Earth travels around the Sun in a slightly elliptical (oval-shaped) path, completing one full loop every 365.25 days at an average speed of about 29.78 kilometers per second, or roughly 67,000 miles per hour. That orbit defines our year, shapes our seasons, and sits at the heart of nearly every natural cycle we experience on this planet.
Shape of the Orbit
Earth’s path around the Sun is not a perfect circle, but it’s close. The orbit is an ellipse, meaning it’s slightly stretched in one direction, like an oval. The degree of stretch is measured by a value called eccentricity, and Earth’s current eccentricity is very low. The distance between Earth and the Sun at the closest point is only about 3% less than the distance at the farthest point, so the orbit is nearly circular to the naked eye if you could see it drawn in space.
At its closest approach to the Sun, called perihelion, Earth sits about 91.4 million miles (147 million km) away. At its farthest point, called aphelion, that distance stretches to roughly 94.5 million miles (152 million km). Perihelion happens around January 3 each year, while aphelion falls near July 4. That three-million-mile difference matters less than you might think for temperature, but it does have subtle effects on how much solar energy reaches us at different times of year.
How Fast Earth Moves
Earth’s average orbital speed is 29.78 km/s, but it doesn’t move at a constant pace. A principle described by Johannes Kepler explains why: a planet moves fastest when it’s closest to the Sun and slowest when it’s farthest away. So in early January, when Earth is at perihelion, it’s zipping along at its top orbital speed. By early July, at aphelion, it has slowed to its minimum. The difference is modest because the orbit is so close to circular, but it’s measurable. It also means that the seasons aren’t perfectly equal in length. Northern Hemisphere winter (when Earth is closer to the Sun and moving faster) is actually a few days shorter than summer.
Why One Orbit Isn’t Exactly 365 Days
A single trip around the Sun takes 365.2422 days by the calendar we use, which is called the tropical year. This is the time between one spring equinox and the next, and it’s what our seasons are synced to. There’s also the sidereal year, which is the time it takes Earth to return to the exact same position relative to distant stars: 365.2564 days, about 20 minutes longer. The difference exists because Earth’s axis slowly wobbles over thousands of years, slightly shifting the point where the equinox occurs along the orbit.
That extra quarter-day in the tropical year is why we add a leap day every four years. Without it, the calendar would drift out of alignment with the seasons by about one full day every four years, and eventually January would fall in what we think of as summer.
Earth’s Tilt, Not Its Distance, Drives Seasons
One of the most common misunderstandings about Earth’s orbit is that seasons are caused by our changing distance from the Sun. They aren’t. Earth is actually closest to the Sun during Northern Hemisphere winter. The real cause is the 23.5-degree tilt of Earth’s rotational axis relative to the plane of its orbit.
During summer in the Northern Hemisphere, Earth’s axis tilts that hemisphere toward the Sun. Sunlight hits the surface at a steeper, more direct angle, concentrating energy over a smaller area. Days are also longer, giving the ground more hours to absorb heat. In winter, the Northern Hemisphere tilts away from the Sun, so light arrives at a shallow angle, spreading the same energy over a larger area, and days are shorter. The Southern Hemisphere experiences the opposite pattern at the same time.
At the spring and fall equinoxes, neither hemisphere is tilted toward or away from the Sun, so both receive roughly equal amounts of sunlight. This tilt is the single biggest factor controlling seasonal temperature changes on Earth, far more important than the small variation in distance caused by the elliptical orbit.
What Keeps the Orbit Stable
Earth’s orbit is governed by the Sun’s gravity, but every other planet in the solar system tugs on Earth too. The strongest influences come from Jupiter and Saturn, the two most massive planets. Their gravitational pull causes small, slow perturbations in the orbits of all the inner rocky planets, including Earth. However, studies of long-term orbital stability show that the changes to Earth’s orbit are small. Earth’s path is remarkably stable over hundreds of millions of years, which has been a key factor in allowing complex life to develop.
Mercury, by contrast, shows much larger swings in its orbital shape and tilt over time. But because Mercury is so small, its instability doesn’t threaten the rest of the solar system.
How the Orbit Changes Over Millennia
Earth’s orbit isn’t frozen. Over tens of thousands of years, three slow, cyclical changes alter how Earth moves around the Sun and how sunlight is distributed across its surface. These are collectively known as Milankovitch cycles, and they’ve played a major role in triggering ice ages.
The first cycle involves eccentricity. The shape of Earth’s orbit shifts from nearly circular to slightly more elliptical and back again over a roughly 100,000-year cycle, driven primarily by the gravitational pull of Jupiter and Saturn. When the orbit is at its most elliptical, the difference in solar energy received at perihelion versus aphelion can reach about 23%, a much larger variation than the roughly 6% difference we see today.
The second cycle is obliquity: the tilt of Earth’s axis shifts between about 22.1 and 24.5 degrees over a roughly 41,000-year cycle. A greater tilt means more extreme seasons, with hotter summers and colder winters. A smaller tilt produces milder seasons.
The third cycle is precession, a slow wobble in the direction Earth’s axis points, completing a full rotation roughly every 26,000 years. Precession determines which hemisphere is tilted toward the Sun at perihelion. Right now, the Northern Hemisphere’s winter coincides with perihelion. In about 13,000 years, it will be the Northern Hemisphere’s summer that coincides with the closest solar approach.
Together, these three cycles can cause variations of up to 25% in the amount of solar energy reaching Earth’s mid-latitudes. That’s enough to push the climate into or out of glacial periods over tens of thousands of years, even though any single cycle’s effect is gradual.