The Sun’s angle, formally known as solar altitude, measures how high the Sun appears above the horizon at a given time and location. Measured in degrees, the angle ranges from \(0^\circ\) at sunrise or sunset up to \(90^\circ\) when the Sun is directly overhead. Solar altitude is a fundamental parameter that directly determines the intensity of sunlight reaching the Earth’s surface. Understanding why this angle constantly shifts is key to comprehending daily weather patterns, seasonal changes, and global heat distribution. The two primary causes of this change are the Earth’s daily rotation and its yearly revolution around the Sun.
The Daily Shift: Earth’s Rotation
The most noticeable change in the Sun’s angle is the daily cycle from dawn to dusk. This predictable movement is the result of the Earth rotating on its axis, not the Sun moving. Our planet completes one full rotation approximately every 24 hours, creating the cycle of day and night.
As the Earth spins, our location is continuously carried toward, under, and away from the Sun. This rotation makes the Sun appear to rise in the east, arc across the sky, and set in the west. At noon, the Sun reaches its maximum solar altitude for the day.
The Seasonal Shift: Earth’s Tilt and Orbit
The seasonal change in the Sun’s noon angle is caused by the Earth’s axial tilt combined with its revolution around the Sun. The Earth’s axis is tilted by about \(23.5^\circ\) relative to its orbit. This tilt, known as axial obliquity, remains fixed in the same direction in space as the Earth orbits the Sun yearly.
This fixed tilt causes the Northern and Southern Hemispheres to alternately lean toward and away from the Sun. When a hemisphere tilts toward the Sun, it receives more direct sunlight, resulting in a higher maximum solar angle and summer. When it tilts away, the Sun’s angle is lower, bringing winter. The axial tilt, not the Earth’s slightly elliptical orbit, is the primary driver of seasonal change.
This seasonal cycle is marked by two solstices and two equinoxes.
Solstices and Equinoxes
- The Summer Solstice (around June 21st) occurs when a hemisphere is maximally tilted toward the Sun, resulting in the highest noon angle.
- The Winter Solstice (around December 21st) marks the maximum tilt away, giving that hemisphere its lowest noon angle.
- The Spring and Autumn Equinoxes (around March 21st and September 21st) happen when the axis is tilted neither toward nor away from the Sun.
- On the equinoxes, the Sun is directly over the equator, and all locations experience roughly 12 hours of day and 12 hours of night.
How the Angle Affects Heating and Daylight Hours
The Sun’s angle has a direct impact on the intensity of solar radiation, which determines temperature and climate. When the Sun is high in the sky, its energy is concentrated over a smaller surface area on the ground. This concentration leads to greater heating and higher temperatures. A higher angle also means the sunlight travels a shorter path through the atmosphere, minimizing energy loss from scattering and absorption.
When the Sun is low, its rays strike the Earth at a shallow angle, causing the solar energy to spread out over a much larger area. This dilution results in less heating per unit of surface area, contributing to cooler temperatures. The longer path through the atmosphere at low angles also increases scattering, further reducing the energy that reaches the ground.
The changing solar angle also dictates the length of daylight hours throughout the year. When a hemisphere is tilted toward the Sun, the higher solar angle is accompanied by a longer duration of daylight. This extended period of heating, combined with the direct angle, makes summer days significantly warmer. Conversely, when a hemisphere is tilted away, the low solar angle brings shorter daylight hours, compounding the effect of reduced heating and resulting in colder winter conditions.