. Even weeks after the solstice, the sun sets late because the tilt remains significant, keeping the daylight period much longer than the hours of darkness.
How Latitude and Time Zones Determine Local Sunset
While the axial tilt explains the seasonality of late sunsets, latitude determines the extent of that late timing. Locations closer to the poles (higher latitudes) experience a far greater seasonal change in daylight hours than those near the equator. For instance, equatorial cities have sunset times that barely change all year, while cities at high latitudes see the sun set extremely late in summer.
This variation occurs because at higher latitudes, the sun’s path across the sky is much shallower relative to the horizon. This shallow angle allows the sun to spend many more hours above the horizon during the summer. The most dramatic example is the “Midnight Sun,” where locations north of the Arctic Circle experience 24 hours of continuous daylight during peak summer.
Beyond the geographical factor of latitude, the clock time of sunset is heavily influenced by human conventions like time zones and Daylight Saving Time (DST). International time zones are broad, often spanning hundreds of miles of longitude. A location’s exact position within its zone affects the sunset time. Cities on the western edge of a time zone will naturally have a later sunset than those on the eastern edge, even at the same latitude, because the sun reaches them later.
Daylight Saving Time (DST) shifts the observed sunset time one hour later on the clock during summer in many regions. By advancing the clock, DST artificially pushes the sunset time later, making the evening seem much longer. This convention is a political decision, not an astronomical one, but it is a major factor in the late summer sunset times people notice.
The Difference Between Sunset and Full Darkness
The official sunset time is only the beginning of the evening, not the moment full darkness arrives. The period between the sun’s disappearance and true night is known as twilight, a duration of lingering light caused by the atmosphere scattering sunlight from below the horizon. Astronomers divide this period into three distinct phases based on the sun’s position below the horizon.
Phases of Twilight
The first phase is Civil Twilight, which lasts while the sun is between the horizon and six degrees below it. During this time, there is enough light for most outdoor activities, and the brightest stars become visible.
Following this is Nautical Twilight, which continues until the sun is twelve degrees below the horizon. This phase was historically relevant for navigation because the horizon is no longer clearly discernible at sea.
The final stage is Astronomical Twilight, which ends when the sun is eighteen degrees below the horizon. Once this threshold is crossed, the sky is considered dark enough for astronomical observations, and true night has begun. This entire twilight period is significantly extended in the summer and at higher latitudes because the sun sets at a very shallow angle, taking much longer to reach the eighteen-degree mark than near the equator.