Seasons are defined periods of the year governed by Earth’s constant axial tilt relative to its orbit. While the tilt causes the seasons, the variation in their length results from two astronomical factors. The first is the fixed inclination of the planet’s axis, which establishes the precise astronomical points marking a season’s beginning and end. The second factor is the slight elliptical shape of Earth’s orbit, which causes the planet’s speed to fluctuate as it travels around the Sun. The interplay between this fixed tilt and variable orbital speed determines why one season may last several days longer than another.
Earth’s Axial Tilt: The Primary Cause of Seasons
The fundamental reason seasons occur is the tilt, or obliquity, of the Earth’s rotational axis. This axis is inclined at approximately 23.4 degrees relative to the orbital plane. Because the axis maintains this orientation as Earth revolves, the Northern and Southern Hemispheres receive significantly varying intensities of sunlight throughout the year.
The hemisphere tilted toward the Sun experiences summer because sunlight strikes the surface at a more direct angle of incidence. This concentrated solar energy, combined with longer daylight hours, leads to consistently warmer temperatures. Conversely, the opposite hemisphere is tilted away, resulting in sunlight hitting the surface at a shallower angle, which causes winter.
This fixed 23.4-degree tilt establishes the astronomical timing of the seasons. It dictates the moments when one hemisphere reaches its maximum and minimum solar exposure. These moments are the precise markers that astronomers use to measure seasonal duration, but the tilt itself does not explain why the time spent between these markers is unequal.
Orbital Eccentricity and Velocity Variation
The second factor impacting seasonal length is the elliptical shape of Earth’s orbit, known as orbital eccentricity. If the orbit were a perfect circle, Earth would travel at a constant speed, and all four astronomical seasons would be of equal length. Instead, the distance between Earth and the Sun constantly changes throughout the year.
The point closest to the Sun is perihelion, occurring around early January. The farthest point is aphelion, occurring around early July. This measurable variation in distance governs the planet’s instantaneous speed, a principle described by Kepler’s Second Law of Planetary Motion.
To satisfy this conservation law, Earth must accelerate when nearer the Sun at perihelion and decelerate when farther away at aphelion. This means the planet travels fastest in January and slowest in July.
This periodic change in velocity directly causes unequal seasonal durations. When Earth moves faster, it traverses that orbital segment more quickly, effectively shortening the season. Conversely, moving slower lengthens the season, as the fixed astronomical points are traversed in varying amounts of time.
Defining Seasonal Boundaries: Solstices and Equinoxes
Astronomers define the start and end of the seasons using four specific points in Earth’s orbit: the solstices and the equinoxes. These moments are determined solely by the orientation of the Earth’s axis relative to the Sun, forming the boundaries for measuring seasonal length.
The solstices mark the moments of maximum axial tilt, when one hemisphere is tilted most directly toward or away from the Sun. The summer solstice signals the longest day of the year, while the winter solstice marks the shortest. These events define the beginning of summer and winter, respectively.
The equinoxes occur halfway between the solstices, when the Earth’s axis is neither tilted toward nor away from the Sun. At these two points—the vernal (spring) and autumnal (fall) equinoxes—daytime and nighttime are of nearly equal duration globally.
These four astronomical events provide the fixed, measurable milestones for the orbital journey. The time elapsed between them quantifies the duration of each season.
Observed Differences in Seasonal Duration
The combined effect of the fixed axial tilt and variable orbital speed creates quantifiable differences in seasonal length. This is clearly illustrated by comparing the time spent in each orbital quadrant in the Northern Hemisphere.
Northern Hemisphere summer occurs near aphelion, the farthest point from the Sun, where orbital velocity is slowest. Because Earth moves slowly here, it spends more time traveling between the June solstice and the September equinox. This results in a longer summer season of approximately 93 days.
Conversely, Northern Hemisphere winter occurs near perihelion, the closest point to the Sun, where orbital speed is highest. Earth speeds through this segment, causing the time between the December solstice and the March equinox to be shorter. The winter season, therefore, lasts only about 89 days, a difference of roughly four days compared to summer.
This unequal duration confirms that while the constant axial tilt sets the precise seasonal boundaries, the orbital eccentricity determines the actual number of days a season spans.