The apparent yearly shift in the Sun’s position in the sky is caused by a single, unchanging feature of our planet. The Sun’s altitude—its angle above the horizon—changes throughout the year as a result of Earth’s annual orbit around the Sun (revolution). Although the planet travels a fixed path, the intensity of sunlight received at any given location constantly varies. This cyclical change in solar angle is fundamentally driven by the consistent, fixed tilt of the Earth’s rotational axis.
The Constant Tilt of Earth’s Axis
The Earth does not spin perfectly upright relative to its orbital plane, the flat path it traces around the Sun. Instead, its axis is consistently tilted at approximately 23.5 degrees from the perpendicular to this plane, known as the obliquity of the ecliptic. This tilt is a permanent feature of our planet’s orientation in space.
The axis maintains a fixed orientation in space throughout the year, a concept called axial parallelism. As the Earth revolves, its North Pole always points toward the same distant point in the sky, near the North Star (Polaris). The axis never changes its direction as the planet moves around the Sun.
Because the axis is both tilted and maintains its orientation, different hemispheres face the Sun at varying angles over the course of the year. The 23.5-degree tilt itself does not change. Instead, the Earth’s orbital position determines which hemisphere is leaning toward the Sun, which drives the observed angle change.
How the Tilt Changes Solar Angle
The axial tilt changes the angle at which sunlight strikes the Earth’s surface, dictating the concentration of solar energy (insolation). When a hemisphere tilts toward the Sun, solar rays strike the planet at a high, direct angle. Direct light concentrates energy over a smaller surface area, leading to greater heat absorption and warmer temperatures.
Conversely, when a hemisphere tilts away from the Sun, incoming solar rays hit the surface at a shallower, more oblique angle. This oblique light spreads the same solar energy over a larger area, reducing intensity and leading to cooler temperatures. This difference in the angle of incoming solar radiation is the primary factor determining seasonal temperature changes.
The angle also dictates the length of daylight hours experienced. When a hemisphere tilts toward the Sun, that region remains in sunlight for a longer portion of the daily rotation. This combination of concentrated sunlight and longer daylight hours maximizes the solar energy received, an effect reversed when the hemisphere is tilted away.
Debunking the Distance Myth
A common misconception is that the changing distance between the Earth and the Sun causes the change in solar angle and the seasons. Earth’s orbit is an ellipse, meaning its distance from the Sun varies. The closest point, perihelion, occurs in early January, and the farthest point, aphelion, occurs in early July.
This distance variation is minimal, amounting to only about a three percent difference between the closest and farthest points. The Northern Hemisphere experiences winter when Earth is closest to the Sun at perihelion, demonstrating that distance is not the main factor. The slight increase in solar energy received at perihelion is completely overwhelmed by the effect of the axial tilt.
While the distance change slightly affects the total solar radiation reaching the planet, this effect is negligible compared to the energy concentration caused by the axial tilt. The tilt determines whether the Sun is high or low in the sky, which is the mechanism behind the changing solar angle and seasonal differences.
Markers in the Yearly Cycle: Solstices and Equinoxes
The extremes of the changing solar angle are marked by the solstices, which occur twice a year. The summer solstice (around June 21st) marks when the hemisphere is tilted most directly toward the Sun, resulting in the highest solar angle and the longest daylight. Six months later, the winter solstice (around December 21st) occurs when that hemisphere is tilted maximally away, resulting in the lowest solar angle and shortest day.
Midway between these extremes are the equinoxes, occurring twice a year around March 20th and September 22nd. At these orbital points, the Earth’s axis is tilted neither toward nor away from the Sun. This alignment positions the Sun directly over the Earth’s equator.
During an equinox, both the Northern and Southern Hemispheres receive nearly equal sunlight, leading to approximately 12 hours of day and 12 hours of night across most latitudes. The equinoxes signal the transition where the solar angle is shifting from favoring one hemisphere to the other. These four annual events track the predictable consequence of Earth’s constant 23.5-degree axial tilt.