All of the planets in our Solar System move around the Sun in elliptical orbits. An ellipse is a shape that can be thought of as a "stretched out" circle or an oval. The Sun is not at the center of the ellipse, as it would be if the orbit were circular. Instead, the Sun is at one of two points called "foci" (which is the plural form of "focus") that are offset from the center. This means that each planet moves closer towards and further away from the Sun during the course of each orbit. The point in the orbit where the planet is closest to the Sun is called "perihelion". The point where the planet is furthest from the Sun is called "aphelion".

Earth reaches perihelion in early January each year, and passes through its aphelion point near the start of July. At perihelion, our planet is about 147 million km (91 million miles) from the Sun; it moves outward to around 152 million km (95 million miles) from the Sun at aphelion. Earth is about 3% further from the Sun at aphelion than it is at perihelion. Some people have the mistaken impression that our seasons are caused by changes in Earth's distance from the Sun, but this is not the case. Notice how Earth is actually closest to the Sun in the middle of the (Northern Hemisphere's) winter!

Earth's orbit is almost a perfect circle, so the difference between its distance to the Sun at aphelion and at perihelion is slight. Some planets have orbits that are more elongated; astronomers say their orbits have a greater "eccentricity", which is a technical term for how "stretched out" an orbit is. Mercury and Pluto have the most eccentric orbits of the planets. Mercury is 52% further from the Sun at aphelion than it is at perihelion, while Pluto is 66% further away at aphelion than at perihelion.

Planets, of course, are not the only objects that orbit the Sun. Many asteroids and comets, and some spacecraft, follow elliptical orbits around the Sun. Any object in such an orbit has both a perihelion and an aphelion point along its orbit. As determined by Kepler and stated in his Second Law of Planetary Motion, the speed of an object in its orbit is fastest at perihelion and slowest at aphelion.

The terms perihelion and aphelion apply specifically to objects orbiting the Sun. There are similar terms for the closest and furthest points in orbits around other bodies, such as Earth, the Moon, and other planets and stars. The most commonly used are:

• Earth - perigee and apogee
• Moon - perilune (or periselene) and apolune (or aposelene)
• a star - periastron and apastron
• Jupiter - perijove and apojove
• a generic object - periapsis or apoapsis

Aphelion is derived from the Greek words "apo" (away from) and "helios" (Sun), while perihelion includes the Greek word "peri" (near).

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The planets in our Solar System orbit the Sun. The orbits of some planets are almost perfect circles, but others are not. Some orbits are shaped more like ovals, or "stretched out" circles. Scientists call these oval shapes "ellipses". If a planet's orbit is a circle, the Sun is at the center of that circle. If, instead, the orbit is an ellipse, the Sun is at a point called the "focus" of the ellipse, which is not quite the same as the center.

Since the Sun is not at the center of an elliptical orbit, the planet moves closer towards and further away from the Sun as it orbits. The place where the planet is closest to the Sun is called perihelion. When the planet is furthest away from the Sun, it is at aphelion. The words "aphelion" and "perihelion" come from the Greek language. In Greek, "helios" mean Sun, "peri" means near, and "apo" means away from.

When Earth is at perihelion, it is about 147 million km (91 million miles) from the Sun. When it is at aphelion, it is 152 million km (almost 95 million miles) from the Sun. Earth is about 5 million km (more than 3 million miles) further from the Sun at aphelion than at perihelion!

Some people think that this is why we have seasons, but they are wrong. Earth reaches perihelion, its closest approach to the Sun and when you might think it should be warmest, in January - the middle of winter in the Northern Hemisphere! The difference in distance is not the cause of our seasons. Instead, seasons are caused by the tilt of Earth's axis.

Some planets have very "stretched out" orbits. Pluto, for example, is much further from the Sun at aphelion than it is at perihelion. Astronomers say that a "stretched out" orbit has a high eccentricity, which means it is long and skinny, not round like a circle. Asteroids, many comets, and some spacecraft also travel around the Sun in elliptical orbits. They all have perihelion and aphelion points along their orbits. Anything following an elliptical orbit moves fastest at perihelion and slowest at aphelion.

If an object orbits something other than the Sun, we don't use the terms perihelion and aphelion. Satellites orbiting Earth (including the Moon!) have a close point called perigee and a far point called apogee. If you want to know the terms for objects that orbit other bodies, take a look at the advanced version of this page.

(Note: If you cannot see the animation on this page, or it is not working properly, you may need to download the latest Flash player.)

The planets in our Solar System orbit around the Sun. Some orbits are almost perfect circles. Other orbits are more like "stretched out" circles or ovals. Astronomers call these oval shapes "ellipses".

When a planet orbits the Sun along an ellipse, sometimes it gets closer to the Sun. At other times, the planet moves further away from the Sun. When the planet is closest to the Sun, we say it is at "perihelion". When it is farthest away from the Sun, we say that it is at "aphelion". Earth moves about 5 million km (over 3 million miles) further away from the Sun at aphelion than at perihelion!

You might guess that Earth is warmer at perihelion because it is closer to the hot Sun. You might think that our seasons are caused by Earth moving closer to the Sun and further away, but that is NOT true! Perihelion, when Earth is closest to the Sun, happens in January. That is the middle of winter in the Northern Hemisphere! The real reason for our seasons is that Earth is tilted a bit.

A planet moves at different speeds as it goes around its orbit. It moves fastest at perihelion. It moves slowest at aphelion.

Asteroids, many comets, and some spacecraft also move around the Sun along an ellipse. They all have perihelion and aphelion points along their orbits, too.

Some planets have very "stretched out" orbits. Pluto is much further from the Sun at aphelion than it is at perihelion. Astronomers say that a "stretched out" orbit has a high eccentricity. An orbit with a high eccentricity is long and skinny, not round like a circle.

(Note: If you cannot see the animation on this page, or it is not working properly, you may need to download the latest Flash player.)