The orbits of the planets were something that
astronomers struggled to explain. Planets appeared to wander around the
sky, sometimes reversing direction. There were two main schools of
thought about planetary orbits in the 16^{th} century; one that the planets orbited the Earth
in the Geocentric model, and the other that the planets orbited the
Sun in the Heliocentric model. Unfortunately, both models struggled to
explain the corkscrew like behaviour of planetary motion.
It was not until the 17^{th} Century when Johannes Kepler, working as an
assistant for the famous astronomer Tacho Brahe, was given the task of
explaining the orbit of the planet Mars. After six years of studying
and thousands of pages of calculations, Kepler finally deduced that the
orbit of Mars around the Sun was in fact not perfectly circular
but was elliptical.
The more elliptical an object the higher its eccentricity
The ellipse does not have one
centre like a circle, but two foci as it is the product of two competing
factors (the motion of the planet and the pull of gravity). Because the Sun has
over 98% of all the matter in the solar system its gravitational pull is MASSIVE!
Kepler’s First Law states all planetary orbits are elliptical –
where planets orbit around two foci. This image is greatly
exaggerated, with most planets having an eccentricity of less than 0.2, meaning
they are very close to circular.
Investigate how the spacing
between the foci effects the eccentricity of an ellipse by following the method
shown on this handson video tutorial.
The word eccentric means “off centre”, and although we like
to save the term for our favourite quirky relative, it is originally a
mathematical term. The eccentricity of
an ellipse depends on how flattened it is. The eccentricity ranges
from 0 – 1, where zero has no flattening and is a circle, the closer to 1
it is the more flattened the circle is.
Ellipse with eccentricity of ~ 0.2 similar to the orbit of Mercury. (https://www.mathopenref.com/ellipseeccentricity.html, accessed 29/06/2020)
The simulation on this website allows you to adjust the
distance between the foci and see how this effects eccentricity: Eccentricity and Ellipse
Below is a table of the eccentricity of different planets in
our solar system.
Planet

Major axis (AU)

Eccentricity

Mercury

0.774

0.2056

Venus

1.446

0.0068

Earth

2

0.0167

Mars

3.048

0.0934

Jupiter

10.406

0.0484

Saturn

19.074

0.0542

Neptune

60.14

0.0086

As you can see, there is no relationship between distance
from the Sun and the eccentricity of a planets orbit. In fact, Mercury has the most eccentric
orbit. However, even an ellipse with eccentricity of 0.2 appears very circular.
This is perhaps why it took so long for astronomers to realise that planets
were orbiting the Sun in ellipses.