Eccentricity of Earth's Orbit

Short answer

• The eccentricity of Earth's orbit is about 0.0167 as of November 2026, which means Earth's path around the Sun is an ellipse so close to a circle that you could not tell the difference by eye. Eccentricity is a number from 0 to 1 that measures how far an orbit departs from a perfect circle, where 0 is a perfect circle and values closer to 1 are more stretched out. Because Earth's value is so low, the distance between Earth and the Sun changes by only about 3.4 percent over the year, from about 147.1 million kilometers at perihelion in early January to about 152.1 million kilometers at aphelion in early July. That small change does not cause the seasons. The seasons come mainly from the tilt of Earth's axis, currently about 23.4 degrees, while eccentricity only makes Northern Hemisphere summers a little longer than winters. Over about 100,000 years, Earth's eccentricity slowly varies from nearly 0.0034 to almost 0.058, and it is currently very slowly decreasing toward its most circular state.

What eccentricity means

Eccentricity is a single number that describes the shape of an orbit. It answers a simple question: how far does this orbit depart from a perfect circle? The answer is given as a value called e, and for a closed orbit around the Sun that value always sits between 0 and 1.

A value of 0 means a perfect circle. A value between 0 and 1 means an ellipse, which is a closed oval shape. The closer the value gets to 1, the longer and thinner the ellipse becomes. A value of exactly 1 would describe a parabola, an open path that never returns, and a value greater than 1 would describe a hyperbola, another open path. Planets stay in the closed range, so their eccentricities sit between 0 and 1.

Encyclopaedia Britannica defines the eccentricity of an elliptical orbit as the distance between the two focal points of the ellipse divided by the length of the major axis, which is the longest diameter of the ellipse. That definition is worth unpacking, because it explains what the number is really measuring.

An ellipse has two focal points, usually called foci. You can picture an ellipse as a circle that has been gently pulled in two opposite directions. As it is pulled, the single center point of the circle splits into two foci that move apart. The farther apart the foci sit, relative to the size of the ellipse, the more stretched the shape becomes, and the higher the eccentricity. When the two foci sit on top of each other, the shape is a circle and the eccentricity is 0.

There is a simple formula for this. If c is the distance from the center of the ellipse to one focus, and a is the semi-major axis, which is half the longest diameter, then eccentricity e equals c divided by a. In symbols, e equals c over a. For Earth, a is about 149.6 million kilometers, and e is about 0.0167, so c works out to about 2.5 million kilometers. That is the distance by which the Sun, which sits at one focus of Earth’s orbit, is offset from the center of the orbital ellipse. It sounds large in everyday terms, but against an orbit nearly 300 million kilometers across, it is a small shift, only about 1.7 percent of the semi-major axis.

Eccentricity describes shape only, not size. Two orbits can have the same eccentricity and very different sizes, and two orbits of the same size can have very different eccentricities. The semi-major axis sets the size of the orbit and, through Kepler’s third law, the length of the year. Eccentricity sets only how stretched that orbit is. That distinction matters, because it is easy to confuse a large orbit with a stretched one. Earth’s orbit is large, about 940 million kilometers around, but it is barely stretched at all.

It also helps to separate eccentricity from two other ideas that are often mixed up with it. The first is distance: eccentricity changes how much the Earth-Sun distance varies during the year, but it does not set the average distance, which is about 149.6 million kilometers, or one astronomical unit. The second is speed: eccentricity affects how much Earth’s orbital speed varies during the year, but the average speed, about 29.78 kilometers per second, is set mainly by the size of the orbit and the Sun’s gravity. Keep those three ideas apart, shape, size, and speed, and most of the confusion about eccentricity disappears.

The number for Earth today: about 0.0167

As of November 2026, the eccentricity of Earth’s orbit is about 0.0167. That figure is quoted to four decimal places in most references, and more precise tabulations give values such as 0.0167086 for the J2000.0 epoch, which is a standard reference date used by astronomers. The small differences between these figures come from rounding and from the reference date chosen, not from disagreement about the orbit. For everyday purposes, 0.0167 is the number to remember, and it is the value used in this article.

What does 0.0167 look like? Imagine drawing Earth’s orbit on a page so that it is 1 meter across. A perfect circle would be 1 meter wide in every direction. Earth’s orbit, drawn at the same scale, would be about 1 meter long and about 0.99986 meters wide. The difference between the long and short diameters would be about 0.14 millimeters, roughly the thickness of a sheet of paper. That is why astronomers describe Earth’s orbit as nearly circular, and why diagrams of the solar system almost always exaggerate the stretch: drawn to true scale, the ellipse would look like a circle.

The table below puts Earth’s key orbital figures together, using values published by NASA. All distances are given in both kilometers and miles, because readers in the United States usually think in miles, while scientific sources use kilometers.

Two things in that table deserve attention. First, the perihelion and aphelion distances come directly from the semi-major axis and the eccentricity: perihelion equals the semi-major axis times one minus e, and aphelion equals the semi-major axis times one plus e. With a semi-major axis of 149.6 million kilometers and e of 0.0167, the arithmetic gives about 147.1 and about 152.1 million kilometers, matching the published figures. Second, the 3.4 percent variation is small enough that the amount of sunlight Earth receives changes only modestly through the year for this reason alone. NASA puts that change at about 6.8 percent more incoming solar radiation in January, at perihelion, than in July, at aphelion. That is a real difference, but it is not the engine of the seasons, as the section below explains.

Earth’s eccentricity is low compared with most of the bodies in the solar system, but it is not the lowest. Venus has the most circular planetary orbit, at about 0.007, and Neptune is close behind at about 0.009. Mercury, at the other end of the scale among the planets, has an eccentricity of about 0.206, more than twelve times Earth’s value, so its distance from the Sun varies far more during its year. Mars sits between them at about 0.093. Comets go much further still: Halley’s Comet, for example, has an eccentricity of about 0.967, which is why it swings from close to the Sun out beyond Neptune. Earth’s 0.0167 sits near the circular end of that range, and that near-circular shape is one of the quiet facts that makes Earth’s climate comparatively stable from year to year.

Perihelion and aphelion: the two ends of the ellipse

Every elliptical orbit has two special points on its long axis. The point closest to the Sun is called perihelion, from Greek roots meaning near the Sun. The point farthest from the Sun is called aphelion, meaning away from the Sun. Earth reaches perihelion on or about January 3 each year, and aphelion on or about July 4. The exact day shifts by a day or two from year to year, so those dates should be read as “on or about” rather than as fixed calendar appointments.

The timing surprises many readers. Earth is closest to the Sun in early January, in the middle of winter in the Northern Hemisphere, and farthest from the Sun in early July, in the middle of summer in the Northern Hemisphere. That single fact is enough to show that distance from the Sun is not what makes summer warm and winter cold. If distance were the cause, January would be the warmest month in the Northern Hemisphere and July the coldest, which is the opposite of what happens. The reason January is cold in the north despite the closer Sun is the tilt of Earth’s axis, covered in the seasons section below.

At perihelion, Earth is about 147.1 million kilometers from the Sun. At aphelion, it is about 152.1 million kilometers away. The gap between them, about 5.1 million kilometers, is about 3.4 percent of the average distance. For a sense of scale, 5.1 million kilometers is about thirteen times the distance from Earth to the Moon, so it is large in absolute terms and small in relative terms at the same time. Both descriptions are true, and the relative one is the one that matters for climate: a 3.4 percent change in distance produces only about a 6.8 percent change in incoming sunlight, because sunlight spreads out as the square of the distance.

Earth also moves at different speeds at the two ends of its orbit. This follows from Kepler’s second law of planetary motion, which says that a line joining a planet to the Sun sweeps out equal areas in equal times. In plain terms, a planet moves faster when it is close to the Sun and slower when it is far away, so that it covers the same area of its orbit in the same amount of time. Earth travels at about 30.29 kilometers per second near perihelion and about 29.29 kilometers per second near aphelion, against an average of about 29.78 kilometers per second. The difference is about 1 kilometer per second, or about 3.4 percent, which mirrors the distance variation. Because Earth moves faster near perihelion, it covers the winter part of its orbit a little more quickly, and because it moves more slowly near aphelion, it lingers a little longer in the summer part. That is the mechanical reason Northern Hemisphere summers are slightly longer than winters, as NASA notes: summers in the Northern Hemisphere are currently about 4.5 days longer than winters, and springs about three days longer than autumns.

If you want the full picture of how that distance changes through the year, and what the average distance means in practice, our guide to How Far Is Earth From the Sun explains the astronomical unit, the perihelion and aphelion distances, and why the distance varies at all.

How eccentricity is measured

Eccentricity is not measured with a ruler. It is calculated from observations of where a planet is in the sky, and how fast it is moving, over time. Astronomers record the direction and distance of a planet at many points in its orbit, then fit an ellipse to those observations. The shape of the best-fitting ellipse gives the eccentricity, and the size of the ellipse gives the semi-major axis. Modern measurements use radar ranging, spacecraft tracking, and precise telescopic positions, and the resulting orbital elements are published by institutions such as NASA’s Jet Propulsion Laboratory. For Earth, the calculation is unusually precise, because we live on the planet being measured and can track the Sun’s apparent position against the stars throughout the year.

A worked example shows how the number connects to distances you can picture. Start with the semi-major axis, a, of about 149.6 million kilometers. Multiply it by the eccentricity, e, of 0.0167, and you get about 2.5 million kilometers. That is c, the offset of the Sun from the center of the ellipse. Subtract c from a and you get perihelion, about 147.1 million kilometers. Add c to a and you get aphelion, about 152.1 million kilometers. The whole shape of Earth’s orbit follows from two numbers, a and e, plus the orientation of the ellipse in space. Change e and the orbit stretches or rounds out; change a and the orbit grows or shrinks; change neither and the orbit stays as it is.

Historically, the elliptical shape itself was the discovery. For centuries, astronomers assumed that planets moved in perfect circles, because the circle was regarded as the perfect shape for heavenly motion. In the early 1600s, Johannes Kepler, working from the precise naked-eye observations of the planet Mars made by Tycho Brahe, showed that the orbit of Mars was an ellipse with the Sun at one focus, not a circle with the Sun at the center. That result became Kepler’s first law of planetary motion: the orbit of each planet is an ellipse with the Sun at one of the two foci. Once the ellipse was accepted, eccentricity became the natural way to say how elliptical a given orbit was, and Earth’s small value could be measured rather than assumed.

Why Earth’s orbit is not a perfect circle

If the Sun were the only body in the solar system, Earth’s orbit would be a fixed ellipse, unchanging from year to year. But the Sun is not alone. Every other planet pulls on Earth with its gravity, and the two largest planets, Jupiter and Saturn, pull hardest. Those pulls are tiny compared with the Sun’s gravity, but they never stop, and over thousands of years they add up. They slowly stretch Earth’s orbit into a slightly more elongated ellipse, then let it relax back toward a circle, in a long cycle. NASA describes this directly: the pull of gravity from Jupiter and Saturn causes the shape of Earth’s orbit to vary from nearly circular to slightly elliptical.

The same gravitational nudging affects the other parts of Earth’s motion as well. It changes the tilt of Earth’s axis and the direction the axis points, and those changes combine with eccentricity into the set of long cycles known as the Milankovitch cycles, discussed below. None of these changes is fast enough to notice in a human lifetime. Eccentricity changes by only a tiny amount per century, and the full cycle takes about 100,000 years. That is why the value 0.0167 can be quoted as a stable fact for practical purposes, even though it is, strictly speaking, a snapshot of a slowly moving target.

It is also worth saying plainly what eccentricity is not. It is not a measure of how tilted Earth’s orbit is, or how fast Earth spins, or how far Earth is from the Sun on average. A planet could have a highly eccentric orbit and a small tilt, or a nearly circular orbit and a large tilt. The shape of the path and the tilt of the axis are separate properties, and they affect climate in different ways. Confusing them is one of the most common errors in popular writing about the seasons, and it is the error the next section corrects.

Does eccentricity cause the seasons? No, tilt does

This is the most important correction in the whole topic, so it is worth stating plainly: eccentricity does not cause the seasons. The tilt of Earth’s axis does. NASA and the National Oceanic and Atmospheric Administration (NOAA) both make this point in their educational material, and the reasoning is straightforward.

Earth’s axis is tilted about 23.4 degrees from the perpendicular to its orbital plane, and the axis keeps pointing in the same direction in space as Earth travels around the Sun. In June, the Northern Hemisphere tilts toward the Sun, so sunlight strikes it more directly and days are longer, and it is summer in the north. In December, the Northern Hemisphere tilts away from the Sun, sunlight strikes it at a slant and days are shorter, and it is winter in the north. The Southern Hemisphere experiences the opposite pattern at the same time. That is the whole mechanism: direct sunlight and long days in one hemisphere, slanted sunlight and short days in the other, swapping places every six months.

Eccentricity plays only a supporting role. It changes the Earth-Sun distance by about 3.4 percent, which changes incoming sunlight by about 6.8 percent between perihelion and aphelion. That is enough to matter at the margins, but it cannot explain a seasonal swing of tens of degrees in temperature. Three facts make the point decisive. First, Earth is closest to the Sun in January, when the Northern Hemisphere is coldest, and farthest in July, when it is warmest, which is the opposite of what a distance-driven theory would predict. Second, the two hemispheres have opposite seasons at the same time, even though they are at the same distance from the Sun; distance cannot make one hemisphere warm while the other is cold. Third, the tilt explanation predicts the observed pattern of day length, Sun angle, and temperature through the year, and it does so at every latitude.

What eccentricity does affect is the length of the seasons, not their existence. Because Earth moves faster near perihelion and slower near aphelion, the seasons are not all the same length. NASA states that summers in the Northern Hemisphere are currently about 4.5 days longer than winters, and springs about three days longer than autumns, and that as eccentricity decreases, the length of the seasons gradually evens out. Eccentricity also makes the seasons slightly more or less extreme depending on which hemisphere has summer at perihelion, a slow shift governed by precession. At present, perihelion falls during winter in the Northern Hemisphere and summer in the Southern Hemisphere, which makes Southern Hemisphere summers hotter and moderates Northern Hemisphere seasonal variation, according to NASA. In about 13,000 years, axial precession will flip that arrangement.

To see how the tilt plays out at the turning points of the year, read First Day of Summer for the June solstice, when the Northern Hemisphere tilts most directly toward the Sun, and Shortest Day of the Year for the December solstice, when it tilts farthest away. Those two days are the clearest demonstrations that the calendar of the seasons is set by tilt, while eccentricity only adjusts the fine detail.

How eccentricity changes over time

Earth’s eccentricity is not fixed. It slowly varies between nearly 0.0034, an orbit almost indistinguishable from a perfect circle, and almost 0.058, a slightly more elongated ellipse, in a cycle that spans about 100,000 years. NASA gives both the range and the cycle length, and notes that Earth’s eccentricity is currently very slowly decreasing and approaching its least elliptic, most circular, state.

The range matters because it sets the scale of what eccentricity can do. At its most circular, about 0.0034, the difference between perihelion and aphelion shrinks to less than 1 percent of the average distance, and the seasonal difference in sunlight from distance alone almost disappears. At its most elliptical, about 0.058, the difference grows to about 11.6 percent of the average distance, and the sunlight difference between perihelion and aphelion grows to about 23 percent, according to NASA. That is a much larger effect than the present 6.8 percent, and it is large enough to change how strongly the seasons differ between the two hemispheres. Even so, NASA stresses that the total change in global annual sunlight due to the eccentricity cycle is very small, because the extra sunlight gained near perihelion is largely balanced by the reduced sunlight near aphelion over the course of a full year. Eccentricity redistributes sunlight through the year more than it changes the yearly total.

The 100,000-year figure is the dominant cycle, but orbital calculations also recognize a longer component of about 405,000 years, in which the eccentricity cycle itself grows stronger and weaker. Think of it as a slow modulation laid over the faster 100,000-year rhythm: during some stretches of geological time, the swings between circular and elliptical are larger, and during others they are smaller. The 405,000-year component is a result of long-term orbital calculations rather than a direct NASA headline figure, so it should be read as a calculated pattern, not as a directly observed cycle in the same way as the 100,000-year one.

The direction of change today is also worth recording. NASA states that Earth’s eccentricity is currently very slowly decreasing. That means the orbit is getting slightly more circular, year by year, and the difference between perihelion and aphelion is shrinking by a tiny amount. The change is far too slow to affect weather, farming, or daily life, and it will take thousands of years to become noticeable even in careful records. Its importance is geological, not practical: it is one of the slow rhythms that set the background conditions for climate over tens of thousands of years.

Eccentricity and ice ages: the Milankovitch cycles

Eccentricity is one of three orbital cycles that together are called the Milankovitch cycles, after the Serbian scientist Milutin Milankovitch, who worked out their combined effect on climate in the early twentieth century. The other two are obliquity, the tilt of Earth’s axis, and precession, the slow wobble in the direction the axis points. NASA lists all three as the components of the cycles, and explains that together they cause variations of up to 25 percent in the amount of incoming sunlight at Earth’s mid-latitudes, the regions between about 30 and 60 degrees north and south of the equator.

Each cycle runs on its own timescale. Eccentricity varies over about 100,000 years. Obliquity, the axial tilt, varies between 22.1 and 24.5 degrees in a cycle of about 41,000 years, and Earth’s axis is currently tilted 23.4 degrees, about halfway between its extremes. Axial precession, the wobble of the axis, spans about 25,771.5 years, and combined with the slow turning of the orbital ellipse itself, it gives an overall precession cycle of about 23,000 years on average. These three rhythms overlap and interfere, so the sunlight reaching a given latitude in a given season depends on where all three cycles happen to be at once.

Milankovitch calculated that ice ages occur approximately every 41,000 years, driven mainly by the tilt cycle. Later research confirmed that ice ages did occur at about 41,000-year intervals between one and three million years ago. But about 800,000 years ago, the cycle of ice ages lengthened to about 100,000 years, matching the eccentricity cycle instead. NASA reports that transition plainly and adds that scientists do not yet have a clear answer for why it happened. That honest gap is important: the link between eccentricity and the timing of ice ages is well supported, but the mechanism behind the change in rhythm is still an open question.

The evidence for the cycles comes from the geological record. In 1976, a study in the journal Science by Hays and colleagues, using deep-sea sediment cores, found that Milankovitch cycles correspond with periods of major climate change over the past 450,000 years, with ice ages occurring when Earth was undergoing different stages of orbital variation. Later work using ice cores from Greenland and Antarctica provided further evidence going back many hundreds of thousands of years. Milankovitch himself died in 1958, about ten years before the scientific community began to take his theory seriously, and his work has since been embraced by the National Research Council of the U.S. National Academy of Sciences, according to NASA.

Two cautions belong here. First, eccentricity has the smallest direct effect of the three cycles on total yearly sunlight. Its importance lies mainly in pacing: when eccentricity is high, the effect of precession on the seasons is stronger, and when eccentricity is low, the effect of precession is weaker. Second, these cycles operate over tens of thousands to hundreds of thousands of years. They set the slow background for ice ages, but they cannot explain rapid climate changes that happen over decades or centuries. Anyone who claims that a change in eccentricity explains a warming trend measured over a single lifetime is confusing timescales: the orbital cycles are real and powerful, but they move far too slowly for that.

Eccentricity across the solar system

Earth’s value of 0.0167 is best understood by comparison. The table below gives the orbital eccentricities of the eight planets, rounded to three decimal places. The values are the commonly tabulated figures used in astronomy references, and they show how unusual, and how ordinary, Earth’s orbit is at the same time.

Venus and Neptune have the most circular orbits, at about 0.007 and about 0.009. Mercury has by far the most eccentric orbit of any planet, at about 0.206, which means its distance from the Sun varies by more than 40 percent between perihelion and aphelion, and its surface temperature swings are correspondingly large. Mars, at about 0.093, has an orbit more than five times as eccentric as Earth’s, which contributes to its more extreme seasons. The giant planets sit in the middle, between about 0.046 and 0.057, more eccentric than Earth but far less than Mercury.

Beyond the planets, the range widens dramatically. Many asteroids have eccentricities above 0.3, and comets can exceed 0.9. Halley’s Comet, at about 0.967, spends most of its roughly 76-year orbit far from the Sun and only briefly sweeps close past it. Dwarf planets show the same spread: Pluto has an eccentricity of about 0.244, which is why its orbit crosses inside Neptune’s for part of its year. Against that background, Earth’s 0.0167 is a reminder that the planets that formed close to the Sun, in the crowded inner solar system, mostly settled into nearly circular orbits, while the smaller bodies that were scattered outward kept the stretched shapes of their violent histories.

Eccentricity also matters beyond our solar system. Astronomers measure the eccentricities of planets orbiting other stars, called exoplanets, and find values far higher than anything in our solar system. Some exoplanets have eccentricities above 0.9, which means their climates, if they have atmospheres at all, would swing between extremes every orbit. Our solar system, with its low planetary eccentricities, looks comparatively calm. That calm is not a coincidence: stable, nearly circular orbits are one of the conditions that allow a planet to keep a steady climate for billions of years.

Common misunderstandings about eccentricity

Several misunderstandings about eccentricity appear again and again in popular writing, and each one is easy to correct once the definitions are clear.

The first is that Earth’s orbit is a strongly stretched oval. It is not. With an eccentricity of 0.0167, Earth’s orbit is nearly circular, and any diagram that shows an obvious oval is exaggerating the stretch so that the shape can be seen at all. NASA’s own illustrations of the Milankovitch cycles carry exactly that warning: the changes in eccentricity are exaggerated so the effect can be seen. If you drew the orbit to scale, you would see a circle.

The second is that winter happens because Earth is farther from the Sun. It does not. Earth is closest to the Sun in January, during Northern Hemisphere winter, and the seasons are caused by axial tilt, as explained above. The distance effect is real but small, about a 6.8 percent difference in sunlight, and it is swamped by the effect of tilt.

The third is that eccentricity is a fixed property of Earth’s orbit. It is not. It varies between about 0.0034 and about 0.058 over about 100,000 years, driven by the gravity of Jupiter and Saturn, and it is currently very slowly decreasing. Quoting 0.0167 without a date is like quoting a temperature without saying when it was measured: accurate for now, but not a permanent constant.

The fourth is that a higher eccentricity would simply make summers hotter and winters colder everywhere. It would not. Higher eccentricity makes the difference between perihelion and aphelion larger, which strengthens the seasonal contrast in one hemisphere and weakens it in the other, depending on which hemisphere has summer at perihelion. The effect is opposite in the two hemispheres, not the same in both.

The fifth is that eccentricity is the same thing as the tilt of Earth’s axis. It is not. Eccentricity describes the shape of the orbit, obliquity describes the tilt of the axis, and precession describes the direction the axis points. They are three separate cycles with three separate timescales, and they affect climate in different ways. Keeping them apart is the key to understanding the Milankovitch cycles as a whole.

Why eccentricity matters, even though it is small

A reader could reasonably ask why a number as small as 0.0167 deserves an article of its own. The answer is that small numbers can have large consequences when they act over long times. Eccentricity does three things that matter, even at its present low value.

First, it sets the length of the seasons. The 4.5-day difference between Northern Hemisphere summer and winter is small, but it is a direct consequence of eccentricity, and it is large enough to show up in any careful table of season lengths. As eccentricity decreases toward its most circular state, the seasons will gradually even out.

Second, it paces the ice ages. Eccentricity itself changes total yearly sunlight only slightly, but by strengthening or weakening the effect of precession, it helps set the timing of glacial cycles. The shift from 41,000-year to 100,000-year ice age cycles about 800,000 years ago is one of the most important unsolved problems in paleoclimatology, and eccentricity sits at the center of it.

Third, it is a reminder that Earth’s climate is not set by one factor alone. Tilt, orbit shape, axis direction, atmosphere, oceans, and ice all interact, and the same change in sunlight can have different effects depending on where the other cycles happen to be. Understanding eccentricity is therefore not only about a single number. It is about learning to think in terms of interacting cycles, each moving at its own pace, whose combined effect is the climate we live in.

That is also why the number is worth dating. Saying “Earth’s eccentricity is 0.0167” is accurate in November 2026. Saying it without a date invites the misunderstanding that it is a constant of nature. It is not. It is a measurement of a slowly changing orbit, taken at a particular moment in a 100,000-year cycle, and it will be slightly different for readers a thousand years from now. The orbit is stable enough to trust and slow enough to measure, and that combination is exactly what makes it worth understanding.

FAQ

What is the eccentricity of Earth’s orbit?

The eccentricity of Earth’s orbit is about 0.0167 as of November 2026. That is a low value on the scale from 0, a perfect circle, to 1, a parabola, and it means Earth’s orbit is nearly circular. The value varies slowly over about 100,000 years, from nearly 0.0034 to almost 0.058, and it is currently very slowly decreasing toward its most circular state.

What does eccentricity mean in simple terms?

Eccentricity is a number that measures how far an orbit departs from a perfect circle. A value of 0 is a perfect circle, values between 0 and 1 are ellipses, and the closer the value is to 1, the more stretched the ellipse is. It is calculated as the distance between the two focal points of the ellipse divided by the length of the major axis, or equivalently as the distance from the center of the ellipse to one focus divided by the semi-major axis.

Is Earth’s orbit a circle or an ellipse?

It is an ellipse, but one so close to a circle that the difference is invisible to the naked eye. Earth’s orbit has a semi-major axis of about 149.6 million kilometers and an eccentricity of about 0.0167, so its long and short diameters differ by only about 0.014 percent. Diagrams that show an obvious oval are exaggerating the stretch for clarity. Kepler’s first law states that each planet moves in an ellipse with the Sun at one focus, and Earth’s orbit follows that law with a very small eccentricity.

When is Earth closest to and farthest from the Sun?

Earth reaches perihelion, its closest point to the Sun, on or about January 3 each year, at about 147.1 million kilometers. It reaches aphelion, its farthest point, on or about July 4, at about 152.1 million kilometers. The difference is about 5.1 million kilometers, or about 3.4 percent of the average distance, and Earth receives about 6.8 percent more sunlight in January than in July for that reason alone.

Does eccentricity cause the seasons?

No. The seasons are caused mainly by the tilt of Earth’s axis, currently about 23.4 degrees, which changes how directly sunlight strikes each hemisphere through the year. Eccentricity only adjusts the fine detail: it makes Northern Hemisphere summers about 4.5 days longer than winters, because Earth moves more slowly near aphelion, and it slightly changes how much sunlight each hemisphere receives. The proof is in the dates: Earth is closest to the Sun in January, during Northern Hemisphere winter, which is the opposite of what a distance-driven theory would predict.

How does Earth’s eccentricity change over time?

Earth’s eccentricity varies between nearly 0.0034 and almost 0.058 in a cycle spanning about 100,000 years, driven mainly by the gravitational pull of Jupiter and Saturn. A longer modulation of about 405,000 years is also recognized in orbital calculations. At present, eccentricity is very slowly decreasing and approaching its least elliptic state. Over geological time, these changes form part of the Milankovitch cycles, which pace the timing of ice ages together with changes in axial tilt and precession.

How eccentric are the orbits of the other planets?

Venus has the most circular planetary orbit at about 0.007, followed by Neptune at about 0.009. Mercury has the most eccentric planetary orbit at about 0.206, and Mars is at about 0.093. The giant planets fall between about 0.046 and 0.057. Comets are far more eccentric: Halley’s Comet has an eccentricity of about 0.967. Earth’s value of about 0.0167 is low, which is one reason its year-to-year climate is comparatively stable.

Can eccentricity explain current climate change?

No, not on the timescale involved. The Milankovitch cycles, including eccentricity, operate over tens of thousands to hundreds of thousands of years, and NASA notes that the total change in global annual sunlight due to the eccentricity cycle is very small. Changes measured over decades or a single lifetime happen far too fast to be caused by orbital cycles, which move far too slowly to account for them.

Sources & references

• * NASA Science, "Milankovitch (Orbital) Cycles and Their Role in Earth's Climate" (published 27 February 2020, updated 22 October 2024). Read 5 November 2026. Used for the eccentricity definition, the perihelion and aphelion dates and distances, the 5.1 million kilometer difference, the 6.8 percent sunlight difference, the 23 percent figure at maximum eccentricity, the 100,000-year cycle, the 0.0034 to 0.058 range, the current slow decrease, the season-length differences, the tilt range and 41,000-year cycle, the precession figures, the 25 percent mid-latitude insolation variation, the 800,000-year ice age transition, and the Hays et al. 1976 evidence.
• * Encyclopaedia Britannica, "Eccentricity" (astronomy). Read 5 November 2026. Used for the definition of eccentricity as the distance between the focal points divided by the major axis, the 0 to 1 scale, and Earth's value of approximately 0.0167.
• * National Oceanic and Atmospheric Administration (NOAA) / NASA Space Place, "Why Does Earth Have Seasons?" (published 26 August 2025). Read 5 November 2026. Used for the statement that seasons are caused by axial tilt rather than distance, the tilt of 23.45 degrees, and the perihelion and aphelion distances of 91,400,000 and 94,500,000 miles.
• * NASA Earth Fact Sheet (David R. Williams), as tabulated in the "Earth's orbit" reference table. Read via the tabulated orbital characteristics 5 November 2026. Used for the semi-major axis of 149.60 million kilometers, the perihelion of 147.10 million kilometers, the aphelion of 152.10 million kilometers, the J2000.0 eccentricity of 0.0167086, the orbital period of 365.256 days, the orbital distance of about 940 million kilometers, and the orbital speeds of about 30.29 kilometers per second at perihelion and about 29.29 at aphelion against an average of 29.78.
• * ScienceABC, "Orbital Eccentricity: Why Do Planets Travel in Elliptical Path?" Read 5 November 2026. Used as a cross-check for the tabulated planetary eccentricities of Mercury (0.206), Venus (0.007), and Earth (0.017 rounded).
• * Penn State University, "The Cold of Space" (GEOSC 10 course material). Read 5 November 2026. Used as a cross-check for the eccentricity range of 0.0034 to 0.058 over 100,000-year cycles.