A perihelion is the point in an object’s orbit where it comes closest to the Sun. The word comes from the Greek “peri” (near) and “helios” (Sun), and it applies to anything orbiting the Sun: planets, comets, asteroids. Earth reaches its perihelion every year in early January, when it sits about 91.4 million miles from the Sun.
Why Orbits Have a Closest Point
Planets don’t orbit the Sun in perfect circles. Their paths are ellipses, slightly stretched-out ovals with the Sun sitting off-center at one focus of the ellipse. Because the Sun isn’t in the exact middle, the distance between a planet and the Sun constantly changes throughout the orbit. The closest point is the perihelion. The farthest point is called the aphelion.
How stretched an orbit is depends on its eccentricity. Earth’s orbit is nearly circular, so the difference between its closest and farthest points is relatively small: about 91.4 million miles at perihelion versus 94.5 million miles at aphelion, a gap of roughly 3 million miles. Mercury, by contrast, has a much more eccentric orbit, so the difference between its perihelion and aphelion is far more dramatic.
What Happens at Perihelion
A planet doesn’t just get closer to the Sun at perihelion. It also speeds up. This behavior is described by Kepler’s Second Law, one of three rules the 17th-century astronomer Johannes Kepler worked out by studying planetary motion. The law states that an imaginary line connecting a planet to the Sun sweeps out equal areas of space in equal amounts of time. In practical terms, this means a planet has to move faster when it’s closer to the Sun and slower when it’s farther away, so that the area covered stays constant.
Earth is moving at its fastest orbital speed in early January and its slowest in early July. The speed difference is small because Earth’s orbit is nearly circular, but it’s measurable. For objects with more elongated orbits, like many comets, the speed difference between perihelion and aphelion is enormous. A comet can whip around the Sun at tremendous velocity during its closest approach, then slow to a crawl as it drifts out toward the far edges of its orbit.
Kepler figured out these patterns decades before anyone understood why they happened. It was Isaac Newton who later explained the underlying cause: gravity. The closer an object is to the Sun, the stronger the gravitational pull, and the faster the object moves.
Perihelion and Earth’s Seasons
Here’s the part that surprises most people: Earth is closest to the Sun in January, right in the middle of winter for the Northern Hemisphere. If distance from the Sun drove the seasons, January would be the hottest month. It isn’t, which tells you something important about what actually causes seasons.
Seasons are driven by Earth’s axial tilt, not by distance. Earth’s axis is tilted about 23.5 degrees relative to its orbital path, and that tilt always points in the same direction as Earth moves around the Sun. In June, the North Pole leans toward the Sun, so the Northern Hemisphere gets more direct sunlight and longer days. In December, the North Pole leans away, so the Northern Hemisphere gets less direct sunlight and shorter days. The tilt matters far more than the distance.
That said, perihelion does have a measurable effect on how much solar energy reaches Earth. In January, about 6.8 percent more solar radiation hits our planet than in July. This is a real difference, but it’s overwhelmed by the effect of axial tilt. Interestingly, it does slightly moderate Northern Hemisphere winters (a bit more solar energy arriving) and slightly moderate Northern Hemisphere summers (a bit less). The Southern Hemisphere gets the opposite deal: its summers coincide with perihelion, making them slightly more intense in terms of incoming solar energy.
Long-Term Changes in Earth’s Orbit
Earth’s orbit doesn’t stay the same shape forever. Over cycles of roughly 100,000 years, it stretches and compresses, becoming more or less elliptical. These shifts are part of what scientists call Milankovitch cycles, slow changes in Earth’s orbital geometry that influence climate over tens of thousands of years.
When Earth’s orbit is at its most elliptical, the difference between perihelion and aphelion becomes much larger. At peak eccentricity, about 23 percent more solar radiation reaches Earth at perihelion than at aphelion. Compare that with today’s 6.8 percent difference, and you can see how these orbital shifts could nudge the climate toward or away from ice ages over geological timescales. The effect is gradual and plays out over millennia, but it’s one of the key factors that has shaped Earth’s climate history.
When Earth Reaches Perihelion
Earth’s perihelion falls in early January each year, typically between January 2 and January 5. The exact date and time shift slightly from year to year. In 2026, for example, Earth reaches perihelion on January 3. The timing drifts very slowly over centuries due to gravitational interactions with other planets.
You won’t notice anything different on perihelion day. There’s no visible change in the Sun’s size or brightness that your eyes could detect. The roughly 3.3 percent difference in Earth’s distance from the Sun between perihelion and aphelion translates to about a 3.3 percent difference in the Sun’s apparent size, which is too small to perceive without instruments.
Perihelion Beyond Earth
Every object orbiting the Sun has its own perihelion. The term is specific to solar orbits. For orbits around other bodies, the general term is “periapsis,” and there are specific versions for different central objects: perigee for orbits around Earth, perijove for Jupiter, and so on.
For comets, perihelion is often the most important moment in their orbit. As a comet approaches the Sun, solar heating vaporizes ice on its surface, creating the glowing coma and tail that make comets visible. Many comets are only detectable near perihelion, spending the rest of their orbit as dim, frozen objects in the outer solar system. Some comets pass so close to the Sun at perihelion that they partially or completely break apart from the intense heat.