As the year progresses, many people notice the noticeable changes in the amount of daylight each day. The mornings might feel darker for longer, and evenings arrive sooner, signaling the approach of winter. This cycle prompts a common question: when do the days begin to lengthen again? Understanding this involves Earth’s consistent movements in its orbit.
The Winter Solstice
The turning point when days begin to lengthen in the Northern Hemisphere is marked by the Winter Solstice. This astronomical event occurs around December 21st or 22nd, marking the shortest day and longest night of the year. While it marks the minimum amount of daylight, it is also the moment when the Northern Hemisphere starts to tilt back towards the Sun, leading to increasing daylight hours.
The earliest sunset occurs several weeks before the Winter Solstice. This phenomenon is due to a combination of Earth’s elliptical orbit and its axial tilt. Earth moves faster in its orbit during January, when it is closest to the Sun, compared to July when it is farthest. This varying orbital speed affects the apparent movement of the Sun across the sky, creating a slight misalignment between our standardized 24-hour clock and the actual solar day. As a result, the earliest sunset can happen in early December, while the latest sunrise is in early January, even though the shortest day remains the Winter Solstice.
The Earth’s Axial Tilt
The reason for the changing length of days throughout the year is the Earth’s axial tilt. Our planet’s rotational axis is tilted at approximately 23.5 degrees relative to its orbital plane. This tilt remains constant in direction as Earth revolves around the Sun, always pointing towards the same region of the sky.
This consistent tilt is the driver of the seasons and the varying daylight across the globe, rather than Earth’s changing distance from the Sun. As Earth orbits, different parts are angled either towards or away from the Sun, altering how sunlight strikes the surface. When a hemisphere tilts towards the Sun, sunlight hits it directly, concentrating solar energy and leading to longer daylight periods.
When a hemisphere tilts away from the Sun, sunlight strikes its surface at an oblique angle, spreading solar energy and results in less intense heating and shorter daylight periods. This tilt ensures solar energy reaching different latitudes changes throughout the year, directly influencing day and night length and seasons.
Varying Day Lengths Across the Year
After the Winter Solstice, the Northern Hemisphere experiences a gradual increase in daylight hours each day. This shift begins subtly, with only seconds of added light daily, but the rate accelerates closer to spring, becoming more pronounced by late winter. This progression signifies Earth’s continued journey, as its Northern Hemisphere gradually tilts more directly towards the Sun.
This lengthening of days continues until the Summer Solstice, which falls around June 20th or 21st in the Northern Hemisphere. Here, the Northern Hemisphere receives its maximum direct sunlight, marking the longest day, after which daylight hours progressively shorten. Within this annual cycle, two points are the equinoxes: the Spring (Vernal) Equinox around March 20th or 21st, and the Autumnal Equinox around September 22nd or 23rd.
During these equinoxes, the Earth’s axis is positioned such that neither the Northern nor Southern Hemisphere is tilted towards or away from the Sun, resulting in equal amounts of daylight and darkness across most latitudes. This balance marks the transitional periods between the extreme day lengths experienced during the solstices. This entire pattern of changing day lengths and seasons is mirrored in the Southern Hemisphere. For instance, when the Northern Hemisphere observes its longest day during the June Summer Solstice, the Southern Hemisphere simultaneously experiences its shortest day, the Winter Solstice, and conversely in December.