Latitude, which measures a city’s distance north or south of the equator, directly dictates the annual range of its daylight hours. Cities nearer to the poles experience a significantly greater difference between the longest and shortest days of the year. Conversely, locations closer to the equator maintain a consistent pattern of daylight. This variation results from the Earth’s spherical geometry and its position relative to the sun.
The Fundamental Cause: Earth’s Axial Tilt
The explanation for the changing duration of daylight hours is the Earth’s axial tilt. The planet is tilted on its axis by approximately 23.5 degrees relative to the plane of its orbit around the sun. This permanent tilt means that as the Earth revolves, different hemispheres are exposed to varying amounts of direct solar radiation.
When the Northern Hemisphere is angled toward the sun, it receives sunlight at a more direct angle and for a longer period. The Southern Hemisphere is simultaneously angled away, causing the sun’s rays to strike it more obliquely and for fewer hours. This unequal distribution drives the seasonal differences in day length. If the Earth had no axial tilt, every location would experience a nearly constant 12 hours of daylight and 12 hours of night.
How Seasonal Cycles Change Daylight Hours
The Earth’s orbit features two moments representing the maximum and minimum daylight variation: the solstices. The Summer Solstice marks when one hemisphere is tilted most directly toward the sun, resulting in maximum daylight hours. Six months later, the Winter Solstice occurs when that hemisphere is tilted farthest away, producing minimum daylight hours.
In between these extremes are the two equinoxes, which occur in spring and autumn. During an equinox, the Earth’s axis is perpendicular to the incoming solar rays. This alignment causes the line dividing day and night to pass directly through both the North and South Poles. As a result, nearly every location on Earth experiences an equal duration of daylight and darkness, approximately 12 hours each.
Daylight Extremes: Equator to the Poles
The most consistent daylight pattern is found at the Equator (0 degrees latitude), where the sun’s rays strike nearly perpendicularly year-round. Equatorial regions experience minimal variation, maintaining approximately 12 hours of daylight every day. The slight variation that occurs is often due to atmospheric refraction, which extends the visible period of sunlight slightly beyond the geometric 12 hours.
Moving away from the equator and into the mid-latitudes, the seasonal variation becomes apparent. A city at 30 degrees North latitude, for instance, might see a difference of about four hours between its longest summer day and its shortest winter day. Higher mid-latitude cities, such as those near 60 degrees North, experience a much more dramatic swing, with summer days lasting over 18 hours and winter days dropping below six hours.
The most extreme daylight variations are found at the high latitudes, within the Arctic and Antarctic Circles (approximately 66.5 degrees latitude). In these polar regions, the axial tilt results in the Midnight Sun and the Polar Night. During summer, the pole tilted toward the sun experiences the Midnight Sun, remaining visible above the horizon for 24 continuous hours. Conversely, during winter, the pole tilted away experiences the Polar Night, where the sun does not rise for 24 continuous hours. The duration of this continuous light or darkness increases closer to the North or South Pole, where the sun remains above or below the horizon for nearly six months.