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Planetary Orbits
The planets of our solar system orbit the sun, but not in perfect circles. Instead, their orbits are elliptical, meaning they are closer to the sun at one end of the orbit and farther away at the opposite end. The point at which a planet is closest to the sun is called its perihelion, while its farthest point is known as its aphelion. These terms come from Greek: "helios" means sun, "peri" means near and "apo" means away from.
Earth's Orbit
Earth's orbit also is elliptical, but it is closer to circular than the orbits of other planets. The Earth's closest point to the sun, or perihelion, occurs around January 4. At this point, the Earth is approximately 91 million miles from the sun. The Earth's farthest point from the sun, or aphelion, occurs around July 4. At this point, the Earth is approximately 94 million miles from the sun. The difference between these distances is rather small on an astronomical scale.
Earth's Tilt
In addition to the orbit of the Earth around the sun, there is another factor that impacts the relationship between the two. The Earth has a pronounced tilt. This tilt is approximately 23.5 degrees from vertical, where vertical is defined as perpendicular to the Earth's orbital plane. This varies the part of the planet that is closest to the sun during a solar orbit, or year. The Northern Hemisphere is closest to the sun around June 21, while the Southern Hemisphere is closest around December 21.
Earth's Seasons
If the variance in distance of the Earth's orbit around the sun were responsible for the seasons, the entire Earth would experience summer, at perihelion in January, or winter, at aphelion in July, at the same time. However, this is not the case. The seasons in each hemisphere are diametrically opposed. Winter comes in January for the Northern Hemisphere, but in July for the Southern Hemisphere. This means that the Earth's seasons are caused by its tilt and not its orbit.
Strange but True
The average global temperature is 4 degrees Fahrenheit higher when the planet is farthest from the sun --- summer in the Northern Hemisphere. This is because aphelion occurs when the Northern Hemisphere is pointed toward the sun, and there's much more landmass in the north. This landmass heats more efficiently than the oceans of the Southern Hemisphere, which face the sun during perihelion.