What are perihelion and aphelion?

What are perihelion and aphelion?

Perihelion is the point in Earth's elliptical orbit at which it is nearest the Sun; aphelion is the point at which it is farthest.⁸⁹ Every closed orbit around the Sun has these two extremes, and they sit at opposite ends of the orbit's long axis. The names come from Greek: peri ("near") and apo ("away from"), each joined to helios ("Sun"). The straight line through the Sun connecting the two — the major axis of the ellipse — is the line of apsides, and the two points together are the apsides.¹⁰

For Earth, perihelion falls in early January and aphelion in early July, each about two weeks after the corresponding solstice.¹ The exact instant shifts a little from year to year: in 2026, Earth reached perihelion on 3 January at 17:15 Coordinated Universal Time and aphelion on 6 July at 17:30, dates the U.S. Naval Observatory tabulates alongside the solstices and equinoxes.³ The U.S. Naval Observatory is the institution that publishes the official almanac data for the United States, and its figures are the reference used throughout this site.

The distance between the two extremes is small, because Earth's orbit is very nearly circular. Its eccentricity — the measure of how far an ellipse departs from a perfect circle, running from 0 for a circle toward 1 for an ever more elongated ellipse — is just 0.0167; in the Naval Observatory's words the orbit "is an ellipse that is flattened by 0.014%."² In practical terms the distance to the Sun at perihelion is "only about 3% less than its distance at aphelion," a swing of a few million kilometres on a mean distance of about 150 million kilometres, which is one astronomical unit.¹⁴ "Perihelion" and "aphelion" are the Sun-specific names; the generic terms for any orbit are periapsis and apoapsis, and bodies orbiting the Earth have their own pair, perigee and apogee.

Do perihelion and aphelion cause the seasons?

No. The seasons are caused by the tilt of Earth's rotation axis, not by the small change in its distance from the Sun. The U.S. Naval Observatory states the cause plainly — the seasons arise from "the 23.4° angular offset (obliquity) between the Earth's axis of rotation and a perpendicular to the Earth's orbital plane" — and is equally plain that the distance change "is too weak to cause the seasons."¹ The intuition that a closer Sun means a hotter season is natural, and wrong.

The clinching observation is the timing. Earth reaches perihelion, its closest approach to the Sun, in early January — about two weeks after the December solstice, at the very start of winter in the Northern Hemisphere.¹ If proximity to the Sun set the temperature, January would be the warmest month for the whole planet; instead it is mid-winter across the populous northern half of the world. The misconception survives partly because most of the people who hold it live in the Northern Hemisphere, where the seasons run exactly opposite to what a distance-driven model would predict.

The reconciliation is that both hemispheres pass through perihelion at the same moment, yet their seasons are reversed — so distance cannot be doing the work; the axial tilt is. Whichever hemisphere is leaning toward the Sun receives its light more directly and for more hours each day, and has its summer; the other has winter. The extra sunlight Earth intercepts at perihelion is only a few percent, and it falls on the entire globe at once rather than favouring one hemisphere. (The distance effect grows over geological time: when Earth's orbit is at its most elongated, perihelion delivers up to "23 percent more incoming solar radiation" than aphelion — but the orbit is nowhere near that elongated today.)⁷ The solstice and day length pages work through how the tilt redistributes sunlight across the year.

What does Earth's elliptical orbit actually change?

The orbit's eccentricity is too small to cause the seasons, but it is not nothing: it has three real, measurable consequences, all flowing from the fact that Earth does not travel its orbit at a constant speed. By Kepler's second law — the rule that a line from the Sun to the Earth sweeps out equal areas in equal times — Earth moves fastest when it is nearest the Sun and slowest when it is farthest.² Its angular rate around the Sun varies by almost 7 percent between perihelion in January and aphelion in July.²

The first consequence is that the seasons are unequal in length. Because Earth dawdles through the stretch of orbit around the July aphelion, the season that spans it — the northern summer, from the June solstice to the September equinox — is the longest of the four; because it races through perihelion in January, the northern winter is the shortest. France's official ephemeris office, attached to the Paris Observatory, gives a worked example for the year 1998: winter lasted 89 days, spring 92 days 18 hours, summer 93 days 15 hours, and autumn 89 days 21 hours.⁵ The gap between the longest and shortest seasons is close to five days, and it traces directly to the difference between Earth's speed at the two apsides.

The second consequence is a hemispheric asymmetry. The Southern Hemisphere's seasons are offset by half a year, so its summer coincides with perihelion and its winter with aphelion. Southern summers are therefore both shorter and, in principle, slightly more intense in sunlight than northern ones, and southern winters are longer.⁵ The effect on climate is modest and heavily masked by the different land-sea distribution of the two hemispheres, but the orbital geometry is real and is the mirror image of the season-length figures above.

The third consequence is a contribution to the equation of time — the gap between the time a sundial shows and the time a clock keeps. The varying orbital speed makes the Sun appear to run a little fast and then a little slow against a uniform clock through the year, producing a sinusoidal swing that reaches about ±7.5 minutes; it is one of the two drivers of the equation of time, the other being the axial tilt, and the two are comparable in size.² The mechanics of how the two terms combine — and why the earliest sunset does not fall on the shortest day — belong to the equation-of-time page and are not reproduced here.

Why do the dates of perihelion and aphelion drift?

The dates of perihelion and aphelion are not fixed, and they move for two unrelated reasons on two very different timescales. Over a few years the change is a wobble of a day or two; over millennia it is a steady creep of the date around the whole calendar.

The short-term wobble is the work of the Moon. Perihelion is defined by the distance between the centre of the Sun and the centre of the Earth, but the Earth's centre is not travelling a smooth ellipse: the Earth and Moon both circle their common centre of mass — the Earth–Moon barycentre, on average about 4,700 kilometres from the Earth's centre — once a month.⁶ The instant at which the Earth's centre is genuinely closest to the Sun therefore depends on where Earth sits in that monthly swing, which lands at a different phase each January and shifts the timing of perihelion by up to a day or so in what the Naval Observatory describes as a "quasi-random variation."⁶

The long-term creep is apsidal precession: Earth's entire elliptical orbit slowly rotates in space, mainly under the gravitational tugs of Jupiter and Saturn, so the line of apsides — and with it the calendar date of perihelion — turns gradually forward.⁷ Measured against the stars this rotation takes about 112,000 years, but measured against the seasons it is faster, because the other reference point, the equinox, is itself drifting the opposite way as Earth's axis precesses over roughly 25,800 years.⁷ One signature of the creep is that the interval between successive perihelions — the anomalistic year — runs about 25 minutes longer than the tropical year of the seasons, which is why the perihelion date slips later by about a day every 57 or 58 years.¹

The two precessions combine into a cycle of roughly 21,000 to 23,000 years over which perihelion migrates through all four seasons. NASA puts the present configuration and its reversal succinctly: "Currently perihelion occurs during winter in the Northern Hemisphere and in summer in the Southern Hemisphere," but "in about 13,000 years, axial precession will cause these conditions to flip," so that the Northern Hemisphere will then have its summer at closest approach.⁷ This wandering of the apsides, together with the slow 100,000-year breathing of the orbit's eccentricity, is one of the Milankovitch cycles — the long orbital rhythms that pace the ice ages.⁷

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