Perihelion and Aphelion
Perihelion is the point in a planet’s (or comet’s or spacecraft’s) orbit at which it is closest to the Sun. Aphelion is the point at which it is farthest. The two terms define the extremes of an elliptical orbit, and their existence explains why Earth’s seasons are not driven by distance from the Sun.
What They Are
Planetary orbits are ellipses, not circles. Johannes Kepler established this in his first law of planetary motion (1609): every planet moves in an ellipse with the Sun at one focus. Because the orbit is elliptical, the planet-Sun distance is not constant — it varies between a minimum (perihelion) and a maximum (aphelion).
For Earth, perihelion currently occurs around January 3–5 each year. Earth is closest to the Sun in early January — roughly 147.1 million kilometers. Aphelion occurs around July 3–5. Earth is farthest from the Sun in early July — roughly 152.1 million kilometers. The difference is about 3.4%.
This immediately confounds the intuitive explanation for seasons that many people carry from childhood. Earth is closest to the Sun in January — Northern Hemisphere winter. It is farthest in July — Northern Hemisphere summer. Distance is not the driver. Seasons are caused by axial tilt: when a hemisphere is tilted toward the Sun, it receives solar energy at a steeper angle and experiences longer days. Distance plays a negligible role.
The terms generalize. Any object orbiting the Sun — asteroids, comets, spacecraft — has a perihelion and aphelion. Objects orbiting other bodies use analogous terms: perigee/apogee for Earth orbit, perilune/apolune for lunar orbit, periapsis/apoapsis as the generic forms for any body.
Etymology
Both words are built from Greek. Perihelion combines peri (around, near) + helios (Sun): the point around or near the Sun. Aphelion combines apo (away from) + helios: the point away from the Sun. The terms entered astronomical vocabulary through Latin translations of Greek astronomical texts and have been standard since the seventeenth century.
The pronunciation of “aphelion” trips readers: the ph is pronounced as f, giving “ap-HEE-lee-on,” not “ap-HEL-ee-on.” The heli root (Sun) is the same as in heliocentric, heliograph, and helium — the element named for the Sun because it was first detected in the solar spectrum.
A Concrete Example
Comet Halley’s orbit is highly elliptical. At perihelion, it passes inside the orbit of Venus — approximately 87.6 million kilometers from the Sun, well inside Earth’s orbit. At aphelion, it reaches beyond Neptune — approximately 5.2 billion kilometers from the Sun, more than 35 times Earth’s distance. The extreme eccentricity of the orbit explains Halley’s 75–79 year period and its dramatic brightness increase as it approaches the Sun, sublimating ice into the visible coma and tail.
Common Misconception
Many people believe Earth is closest to the Sun in summer (in the Northern Hemisphere) because summer is hot. The opposite is true. Earth’s perihelion falls in early January; Northern Hemisphere winter occurs simultaneously. The counterintuitive fact — that Earth is nearest the Sun when the Northern Hemisphere is coldest — consistently surprises people who have never considered that distance and axial tilt are independent variables. The Southern Hemisphere summer, occurring in December and January, actually receives slightly more solar energy than the Northern Hemisphere summer because it coincides with perihelion.