The question of whether days are getting longer requires clarifying what “day” means. For most people, a day refers to the period of daylight, which lengthens and shortens throughout the year due to the Earth’s orbital mechanics. From a scientific perspective, however, a “day” is the precise duration of one complete 360-degree rotation of the planet, and this period is also slowly increasing. The answer is yes in both contexts, but the timescales are vastly different: one is an annual cycle and the other spans eons.
The Annual Cycle of Daylight Hours
The most noticeable change in the length of the day is the annual cycle of daylight hours, which governs the seasons. In the Northern Hemisphere, increasing daylight begins immediately following the Winter Solstice, typically around December 21st. After this point, the Sun appears to rise earlier and set later each day, incrementally increasing the time between sunrise and sunset.
This lengthening continues until the Summer Solstice, usually around June 21st, which marks the peak of daylight hours. At this solstice, the hemisphere receives its maximum solar energy and longest day, after which the process reverses. The daylight hours then begin to shorten, leading toward the next Winter Solstice.
The two Equinoxes, occurring in spring and autumn, represent the moments when the day and night are of nearly equal length. For the Southern Hemisphere, this cycle is precisely inverted, experiencing its longest day during the Northern Hemisphere’s winter. This seasonal variation is purely a change in the distribution of sunlight, not a change in the 24-hour period it takes for the Earth to complete a rotation.
The Role of Axial Tilt in Seasonal Shifts
The underlying cause of the annual cycle is the Earth’s axial tilt, a constant slant of approximately 23.5 degrees relative to the plane of its orbit. This tilt means that as the Earth revolves, different hemispheres are exposed to more direct sunlight at different times of the year. The planet does not orbit perfectly upright, causing the Sun’s most intense rays to shift their focus between the Tropic of Cancer and the Tropic of Capricorn.
When the Northern Hemisphere is tilted toward the Sun, it receives a higher concentration of solar radiation, resulting in summer and longer daylight hours. The Sun’s path across the sky is higher and longer during this time. Conversely, when the Northern Hemisphere is tilted away, the angle of the Sun’s rays is lower and more oblique, spreading the same amount of energy over a larger area, which causes winter and shorter days.
The angle of the Sun’s path determines how long the Sun remains above the horizon each day, directly influencing the duration of daylight. This consistent axial tilt dictates the predictable pattern of the solstices and equinoxes. Without this constant tilt, every day would resemble an equinox, with little to no seasonal variation in daylight length.
The Astronomical Slowdown of Earth’s Rotation
Beyond the seasonal changes, the actual duration of the 24-hour day, defined by one complete rotation of the Earth, is increasing on a geological timescale. Measurements based on ancient astronomical records, such as eclipse observations, show that the Earth’s rotation has slowed by an average of 1.8 to 2.3 milliseconds per century since the 8th century BCE. Modern atomic clocks confirm this long-term trend, showing that a modern day is slightly longer than a day a century ago.
This phenomenon creates a discrepancy between two measures of time: Universal Time (UT1) and Coordinated Universal Time (UTC). UT1 is the astronomical time scale based on the Earth’s actual rotation, while UTC is the stable civil time scale maintained by atomic clocks. Because the Earth’s rotation is slowing, the clock-based UTC begins to run ahead of the rotation-based UT1.
To reconcile these two time scales, an occasional adjustment known as a “leap second” is necessary. The International Earth Rotation and Reference Systems Service (IERS) is responsible for inserting an extra second into UTC whenever the difference between it and UT1 approaches 0.9 seconds. All leap seconds introduced since 1972 have been positive, confirming the long-term trend of a slowing Earth rotation.
Tidal Friction and Lunar Influence
The force driving this astronomical lengthening of the day is the gravitational interaction between the Earth and the Moon, a process known as tidal friction. The Moon’s gravitational pull raises bulges of water in the Earth’s oceans, creating high tides on both the near and far sides of the planet. Because the Earth rotates much faster than the Moon orbits, the planet’s rotation drags these tidal bulges slightly ahead of the Moon’s direct line of sight.
The gravitational attraction of the Moon pulls back on these offset bulges, acting as a continuous brake on the Earth’s spin. This friction dissipates the Earth’s rotational energy, which slows the rotation and lengthens the day. The angular momentum lost by the Earth is transferred to the Moon.
This transfer of momentum has a reciprocal effect on the Moon’s orbit, causing it to spiral slowly away from the Earth. The Moon is currently receding at a rate of approximately 3 to 4 centimeters (1.2 to 1.5 inches) per year. This interconnected system ensures that the slowing of the Earth’s rotation and the increasing distance of the Moon are two sides of the same tidal-driven process.