Daylight hours represent the period of time each day when a specific location on Earth experiences natural illumination from the Sun. This duration is calculated as the interval between the official time of sunrise and the official time of sunset. Unlike the fixed length of the 24-hour day, the total number of daylight hours is not constant and changes predictably throughout the year. This variation is a fundamental characteristic of life on Earth, influencing everything from biological cycles to human activity.
Defining the Boundaries of Daylight
The official definition of sunrise and sunset is precisely determined for scientific and civil use. Sunrise occurs at the exact moment the upper edge of the Sun’s disk appears to cross the horizon in the morning, while sunset is when the upper edge finally disappears below the horizon in the evening. This calculation assumes a level, unobstructed horizon and accounts for the bending of light, or atmospheric refraction, which makes the Sun visible slightly before it is geometrically above the horizon.
Functional daylight extends beyond these direct sunlight hours due to twilight, the period before sunrise and after sunset when the atmosphere scatters sunlight. The brightest phase is civil twilight, which lasts until the Sun is 6 degrees below the horizon. During this time, there is typically enough natural light for most ordinary outdoor activities without artificial illumination.
Following civil twilight is nautical twilight, ending when the Sun reaches 12 degrees below the horizon. This phase is historically defined as the point at which the horizon becomes difficult to distinguish at sea. The darkest phase is astronomical twilight, ending when the Sun is 18 degrees below the horizon, after which the sky is dark enough for astronomers to observe the faintest celestial objects.
The Astronomical Cause of Changing Length
The primary reason daylight hours change throughout the year is the Earth’s axial tilt, combined with its orbit around the Sun. The planet’s axis of rotation is tilted by approximately 23.5 degrees relative to its orbital plane. This tilt remains pointed in the same direction in space as the Earth revolves in its year-long path around the Sun.
The tilt means that for half the year, one hemisphere is angled toward the Sun, while the other is angled away. When a hemisphere is tilted toward the Sun, it receives more direct sunlight and experiences a longer path of the Sun above the horizon, resulting in longer daylight hours and warmer temperatures. Conversely, when tilted away, the Sun traces a shorter, lower arc across the sky, leading to shorter days and less direct solar energy.
This mechanism ensures that the two hemispheres experience opposite seasons and variations in daylight length. This constant angle of tilt, known as obliquity, is the primary factor driving the seasonal changes in daylight and temperature across the globe.
How Location and Time of Year Affect Daylight
The magnitude of the change in daylight hours is directly linked to a location’s latitude, or its distance north or south of the equator. At the equator, the variation in daylight is minimal, with the day length remaining close to 12 hours year-round. The Sun’s path here is nearly perpendicular to the horizon, which keeps the day and night portions of the rotation stable.
Moving away from the equator, the seasonal difference in day length becomes increasingly pronounced. In the middle latitudes, the difference between the longest and shortest days is noticeable, with summer days stretching significantly longer than winter days. For example, a location at 60 degrees north latitude may experience over 18 hours of daylight in summer but fewer than six hours in winter.
Solstices and Equinoxes
The extremes of this variation occur at the solstices and equinoxes. The summer solstice marks the longest period of daylight and the winter solstice the shortest, as these are the moments when one hemisphere is maximally tilted toward or away from the Sun.
The equinoxes, occurring in spring and autumn, are the two times each year when the Earth’s axis is not tilted toward or away from the Sun. This results in nearly equal periods of day and night—approximately 12 hours each—across the entire globe. Near the poles, this effect is maximized, leading to polar days, where the Sun remains above the horizon for months, and polar nights, where it remains below the horizon for extended periods.