Motion

Aphelion & Perihelion

8.5 -Understand the terms ‘aphelion’ and ‘perihelion’ (solar orbits), ‘apogee’ and ‘perigee’ (Earth orbits) for an elliptical orbit

Planets and comets do not orbit in perfect circles. They orbit in ellipses.

The point at which they are nearest the Sun (or focus, such as a moon around a planet) is called perihelion.

The point at which they are furthest from the Sun is called aphelion.

An orbit is the path of a body (such as a planet, moon or comet etc.) around another. The Earth travels around the Sun in 365.25 days.

The orbit of the Earth is not quite circular. The shape of the Earth's orbit is an ellipse. It is an elliptical orbit.

This is like an oval orbit. Most objects in the Solar System follow an elliptical orbit.

One part of an ellipse is nearer to the Sun than the other.

PERIHELION = Body NEARER to Sun
APHELION = Body FURTHER from Sun

When the Earth is nearest to the Sun (in January) it is in PERIHELION or perigee and is 147,000,000 km from it.

When it is furthest from the Sun (in July) it is in APHELION or apogee. It is then 153,000,000 km from the Sun.

The mean average distance is approximately 150,000,000 km. This is called the Astronomical Unit, or AU.

We use the terms Perigee and Apogee to describe one objects relationship to another. So at one point of the Moon's elliptical orbit around Earth it will be in perigee at its closest and apogee at its furthest point from Earth.

PERIGEE = Body NEARER to orbiting body
APOGEE = Body FURTHER from orbiting body