The distance between the Earth and the Sun is not fixed; it constantly changes as our planet moves through space. For convenience in astronomical calculations, scientists use the Astronomical Unit (AU). The AU represents the average distance between the center of the Earth and the Sun, which is approximately 93 million miles or 150 million kilometers. This average figure provides a standard reference point for measuring distances within our solar system, though the actual separation varies predictably over the course of Earth’s annual journey.
Why the Distance Between Earth and the Sun Changes
The variation in the Earth-Sun distance results from the shape of Earth’s orbit, which is not a perfect circle. Our planet follows an elliptical, or oval-shaped, path around the Sun, defined by Kepler’s first law (the Law of Ellipses). An ellipse has two focal points, and the Sun is positioned at one of these foci.
Because the Sun is not perfectly centered, the distance between the Earth and the Sun fluctuates during its yearly revolution. The orbit’s deviation from a perfect circle is called eccentricity. For Earth, this eccentricity is small, meaning the orbit is very close to circular, but the slight oval shape creates a noticeable difference in solar distance.
This orbital geometry also governs the Earth’s speed, as described by Kepler’s second law (the Law of Equal Areas). This law requires that the Earth travel faster when closer to the Sun and slower when farther away. This constantly changing orbital velocity is a consequence of the elliptical path.
Defining the Closest and Farthest Orbital Points
The two extreme points in Earth’s orbit describe the varying distance from the Sun. The closest point is called perihelion, and the farthest point is called aphelion. These events occur reliably at the same points in the calendar each year, though the exact date can shift slightly.
The Earth reaches perihelion around January 3rd, when the distance is approximately 91.4 million miles. Conversely, the Earth reaches aphelion in early July, typically around the 4th or 6th. At aphelion, the distance increases to approximately 94.5 million miles.
The Earth-Sun separation varies by about 3 million miles over the course of a year. This variation is small compared to the overall average distance of 93 million miles.
The timing of these extremes is noteworthy for the Northern Hemisphere. The Earth is closest to the Sun during the Northern Hemisphere’s winter and farthest during its summer. This timing suggests that distance variation is not the primary driver of seasonal temperature changes.
The Actual Cause of Earth’s Seasons
The common assumption that changing distance causes the seasons is contradicted by the timing of perihelion and aphelion. If distance were the main factor, both hemispheres would experience their warmest temperatures in January, which is not the case for the Northern Hemisphere. The slight difference in solar intensity from the 3-million-mile variation is insufficient to override atmospheric and geographical factors.
The true cause of the seasons is the Earth’s axial tilt, or obliquity, which is approximately 23.5 degrees. This tilt is fixed in space, meaning the axis always points in the same direction as the Earth revolves around the Sun. As a result, different hemispheres are tilted either toward or away from the Sun at various points in the orbit.
When a hemisphere is tilted toward the Sun, it experiences summer because it receives the Sun’s rays at a more direct, higher angle. This direct angle concentrates solar radiation over a smaller surface area, leading to more intense heating. Conversely, when a hemisphere is tilted away, the solar rays strike the surface at a shallower, lower angle.
The lower angle spreads the incoming solar energy over a larger area, reducing the heat absorbed. The tilt also dictates the length of daylight hours. The hemisphere tilted toward the Sun experiences longer days, allowing for more heat absorption, while the other hemisphere experiences shorter days and less solar heating.
The combination of a higher solar angle and longer daylight hours defines the summer season. The Earth’s 23.5-degree tilt is the determining factor that drives the annual cycle of temperature variation and defines our planet’s seasons.