Perihelion Facts For Kids

Apsis is a word for the very closest or very farthest point of something that moves around another thing, like a planet moving around the Sun. The path the planet follows is called an orbit. Every orbit that is a stretched circle (an ellipse) has two special places: one nearest the thing it goes around and one farthest away.

These two places are important because they help us measure how close objects get and how their speed can change. We also draw an imaginary straight line between them to show the directions of closest and farthest points.

Every stretched-circle orbit has two named points: one close and one far. Around the Sun, the nearest point is called perihelion and the farthest point is called aphelion. Around Earth, people often say perigee (nearest) and apogee (farthest). These names tell us which object is being orbited.

The straight connection between the near and far points is the line of apsides. This line helps astronomers see where an orbit is longest or shortest. For example, Earth reaches perihelion (closest to the Sun) in early January and aphelion (farthest) in early July.

A long time ago a scientist named Johannes Kepler used and shaped these words while he studied how planets move. The beginnings of the words come from Greek: peri- means "near" and apo- (also seen as ap-) means "away" or "off".

The ends of the words (the suffixes) tell which body is being orbited. For example, -helion points to the Sun, and -gee points to Earth. Scientists make names by mixing a prefix (peri- or apo-) with a fitting suffix, so the name tells both "near or far" and "near what."

You can make a name for an orbit point by using either peri- (near) or apo- (far) plus a part that names the central body. If no special name exists, people use the general suffix -apsis (so periapsis and apoapsis).

Some common special names are used for the Sun, Earth, and Moon. For other objects, scientists might use endings like -astron for other stars or just use the generic -apsis. This system helps everyone know exactly which "near" or "far" point someone means.

Perihelion is the point where a planet comes closest to the Sun, and aphelion is where it is farthest. Inner planets (Mercury, Venus, Earth, Mars) travel in smaller paths, so their closest and farthest points happen nearer to the Sun and change position faster around the sky. Outer planets (Jupiter, Saturn, Uranus, Neptune) move much farther away, so even though their difference between closest and farthest can be huge in kilometres, it is often a smaller part of their very large orbit.

Because inner planets circle the Sun more quickly, the order and place of their perihelion and aphelion shift more noticeably from year to year than those of outer planets.

The line of apsides is an invisible straight line that connects a planet’s perihelion and aphelion. Think of it like drawing a line across a racetrack to show the very shortest and longest stretches. This line shows the full range of how close and how far a planet gets from the Sun.

Astronomers use that line to compare orbits. On charts, that range often looks like a bar: one end marks perihelion and the other marks aphelion. Seeing many bars together helps scientists compare how stretched or circular different orbits are.

Earth reaches its perihelion in early January and its aphelion in early July. Right now, perihelion falls about 14 days after the December solstice and aphelion about 14 days after the June solstice. At perihelion Earth is about 0.9833 AU from the Sun (roughly 147,100,000 km). At aphelion it is about 1.01664 AU (roughly 152,087,000 km).

These distances and dates are not fixed forever. Over many years the timing slowly changes because Earth’s orbit twists and wobbles a little. The Moon also nudges Earth, so dates can move by a few days from one year to the next.

Planets do not move in perfect, unchanging paths. Tiny pulls from other planets and the Moon make small shifts called perturbations. For example, the day of aphelion for Earth can jump by a few days from one year to the next because of the Moon’s pull and the way Earth and Moon move together.

The Earth–Moon system actually orbits the Sun around a shared center called the barycenter. Earth’s center can sit up to about 4,700 km from that point, and this wobble changes when perihelion and aphelion happen. Over longer times—decades and centuries—gravitational tugs and slow wobbling move those dates even more, so astronomers watch and predict these changes carefully.

Perihelion is the moment a planet comes nearest to the Sun. The opposite moment, aphelion, is when it is farthest away. Distances change a lot from world to world. For example, Earth is about 147 million kilometers from the Sun at perihelion and about 152 million kilometers at aphelion — a small change. Mercury, by contrast, swings from about 46 million to 70 million kilometers. Dwarf worlds like Pluto move much farther away and back, so their distance changes are very large.

These distance swings change how much sunlight, or insolation, a planet gets. Earth gets roughly 7% more sunlight at perihelion, so distance has only a small effect on our seasons. But on Mercury the Sun can shine more than twice as brightly at perihelion, which makes a big difference in surface heating. On very distant bodies like Pluto, sunlight is always very faint, so the difference mostly matters for brightness rather than warmth.