Earth’s orbital speed varies throughout the year, moving fastest around the beginning of January and slowest in early July. This fluctuation is a predictable outcome of the laws of physics governing celestial mechanics. The difference in speed is explained by the geometry of Earth’s path around the Sun and the gravitational forces involved.
Defining Earth’s Elliptical Orbit
Earth does not travel in a perfect circle around the Sun, but rather in an oval-shaped path known as an ellipse. This elliptical shape means that the distance between Earth and the Sun is constantly changing across the year. An ellipse is defined by two focal points, and the Sun is located at one of these two foci.
The point in the orbit where Earth is closest to the Sun is called Perihelion. Conversely, the point where Earth is farthest from the Sun is called Aphelion. Earth’s orbit is nearly circular, possessing a low eccentricity of about 0.0167, yet this slight deviation is enough to cause variations in speed.
At Perihelion, the distance between the centers of the Earth and the Sun is approximately 147.1 million kilometers (91.4 million miles). The distance increases to about 152.1 million kilometers (94.5 million miles) when the planet reaches Aphelion. This difference of about 5 million kilometers, or just over 3% of the average distance, determines the fluctuations in orbital velocity.
The Principle Governing Orbital Speed
The reason this varying distance causes a change in speed is explained by Johannes Kepler’s Second Law of Planetary Motion, often referred to as the Law of Equal Areas. This principle states that a line connecting a planet to the Sun sweeps out equal areas of space in equal amounts of time. To maintain this equal area rule, the planet’s speed must adjust in a precise manner.
When Earth is closer to the Sun, the line connecting the two is shorter, creating a narrower but longer triangular area in the orbit. To sweep out the same area as a wider, shorter triangle created farther away from the Sun, the planet must travel a greater distance along its path. Therefore, the planet must accelerate as it approaches the Sun.
This change in speed is a manifestation of the conservation of angular momentum. Angular momentum must remain constant in a system like the Earth-Sun pair. Since angular momentum depends on the distance from the central object, a decrease in distance requires a corresponding increase in velocity to keep the overall value the same.
Linking Distance and Speed to Specific Dates
Earth reaches its Perihelion, the closest point to the Sun, around January 3rd each year. According to the law of equal areas, this is the moment when Earth’s orbital velocity is maximized.
Conversely, Earth reaches its Aphelion, the farthest point from the Sun, around July 4th. At this maximum distance, the Sun’s gravitational influence is at its weakest for the year, and the planet must slow down to conserve angular momentum. This results in the minimum orbital velocity for the year.
The specific dates for Perihelion and Aphelion are not fixed and can vary slightly because the calendar year does not perfectly align with the planet’s orbit. This distance variation does not cause the seasons, which are instead determined by the tilt of Earth’s axis. The speed difference does, however, slightly affect the length of seasons, making the Northern Hemisphere’s summer slightly longer than its winter.