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.)
