Aphelion

When something is orbiting the Sun, this is the point of the orbit that is furthest from the Sun. It applies to anything in solar orbit – planet, comet, minor planet, dust particle …

For the Earth, aphelion is around July 4th, when the Northern Hemisphere is in summer.  The word aphelion derives from the Greek words, apo meaning away, off, apart and Helios (the Greek god of the sun).

The reason why the this occurs is because orbits are elliptical and not circular. Kepler realised this and published the information in 1609. An ellipse has two foci which I suppose can be regarded as the equivalent of the centre of a circle and are used to construct the ellipse. In terms of an orbit, the Sun sits at one of the foci therefore as a body orbits the Sun it will have a varying distance.

Illustration of aphelion and perihelion

Illustration of aphelion and perihelion

There is a further explanation and animation at http://www.windows2universe.org/physical_science/physics/mechanics/orbit/perihelion_aphelion.html

 

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