The Earth’s annual journey around the Sun is not a perfect circle, leading to a constant variation in the distance between the two bodies. This predictable orbital path means our planet regularly reaches its closest and farthest points from the Sun. The term for the point when any celestial object is at its maximum distance from the Sun is known as aphelion. This astronomical position is a direct consequence of the laws governing planetary motion.
Defining Aphelion and Orbital Mechanics
Aphelion is the specific spot in Earth’s orbit, or the orbit of any solar system body, where it is most distant from the Sun. The word itself comes from the ancient Greek words apo, meaning “away from,” and helios, meaning “Sun.” This variation in distance exists because planetary orbits are not perfectly circular but are instead slightly elliptical.
Kepler’s First Law states that a planet’s path around the Sun is an ellipse, with the Sun situated at one of the two focal points, not in the exact center. Because the Sun is off-center, the distance to the orbiting body must change as it travels along the elliptical path. For Earth, aphelion typically occurs in early July, about two weeks after the June solstice.
During this time, the Earth is approximately 94.5 million miles (152.1 million kilometers) away from the Sun. This is roughly 3 million miles farther than the average Earth-Sun distance.
The Counterpart: Understanding Perihelion
To understand aphelion, it is helpful to know its opposite, a point called perihelion. Perihelion is the position in an orbit where the celestial body is closest to the Sun. The term uses the Greek prefix peri, meaning “near” or “around,” paired with helios.
For Earth, perihelion occurs annually in early January, roughly two weeks after the December solstice. At this closest point, Earth is approximately 91.4 million miles (147.1 million kilometers) from the Sun.
The difference in distance between aphelion and perihelion is about 3 million miles. This small orbital eccentricity causes Earth to move slightly faster in its orbit when it is at perihelion in January and slightly slower when it reaches aphelion in July. This variation in speed is a consequence of Kepler’s Second Law.
Aphelion’s Minimal Effect on Global Climate
A common misconception is that the changing distance between Earth and the Sun causes the seasons. However, the variation in distance has only a minimal effect on global temperatures. In fact, the Northern Hemisphere experiences summer, its warmest season, when Earth is at aphelion in July and farthest from the Sun.
The true cause of the seasons is the 23.5-degree tilt of the Earth’s axis. This tilt determines the angle at which sunlight strikes the planet’s surface and the length of daylight hours. When the Northern Hemisphere is tilted toward the Sun, it receives more direct sunlight, resulting in summer.
Conversely, when the Northern Hemisphere is tilted away from the Sun, the sunlight is less direct, leading to winter. The distance change contributes only a minor difference to the total solar energy received. The angle and duration of solar radiation due to axial tilt are the overwhelming drivers of Earth’s seasonal cycles.