Earth’s path around the Sun is not a perfect circle, meaning our distance from the star changes throughout the year. The month when Earth is farthest from the Sun is July. This point, where the planet reaches the greatest separation from the star, is a natural consequence of the elliptical shape of Earth’s orbit.
Defining Aphelion and Perihelion
The extremes of Earth’s orbit are described by two terms: aphelion and perihelion. Aphelion is the point where Earth is farthest from the Sun, typically occurring in early July, about two weeks after the June solstice. Conversely, perihelion is the orbital point where Earth is closest to the Sun, which happens in early January, roughly two weeks after the December solstice.
The difference in distance between these two points is significant. At aphelion, Earth is approximately 94.5 million miles (152 million kilometers) from the Sun, while at perihelion, the distance shrinks to about 91 million miles (147 million kilometers). This results in a difference of about 3 million miles (4.8 to 5 million kilometers) between the two extremes. This distance change means that at aphelion, Earth receives about 6.7% less solar energy than it does at perihelion.
The Mechanics of Earth’s Elliptical Orbit
The existence of closest and farthest points is explained by the laws of planetary motion, which state that Earth’s path is an ellipse, not a circle. The Sun is not located at the center of this path but rather at one of the two focal points of the ellipse. Since the Sun is offset, the distance between Earth and the Sun continuously changes as the planet completes its yearly revolution.
The degree to which Earth’s orbit deviates from a perfect circle is known as its eccentricity. Earth’s orbit has a current eccentricity value of about 0.0167, which is quite close to zero, meaning the orbit is only slightly elongated. This small amount of flattening is enough to create the measurable difference in distance between aphelion and perihelion. Over thousands of years, the gravitational influence of other large planets, like Jupiter and Saturn, causes this eccentricity to slowly vary, changing the shape of Earth’s elliptical path.
Distance Does Not Determine Seasons
The fact that Earth is farthest from the Sun in July, during Northern Hemisphere summer, demonstrates that distance does not determine the seasons. The true cause of seasonal variation is the tilt of Earth’s rotational axis, angled at approximately 23.4 degrees relative to the plane of its orbit. This tilt, known as obliquity, remains fixed as Earth revolves around the Sun.
The axial tilt affects how directly sunlight strikes the surface and determines the length of daylight hours. When the Northern Hemisphere is tilted toward the Sun, which occurs around June, that region receives more direct, concentrated solar rays. Simultaneously, the day length is longer, allowing for more time to absorb heat, resulting in warmer summer temperatures.
When the Northern Hemisphere is tilted away from the Sun, around December, the incoming solar rays strike the surface at a much lower angle. This causes the light to be spread out over a larger area, reducing the intensity of the heat. Combined with shorter daylight hours, this leads to the lower average temperatures experienced during winter. The Southern Hemisphere experiences the opposite season, confirming that the tilt, not the distance, is the primary driver of the seasons.