Perihelion and Aphelion

The closest point to the Sun in a planet's orbit is called perihelion. The furthest point is called aphelion. Notice how the planet moves fastest at perihelion and slowest at aphelion.
Randy Russell).

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.)

Interactive animation illustrating shapes of orbits

Elliptical orbits

Eccentricity of an orbit

Last modified December 14, 2005 by Randy Russell.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

What Causes the Seasons?

Let's get rid of some common misconceptions about the seasons. The Earth's orbit is in the shape of an ellipse, so that sometimes the Earth is a little bit closer to the Sun than at other times. Is this...more


Pluto is a frigid ball of ice and rock that orbits far from the Sun on the frozen fringes of our Solar System. Considered a planet, though a rather odd one, from its discovery in 1930 until 2006, it was...more

Eccentricity of an Orbit

You may think that most objects in space that orbit something else move in circles, but that isn't the case. Although some objects follow circular orbits, most orbits are shaped more like "stretched...more

Kepler's 2nd Law: The Speeds of Planets

Kepler's second law he again discovered by trial and error. Kepler realized that the line connecting the planet and the Sun sweeps out equal area in equal time. Look at the diagram to the left. What Kepler...more

Elliptical Orbits

You may think that most objects in space that orbit something else move in circles, but that isn't the case. Although some objects follow circular orbits, most orbits are shaped more like "stretched...more

Eris - a dwarf planet

Eris is a dwarf planet in our Solar System. Eris was one of the first three objects classified as a dwarf planet, along with Pluto and Ceres. Eris was first spotted in January 2005. Eris is a large sphere...more

Makemake: a Dwarf Planet

Makemake is a dwarf planet in our Solar System. Makemake was discovered in March 2005 by a team of astronomers led by Mike Brown. Makemake officially became a dwarf planet in July 2008. There were three...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA