Daylight hours refer to the duration of natural light experienced at a specific location, spanning the time between sunrise and sunset. This period of illumination is influenced by Earth’s interaction with the Sun.
The Earth’s Tilt and Orbit
The primary reason for the variation in daylight hours across the globe is Earth’s axial tilt combined with its revolution around the Sun. Earth spins on an imaginary axis that is tilted approximately 23.5 degrees relative to its orbital plane. This tilt remains constant in its orientation as the planet journeys through space.
As Earth orbits the Sun, different parts of the planet are angled more directly towards or away from the Sun’s rays. When a hemisphere is tilted towards the Sun, it receives more direct sunlight. Conversely, when it tilts away, sunlight strikes it at a lower angle and is spread over a larger area, leading to less intense heating.
This constant tilt, coupled with Earth’s annual orbit, means that the distribution of solar energy across the planet changes throughout the year. The distance between Earth and the Sun, which changes due to Earth’s elliptical orbit, has a minimal effect on the length of daylight compared to the axial tilt.
How Location and Time of Year Change Daylight
Geographic latitude significantly influences how daylight hours change throughout the year. Areas closer to the equator experience minimal variation, with daylight hours remaining close to 12 hours daily year-round. This occurs because the Sun’s path appears more directly overhead for these regions.
Moving away from the equator towards the poles, the seasonal variations in daylight become more pronounced. In mid-latitude regions, summer days are considerably longer, while winter days are much shorter. For instance, locations in the mid-latitudes can see daylight ranging from about 15 hours in summer to around 9 hours in winter.
The solstices and equinoxes mark specific points in Earth’s orbit that define these changes. The summer solstice, occurring around June 21st in the Northern Hemisphere, signifies the longest day of the year when that hemisphere is tilted most directly towards the Sun. Conversely, the winter solstice, around December 21st, marks the shortest day when the Northern Hemisphere is tilted farthest away.
During the equinoxes, occurring around March 20th (spring) and September 22nd (autumn), neither hemisphere is tilted towards or away from the Sun. On these dates, day and night are approximately equal in length across most of the globe, with nearly 12 hours of daylight everywhere. Near the poles, these variations become extreme, with continuous daylight during their respective summers and prolonged periods of darkness during winter.