The period of “daylight” is defined as the time between sunrise and sunset, and its duration changes noticeably over the course of a year. This annual shift causes the familiar sensation of days growing shorter in the autumn and longer in the spring.
The Short Answer: Why the Rate is Not Constant
The number of minutes of daylight gained or lost each day is not a fixed amount, but rather fluctuates throughout the year in a predictable pattern. The rate of change is slowest around the summer and winter solstices, which represent the longest and shortest days, respectively. For a few days surrounding a solstice, the change in daylight duration can be subtle, sometimes amounting to only a minute or two per day at mid-latitudes.
The speed of change accelerates significantly as the Earth moves toward the equinoxes, which occur in spring and autumn. Around the equinox, the daily change in daylight is at its most rapid, with many mid-latitude locations gaining or losing three to four minutes of daylight in a single 24-hour period. The curve representing the change in day length over the year is not a straight line, but a smooth, sinusoidal curve where the slope, or rate of change, is steepest at the equinoxes.
The Astronomy Behind Changing Day Length
The fundamental driver of changing day length is the Earth’s axial tilt combined with its revolution around the sun. The planet spins on an axis that is tilted by approximately 23.5 degrees relative to the plane of its orbit, and this tilt remains pointed in the same direction in space as the Earth travels. As the Earth orbits, this constant tilt means that the Northern and Southern Hemispheres alternately receive varying amounts of direct solar radiation over the year. When one hemisphere is tilted toward the sun, it experiences summer, characterized by longer daylight hours.
The solstices mark the two points in the orbit when the tilt is at its maximum inclination toward or away from the sun, resulting in the longest and shortest days of the year. Conversely, the equinoxes occur when the Earth’s axis is not tilted toward or away from the sun, causing both hemispheres to receive nearly equal amounts of sunlight and resulting in a day and night that are almost exactly 12 hours long globally. This constant tilt, rather than the Earth’s slightly elliptical orbit, is the primary mechanism that produces the seasons and the cycle of daylight change. Without this 23.5-degree inclination, every location on Earth would experience a constant 12 hours of daylight and 12 hours of darkness every single day.
The Role of Latitude and Location
A person’s geographical latitude plays a significant role in determining the magnitude of the daily change in daylight hours. Locations situated near the equator experience the least amount of variation throughout the year. For instance, a place like Quito, Ecuador, which is nearly on the equator, maintains a day length of approximately 12 hours year-round, with only a few minutes of total change between the solstices. Moving away from the equator toward the poles, the variation in daylight hours becomes dramatically greater.
In mid-latitude cities, such as those in the continental United States or Europe, the difference between the longest and shortest day can span six to eight hours. The most extreme changes occur at very high latitudes, close to the Arctic and Antarctic Circles, where the daily change can be immense. In these polar regions, the cycle culminates in the phenomena of 24 hours of continuous daylight during the summer and 24 hours of continuous darkness during the winter.