Earlier this month, the Earth had its aphelion - our furthest point to the sun. This is not the same as the solstice - which is the point of maximum tilt of the Earth towards the sun and creates the seasons.
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For Earth, the average distance to the sun is 149,600,000km. At aphelion, our furthest point from the sun, we are about 152,000,000km. We also have perihelion, our closest point. As our orbit is elliptical, just as we have the furthest point, we have the closest point, which is about 147,000,000km.
This means our distance to the sun varies by about 5 million kilometres, or a few per cent.
We may picture the orbit of the Earth around the sun as a perfect, but it is not, it is slightly elliptical. Nearly every orbit in space - the Earth around the sun, the moon around the Earth, and even our satellites, are elliptical, not circular.
A circular orbit is just a type of an elliptical orbit. They are both stable, and orbit or move in a regular, predictable motion.
Elliptical orbits offer more options for planets to form around their stars, or moons around their planets. This all has to do with the original formation of the star and planets.
The sun as well as the planets and other objects in our solar system, were formed from the same, spinning, gas cloud. As bits of gas and dust start to come together through gravity, they conserve angular momentum (like a figure skater spinning and moving their arms in and out).
This conservation means as mass is pulled together, it balances with the momentum of the objects moving around the star. This balance is rarely a circle - easier to balance into an ellipse.
This balance creates a range of sizes and densities of planets to form, and the elliptical nature of an orbit of the planet to vary.
Mars, which is about half the size of Earth and only 10 per cent the mass of Earth, has a more eccentric orbit than the Earth. The average distance of Mars from the sun is about 228 million kilometres.
However, Mars at aphelion is about 249 million kilometres, and perihelion is 206 million kilometres. That is a variation of over 43 million kilometres, over 18 per cent of its orbit.
Jupiter has an average distance of 778 million kilometres, with an aphelion of 816 million and perihelion of 741 million kilometres. This variation of 75 million kilometres is around 9 per cent of its orbit.
Most of the planets have a range between 2-3 per cent to 15 per cent in their orbit, between perihelion and aphelion.
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Pluto is very different to most of the planets. Pluto's closest point to the sun is 4.4 billion kilometres, while its aphelion 7.4 billion kilometres - a difference of 3 billion kilometres or a variation of 50 per cent or so.
Our moon also does this. The moon is on average 384,400km away but ranges in apogee (furthest point to Earth) at 405,500 km on average to 363,000km at perigee (closest point to Earth).
Nothing is perfect in the universe - the orbit of the Earth is not a perfect circle, the Earth is not a perfect sphere, and the same for all the other planets and moons.
But it is the imperfection that makes them all unique.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU.
