Shop Windows to the Universe

The Spring 2011 issue of The Earth Scientist is focused on modernizing seismology education. Thanks to IRIS, you can download this issue for free as a pdf. Print copies are available in our online store.

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).

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).

(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.

Shop Windows to the Universe Science Store!

Learn about Earth and space science, and have fun while doing it! The games section of our online store includes a climate change card game and the Traveling Nitrogen game!

Windows to the Universe Community



You might also be interested in:

Kepler's 1st Law: Orbits are Elliptical

With Tycho Brahe's observations in hand, Kepler set out to determine if the paths of the planets against the background stars could be described with a curve. By trial and error, he discovered that an...more

The Kuiper Belt

Thousands of frozen worlds hover at the edge of our Solar System. These frigid balls of ice and rock, similar in location and size to the planet Pluto, orbit the Sun in a distant region called the Kuiper...more

Asteroid (or Comet?) 3200 Phaethon

In 1983, a satellite named IRAS (Infrared Astronomical Satellite) discovered a strange, dark object orbiting the Sun. The object, which was given the name 3200 Phaethon, turned out to be an asteroid. It...more

Haumea (dwarf planet)

Haumea is a dwarf planet in our Solar System. Haumea is the fifth object officially classified as a dwarf planet; the previous four are Pluto, Eris, Ceres, and Makemake. The International Astronomical...more

Images & Multimedia

Here you will find links to all sorts of pictures, animations, videos, sounds, and interactive multimedia that are on Windows to the Universe Explore collections of images in the Image Galleries. Watch...more

What does falling have to do with keeping a satellite in orbit?

The key to understanding satellite motions is in recognizing that gravity acts only in the downward direction - forward motion and falling are totally independent of each other. If there were no gravity...more

The Heart of Satellite Orbits

Starting at the same altitude above the surface of a small moon, balls are thrown at different speeds. Regardless of the horizontal speed of the balls, they all fall at the same rate and hit the surface...more


PAH is the short name for a "polycyclic aromatic hydrocarbon". A PAH is a stable structure made up of multiple rings that are fused together. The rings are made of carbon and hydrogen (though other elements...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