The annual change in the duration of sunlight varies dramatically across the planet, primarily depending on a location’s latitude. The variation is measured by comparing the maximum daylight hours experienced during the summer solstice with the minimum daylight hours during the winter solstice. The difference between these two extremes determines where the amount of sunlight changes the most throughout the year.
The Reason for Changing Daylight Hours
The fundamental mechanism driving the annual change in daylight hours is the Earth’s axial tilt, known as obliquity. The Earth’s axis of rotation is tilted approximately $23.4$ degrees relative to the plane of its orbit around the Sun. This constant lean means that as the planet completes its yearly journey, different hemispheres are alternately angled toward or away from the Sun.
When the Northern Hemisphere is tilted toward the Sun in June, it receives sunlight over a broader area and for a longer period each day, resulting in summer and longer days. Conversely, during the December solstice, the Northern Hemisphere tilts away, leading to shorter days and winter. Without this tilt, every location on Earth would experience a nearly constant 12 hours of daylight and 12 hours of darkness every single day.
The Equatorial Zone of Minimal Change
The area where the annual variation in daylight duration is the least pronounced is the equatorial zone, specifically at $0$ degrees latitude. Because the Equator is positioned midway between the poles, it does not experience the dramatic tilt toward or away from the Sun that higher latitudes do. The Sun’s apparent path is nearly perpendicular to the horizon in this region year-round.
As a result, the Equator maintains a day length that is remarkably consistent, staying close to $12$ hours every day of the year. The duration of sunlight is actually slightly longer than $12$ hours—around $12$ hours and $7$ minutes—due to the effect of atmospheric refraction. The minimal annual difference of only a few minutes between the longest and shortest days defines the location of minimum variation on the planet.
The Polar Zones of Maximum Variation
The maximum annual swing in daylight hours occurs in the high-latitude regions, specifically within the Arctic and Antarctic Circles, which sit at approximately $66.5$ degrees North and South latitude. These regions experience the Earth’s axial tilt most acutely, leading to the phenomena known as “polar day” and “polar night”.
The Arctic and Antarctic Circles mark the boundary where the Sun remains continuously above the horizon for at least one $24$-hour period during the summer solstice. This period of continuous daylight, called the midnight sun or polar day, increases in duration as one travels closer to the geographic poles. At $78$ degrees North, such as in Svalbard, Norway, the Sun remains above the horizon for approximately four months.
Conversely, during the winter, the same high latitudes experience polar night, where the Sun remains below the horizon for more than $24$ consecutive hours. At the geographic poles, the annual variation reaches its theoretical maximum, with the Sun rising and setting only once per year. The North Pole, for instance, experiences approximately $186$ continuous days of sunlight followed by $179$ days of darkness.