GLOSSARY

ecliptic

The plane containing the Earth’s orbit.

The planets do not all orbit in the same plane as each other: the solar system is not flat. Their orbits make angles with each other. The ecliptic is the plane that contains the orbit of the Earth — so it contains the centre of the Sun and the path of the centre of the Earth as it moves around the Sun.

Interestingly, the Earth’s equator does not lie in the ecliptic: it makes an angle with it. In other words, the Earth is tilted. And, in fact, the Sun’s equator does not lie in the ecliptic, either. The axes of rotation of the Earth and the Sun are not parallel.

You can see in the diagram above that the axis of rotation of Uranus nearly lies in its orbital plane — in other words, it’s almost as if Uranus rolls around in its orbit. (It doesn’t actually do this, of course, because it’s not sitting on a surface as it moves.)

You may also have noticed that the arrow representing the axis of rotation of Venus points downwards. This indicates that Venus rotates around its own axis in the opposite direction to that of the Sun around its axis and, indeed, to the direction the other planets rotate (except Uranus, which is discussed in the previous paragraph). We call such rotation retrograde.

It is clear from the diagram that the orbital plane of Pluto’s orbit is very different from that of the eight planets — this was one of the motivations for ‘downgrading’ Pluto from being a planet to being a dwarf planet: there is clearly something very different about it. Indeed, the eccentricity of Pluto’s orbit is so large (0.25) that its orbit actually crosses the orbit of Neptune — so sometimes Pluto is nearer to the Sun than Neptune is. This was last true between 1979 and 1999; but it will not be true again for over 200 years.