Daylight refers to the period each day between sunrise and sunset when the sun is visible above the horizon. The amount of daylight changes throughout the year, with longer days in summer and shorter ones in winter. These shifts are governed by specific astronomical principles, influencing the daily gain or loss of sunlight across different times and locations.
Earth’s Tilt and Orbit
The variation in daylight hours stems from the Earth’s orientation as it travels around the Sun. Earth’s axis, an imaginary line running through the North and South Poles, is not perpendicular to its orbital path. Instead, it is tilted at an angle of approximately 23.5 degrees relative to its orbital plane. This axial tilt remains constant as the Earth orbits the Sun.
As the Earth orbits the Sun, different parts of our planet are angled towards or away from the Sun. When the Northern Hemisphere is tilted towards the Sun, it receives more direct sunlight, leading to longer days and summer. Conversely, when the Northern Hemisphere is tilted away, it receives less direct sunlight, resulting in shorter days and winter. This same principle applies to the Southern Hemisphere, but with opposite seasons.
The Daily Change in Daylight
The amount of daylight gained or lost each day varies significantly throughout the year. This rate of change is slowest around the solstices, which occur in summer and winter. At these times, the Sun’s apparent position in the sky leads to minimal changes in the length of day. For instance, near the summer solstice, the day length changes by only a few seconds per day.
In contrast, the rate of change in daylight is fastest around the equinoxes, which happen in spring and autumn. During these periods, the Sun’s apparent path across the sky is steepest relative to the horizon. This steep angle causes sunrise and sunset times to shift more rapidly each day, resulting in a greater gain or loss of daylight. At mid-latitudes, the daily change can be several minutes per day, sometimes reaching up to three or four minutes around the equinoxes.
How Latitude and Season Affect Daylight
Latitude significantly influences daily daylight changes. Regions closer to the Earth’s poles experience more dramatic fluctuations in daylight hours throughout the year. For example, areas within the Arctic and Antarctic Circles can experience periods of 24 hours of daylight during their respective summers and 24 hours of darkness during their winters. This extreme variation contrasts with locations near the equator.
Near the equator, the length of day and night remains relatively consistent throughout the year, with 12 hours of daylight and 12 hours of darkness every day. The Sun’s path is always high overhead, leading to minimal seasonal changes in day length.
The winter solstice, around December 21st in the Northern Hemisphere, marks the shortest day of the year and the beginning of daily daylight gains. The spring equinox, around March 20th, signifies equal day and night hours and represents the period of the fastest daily daylight gain. Similarly, the summer solstice, around June 21st, is the longest day, after which daily daylight hours begin to decrease. Finally, the autumnal equinox, around September 22nd, brings equal day and night, marking the period of the fastest daily daylight loss.